internal/api/admin_duplicates.go

@@ -2,11 +2,14 @@ package api
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
+	"io"
 	"net/http"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
@@ -228,3 +231,86 @@ func (h *handlers) handleDismissDuplicateGroup(w http.ResponseWriter, r *http.Re
 	}
 	writeJSON(w, http.StatusOK, map[string]string{"status": "dismissed"})
 }
+
+// mergeDuplicateRequest chooses the copy to keep. An empty survivor_track_id
+// keeps the report's proposal.
+type mergeDuplicateRequest struct {
+	SurvivorTrackID string `json:"survivor_track_id"`
+	Unmonitor       bool   `json:"unmonitor"`
+}
+
+// mergeDuplicateResponse reports what the merge removed. RemovedPaths are files
+// deleted from disk; the operator reads them to know exactly what went.
+type mergeDuplicateResponse struct {
+	SurvivorTrackID       string   `json:"survivor_track_id"`
+	RemovedPaths          []string `json:"removed_paths"`
+	LidarrUnmonitorFailed *bool    `json:"lidarr_unmonitor_failed,omitempty"`
+}
+
+// mergeRequestBodyLimit bounds the request body. It holds one id and a flag.
+const mergeRequestBodyLimit = 1 << 16
+
+// handleMergeDuplicateGroup implements POST /api/admin/library/duplicates/{id}/merge
+// (#3911): keep one copy, move the others' likes, plays and playlist entries onto
+// it, and delete the others' files and rows.
+//
+// Errors:
+//   - 409 library_not_writable / 500 file_delete_failed when a file could not be
+//     removed — nothing was changed (fileRemoveAPIError)
+//   - 404 duplicate_group_not_pending when the group was already resolved
+//   - 400 survivor_not_in_group, invalid_id, invalid_body
+func (h *handlers) handleMergeDuplicateGroup(w http.ResponseWriter, r *http.Request) {
+	admin, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	groupID, ok := parseUUID(chi.URLParam(r, "id"))
+	if !ok {
+		writeAdminJSONErr(w, http.StatusBadRequest, "invalid_id")
+		return
+	}
+	var body mergeDuplicateRequest
+	if err := json.NewDecoder(http.MaxBytesReader(w, r.Body, mergeRequestBodyLimit)).Decode(&body); err != nil && !errors.Is(err, io.EOF) {
+		writeAdminJSONErr(w, http.StatusBadRequest, "invalid_body")
+		return
+	}
+	var survivorID pgtype.UUID // invalid: keep the proposal
+	if body.SurvivorTrackID != "" {
+		if survivorID, ok = parseUUID(body.SurvivorTrackID); !ok {
+			writeAdminJSONErr(w, http.StatusBadRequest, "invalid_id")
+			return
+		}
+	}
+
+	res, unmonitorFailed, err := h.tracks.MergeDuplicates(r.Context(), groupID, survivorID, admin.ID, body.Unmonitor)
+	if err != nil {
+		if apiErr, ok := fileRemoveAPIError(err); ok {
+			logFileRemoveFailure(h.logger, apiErr, "group_id", uuidToString(groupID))
+			writeErr(w, apiErr)
+			return
+		}
+		switch {
+		case errors.Is(err, library.ErrDuplicateGroupNotPending):
+			writeAdminJSONErr(w, http.StatusNotFound, "duplicate_group_not_pending")
+		case errors.Is(err, library.ErrSurvivorNotInGroup):
+			writeAdminJSONErr(w, http.StatusBadRequest, "survivor_not_in_group")
+		default:
+			h.logger.Error("admin: merge duplicate group", "group_id", uuidToString(groupID), "err", err)
+			writeAdminJSONErr(w, http.StatusInternalServerError, "server_error")
+		}
+		return
+	}
+
+	resp := mergeDuplicateResponse{
+		SurvivorTrackID: uuidToString(res.Survivor.TrackID),
+		RemovedPaths:    make([]string, 0, len(res.Removed)),
+	}
+	for _, c := range res.Removed {
+		resp.RemovedPaths = append(resp.RemovedPaths, c.FilePath)
+	}
+	if body.Unmonitor && unmonitorFailed {
+		failed := true
+		resp.LidarrUnmonitorFailed = &failed
+	}
+	writeJSON(w, http.StatusOK, resp)
+}

internal/api/api.go

@@ -217,10 +217,12 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				admin.Get("/library/coverage", h.handleGetLibraryCoverage)
 				admin.Get("/library/fingerprints", h.handleGetFingerprintCoverage)
 				// Duplicates report (#3912): proposals from the duplicate sweep, a
-				// trigger to sweep now, and dismissal. Nothing here merges or deletes.
+				// trigger to sweep now, dismissal, and the merge (#3911), which deletes
+				// the removed copies' files after moving their history onto the kept one.
 				admin.Get("/library/duplicates", h.handleListDuplicates)
 				admin.Post("/library/duplicates/sweep", h.handleRunDuplicateSweep)
 				admin.Post("/library/duplicates/{id}/dismiss", h.handleDismissDuplicateGroup)
+				admin.Post("/library/duplicates/{id}/merge", h.handleMergeDuplicateGroup)
 
 				admin.Get("/invites", h.handleListInvites)
 				admin.Post("/invites", h.handleCreateInvite)

internal/audit/audit.go

@@ -55,6 +55,11 @@ const (
 	// exercised.
 	ActionSessionRevoke       Action = "session_revoke"
 	ActionSessionRevokeOthers Action = "session_revoke_others"
+
+	// Duplicate merge (#3911). Irreversible: a copy's file and row are removed
+	// and its history moved onto the copy kept. The metadata names both, so the
+	// log can answer "where did that file go" long after the report is gone.
+	ActionDuplicateMerge Action = "duplicate_merge"
 )
 
 // Write inserts one audit_log row. metadata is marshaled as JSON;

internal/audit/audit_test.go

@@ -168,6 +168,7 @@ func TestWrite_AllActionConstantsArePersisted(t *testing.T) {
 		audit.ActionTokenRegenerate,
 		audit.ActionForgotPasswordInit,
 		audit.ActionPasswordResetByEmail,
+		audit.ActionDuplicateMerge,
 	}
 	for _, a := range actions {
 		if err := audit.Write(context.Background(), pool, nilUUID, nilUUID, a, nil); err != nil {

internal/db/dbq/merge.sql.go

@@ -0,0 +1,339 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: merge.sql
+
+package dbq
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+const listDuplicateGroupMergeMembers = `-- name: ListDuplicateGroupMergeMembers :many
+SELECT t.id, t.file_path, t.file_format, t.file_size, t.added_at, t.album_id,
+       t.mbid, albums.mbid AS album_mbid
+  FROM duplicate_group_members m
+  JOIN tracks t  ON t.id = m.track_id
+  JOIN albums    ON albums.id = t.album_id
+ WHERE m.group_id = $1
+ ORDER BY t.id
+`
+
+type ListDuplicateGroupMergeMembersRow struct {
+	ID         pgtype.UUID
+	FilePath   string
+	FileFormat string
+	FileSize   int64
+	AddedAt    pgtype.Timestamptz
+	AlbumID    pgtype.UUID
+	Mbid       *string
+	AlbumMbid  *string
+}
+
+func (q *Queries) ListDuplicateGroupMergeMembers(ctx context.Context, groupID pgtype.UUID) ([]ListDuplicateGroupMergeMembersRow, error) {
+	rows, err := q.db.Query(ctx, listDuplicateGroupMergeMembers, groupID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListDuplicateGroupMergeMembersRow
+	for rows.Next() {
+		var i ListDuplicateGroupMergeMembersRow
+		if err := rows.Scan(
+			&i.ID,
+			&i.FilePath,
+			&i.FileFormat,
+			&i.FileSize,
+			&i.AddedAt,
+			&i.AlbumID,
+			&i.Mbid,
+			&i.AlbumMbid,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const lockDuplicateGroupForMerge = `-- name: LockDuplicateGroupForMerge :one
+
+SELECT id, tier, status
+  FROM duplicate_groups
+ WHERE id = $1
+   FOR UPDATE
+`
+
+type LockDuplicateGroupForMergeRow struct {
+	ID     pgtype.UUID
+	Tier   string
+	Status string
+}
+
+// Duplicate merge (Scribe #3911). Every statement here runs inside the one
+// transaction library.MergeDuplicateGroup opens, after the removed copy's file
+// is already gone. The loser's own track row is deleted last with DeleteTrack;
+// what these do is move everything it carries onto the survivor first, so that
+// delete's CASCADE finds nothing left to destroy.
+// Locks the group for the rest of the transaction, so two merges of one group
+// cannot run at once.
+func (q *Queries) LockDuplicateGroupForMerge(ctx context.Context, id pgtype.UUID) (LockDuplicateGroupForMergeRow, error) {
+	row := q.db.QueryRow(ctx, lockDuplicateGroupForMerge, id)
+	var i LockDuplicateGroupForMergeRow
+	err := row.Scan(&i.ID, &i.Tier, &i.Status)
+	return i, err
+}
+
+const markDuplicateGroupMerged = `-- name: MarkDuplicateGroupMerged :execrows
+UPDATE duplicate_groups
+   SET status = 'merged', resolved_at = now()
+ WHERE id = $1 AND status = 'pending'
+`
+
+func (q *Queries) MarkDuplicateGroupMerged(ctx context.Context, id pgtype.UUID) (int64, error) {
+	result, err := q.db.Exec(ctx, markDuplicateGroupMerged, id)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
+const mergeCopyGeneralLikes = `-- name: MergeCopyGeneralLikes :many
+
+INSERT INTO general_likes (user_id, track_id, liked_at)
+SELECT user_id, $1::uuid, liked_at
+  FROM general_likes
+ WHERE track_id = $2::uuid
+ON CONFLICT (user_id, track_id) DO UPDATE
+   SET liked_at = LEAST(general_likes.liked_at, EXCLUDED.liked_at)
+RETURNING user_id
+`
+
+type MergeCopyGeneralLikesParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+// Collision-safe merges: a unique key includes track_id, so the survivor may
+// already hold a matching row. Copy what it lacks; DeleteTrack's CASCADE then
+// removes the loser's originals.
+// One like per user. A user who liked both copies keeps a single like, dated to
+// the earlier of the two.
+func (q *Queries) MergeCopyGeneralLikes(ctx context.Context, arg MergeCopyGeneralLikesParams) ([]pgtype.UUID, error) {
+	rows, err := q.db.Query(ctx, mergeCopyGeneralLikes, arg.SurvivorID, arg.LoserID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []pgtype.UUID
+	for rows.Next() {
+		var user_id pgtype.UUID
+		if err := rows.Scan(&user_id); err != nil {
+			return nil, err
+		}
+		items = append(items, user_id)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const mergeCopyTrackSimilarity = `-- name: MergeCopyTrackSimilarity :execrows
+INSERT INTO track_similarity (track_a_id, track_b_id, score, source, fetched_at)
+SELECT CASE WHEN track_a_id = $1::uuid THEN $2::uuid ELSE track_a_id END,
+       CASE WHEN track_b_id = $1::uuid THEN $2::uuid ELSE track_b_id END,
+       score, source, fetched_at
+  FROM track_similarity
+ WHERE (track_a_id = $1::uuid OR track_b_id = $1::uuid)
+   AND (CASE WHEN track_a_id = $1::uuid THEN $2::uuid ELSE track_a_id END)
+    <> (CASE WHEN track_b_id = $1::uuid THEN $2::uuid ELSE track_b_id END)
+ON CONFLICT (track_a_id, track_b_id, source) DO NOTHING
+`
+
+type MergeCopyTrackSimilarityParams struct {
+	LoserID    pgtype.UUID
+	SurvivorID pgtype.UUID
+}
+
+// Rewrites the loser to the survivor on either side of an edge. An edge between
+// the two copies would become a track similar to itself — the table forbids
+// that, and it means nothing — so it is dropped. An edge the survivor already
+// has from the same source is kept as it is.
+func (q *Queries) MergeCopyTrackSimilarity(ctx context.Context, arg MergeCopyTrackSimilarityParams) (int64, error) {
+	result, err := q.db.Exec(ctx, mergeCopyTrackSimilarity, arg.LoserID, arg.SurvivorID)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
+const mergeCopyTrackTags = `-- name: MergeCopyTrackTags :execrows
+INSERT INTO track_tags (track_id, tag, weight)
+SELECT $1::uuid, tag, weight
+  FROM track_tags
+ WHERE track_id = $2::uuid
+ON CONFLICT (track_id, tag) DO NOTHING
+`
+
+type MergeCopyTrackTagsParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+func (q *Queries) MergeCopyTrackTags(ctx context.Context, arg MergeCopyTrackTagsParams) (int64, error) {
+	result, err := q.db.Exec(ctx, mergeCopyTrackTags, arg.SurvivorID, arg.LoserID)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
+const mergeInheritTrackMbid = `-- name: MergeInheritTrackMbid :exec
+UPDATE tracks AS survivor
+   SET mbid = loser.mbid
+  FROM tracks AS loser
+ WHERE survivor.id = $1::uuid
+   AND loser.id = $2::uuid
+   AND survivor.mbid IS NULL
+   AND loser.mbid IS NOT NULL
+`
+
+type MergeInheritTrackMbidParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+// A recording MBID is what the similarity pipeline keys on. If only the removed
+// copy carried one, the survivor takes it rather than going dark to similarity.
+func (q *Queries) MergeInheritTrackMbid(ctx context.Context, arg MergeInheritTrackMbidParams) error {
+	_, err := q.db.Exec(ctx, mergeInheritTrackMbid, arg.SurvivorID, arg.LoserID)
+	return err
+}
+
+const mergeRepointContextualLikes = `-- name: MergeRepointContextualLikes :execrows
+UPDATE contextual_likes SET track_id = $1::uuid WHERE track_id = $2::uuid
+`
+
+type MergeRepointContextualLikesParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+func (q *Queries) MergeRepointContextualLikes(ctx context.Context, arg MergeRepointContextualLikesParams) (int64, error) {
+	result, err := q.db.Exec(ctx, mergeRepointContextualLikes, arg.SurvivorID, arg.LoserID)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
+const mergeRepointLidarrRequests = `-- name: MergeRepointLidarrRequests :execrows
+UPDATE lidarr_requests SET matched_track_id = $1::uuid
+ WHERE matched_track_id = $2::uuid
+`
+
+type MergeRepointLidarrRequestsParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+func (q *Queries) MergeRepointLidarrRequests(ctx context.Context, arg MergeRepointLidarrRequestsParams) (int64, error) {
+	result, err := q.db.Exec(ctx, mergeRepointLidarrRequests, arg.SurvivorID, arg.LoserID)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
+const mergeRepointPlayEvents = `-- name: MergeRepointPlayEvents :execrows
+
+UPDATE play_events SET track_id = $1::uuid WHERE track_id = $2::uuid
+`
+
+type MergeRepointPlayEventsParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+// Plain repoints: no unique key involves track_id, so moving rows cannot collide.
+func (q *Queries) MergeRepointPlayEvents(ctx context.Context, arg MergeRepointPlayEventsParams) (int64, error) {
+	result, err := q.db.Exec(ctx, mergeRepointPlayEvents, arg.SurvivorID, arg.LoserID)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
+const mergeRepointPlaybackErrors = `-- name: MergeRepointPlaybackErrors :execrows
+UPDATE playback_errors SET track_id = $1::uuid WHERE track_id = $2::uuid
+`
+
+type MergeRepointPlaybackErrorsParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+func (q *Queries) MergeRepointPlaybackErrors(ctx context.Context, arg MergeRepointPlaybackErrorsParams) (int64, error) {
+	result, err := q.db.Exec(ctx, mergeRepointPlaybackErrors, arg.SurvivorID, arg.LoserID)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
+const mergeRepointPlaylistTracks = `-- name: MergeRepointPlaylistTracks :many
+UPDATE playlist_tracks SET track_id = $1::uuid
+ WHERE track_id = $2::uuid
+RETURNING playlist_id
+`
+
+type MergeRepointPlaylistTracksParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+// playlist_tracks is keyed by (playlist_id, position), so repointing keeps every
+// entry exactly where it was. A playlist that held both copies simply holds the
+// survivor twice — the user put two entries there, and both stay.
+func (q *Queries) MergeRepointPlaylistTracks(ctx context.Context, arg MergeRepointPlaylistTracksParams) ([]pgtype.UUID, error) {
+	rows, err := q.db.Query(ctx, mergeRepointPlaylistTracks, arg.SurvivorID, arg.LoserID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []pgtype.UUID
+	for rows.Next() {
+		var playlist_id pgtype.UUID
+		if err := rows.Scan(&playlist_id); err != nil {
+			return nil, err
+		}
+		items = append(items, playlist_id)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const mergeRepointSkipEvents = `-- name: MergeRepointSkipEvents :execrows
+UPDATE skip_events SET track_id = $1::uuid WHERE track_id = $2::uuid
+`
+
+type MergeRepointSkipEventsParams struct {
+	SurvivorID pgtype.UUID
+	LoserID    pgtype.UUID
+}
+
+func (q *Queries) MergeRepointSkipEvents(ctx context.Context, arg MergeRepointSkipEventsParams) (int64, error) {
+	result, err := q.db.Exec(ctx, mergeRepointSkipEvents, arg.SurvivorID, arg.LoserID)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}

internal/db/queries/merge.sql

@@ -0,0 +1,101 @@
+-- Duplicate merge (Scribe #3911). Every statement here runs inside the one
+-- transaction library.MergeDuplicateGroup opens, after the removed copy's file
+-- is already gone. The loser's own track row is deleted last with DeleteTrack;
+-- what these do is move everything it carries onto the survivor first, so that
+-- delete's CASCADE finds nothing left to destroy.
+
+-- name: LockDuplicateGroupForMerge :one
+-- Locks the group for the rest of the transaction, so two merges of one group
+-- cannot run at once.
+SELECT id, tier, status
+  FROM duplicate_groups
+ WHERE id = sqlc.arg(id)
+   FOR UPDATE;
+
+-- name: ListDuplicateGroupMergeMembers :many
+SELECT t.id, t.file_path, t.file_format, t.file_size, t.added_at, t.album_id,
+       t.mbid, albums.mbid AS album_mbid
+  FROM duplicate_group_members m
+  JOIN tracks t  ON t.id = m.track_id
+  JOIN albums    ON albums.id = t.album_id
+ WHERE m.group_id = sqlc.arg(group_id)
+ ORDER BY t.id;
+
+-- Plain repoints: no unique key involves track_id, so moving rows cannot collide.
+
+-- name: MergeRepointPlayEvents :execrows
+UPDATE play_events SET track_id = sqlc.arg(survivor_id)::uuid WHERE track_id = sqlc.arg(loser_id)::uuid;
+
+-- name: MergeRepointSkipEvents :execrows
+UPDATE skip_events SET track_id = sqlc.arg(survivor_id)::uuid WHERE track_id = sqlc.arg(loser_id)::uuid;
+
+-- name: MergeRepointContextualLikes :execrows
+UPDATE contextual_likes SET track_id = sqlc.arg(survivor_id)::uuid WHERE track_id = sqlc.arg(loser_id)::uuid;
+
+-- name: MergeRepointPlaybackErrors :execrows
+UPDATE playback_errors SET track_id = sqlc.arg(survivor_id)::uuid WHERE track_id = sqlc.arg(loser_id)::uuid;
+
+-- name: MergeRepointLidarrRequests :execrows
+UPDATE lidarr_requests SET matched_track_id = sqlc.arg(survivor_id)::uuid
+ WHERE matched_track_id = sqlc.arg(loser_id)::uuid;
+
+-- name: MergeRepointPlaylistTracks :many
+-- playlist_tracks is keyed by (playlist_id, position), so repointing keeps every
+-- entry exactly where it was. A playlist that held both copies simply holds the
+-- survivor twice — the user put two entries there, and both stay.
+UPDATE playlist_tracks SET track_id = sqlc.arg(survivor_id)::uuid
+ WHERE track_id = sqlc.arg(loser_id)::uuid
+RETURNING playlist_id;
+
+-- Collision-safe merges: a unique key includes track_id, so the survivor may
+-- already hold a matching row. Copy what it lacks; DeleteTrack's CASCADE then
+-- removes the loser's originals.
+
+-- name: MergeCopyGeneralLikes :many
+-- One like per user. A user who liked both copies keeps a single like, dated to
+-- the earlier of the two.
+INSERT INTO general_likes (user_id, track_id, liked_at)
+SELECT user_id, sqlc.arg(survivor_id)::uuid, liked_at
+  FROM general_likes
+ WHERE track_id = sqlc.arg(loser_id)::uuid
+ON CONFLICT (user_id, track_id) DO UPDATE
+   SET liked_at = LEAST(general_likes.liked_at, EXCLUDED.liked_at)
+RETURNING user_id;
+
+-- name: MergeCopyTrackTags :execrows
+INSERT INTO track_tags (track_id, tag, weight)
+SELECT sqlc.arg(survivor_id)::uuid, tag, weight
+  FROM track_tags
+ WHERE track_id = sqlc.arg(loser_id)::uuid
+ON CONFLICT (track_id, tag) DO NOTHING;
+
+-- name: MergeCopyTrackSimilarity :execrows
+-- Rewrites the loser to the survivor on either side of an edge. An edge between
+-- the two copies would become a track similar to itself — the table forbids
+-- that, and it means nothing — so it is dropped. An edge the survivor already
+-- has from the same source is kept as it is.
+INSERT INTO track_similarity (track_a_id, track_b_id, score, source, fetched_at)
+SELECT CASE WHEN track_a_id = sqlc.arg(loser_id)::uuid THEN sqlc.arg(survivor_id)::uuid ELSE track_a_id END,
+       CASE WHEN track_b_id = sqlc.arg(loser_id)::uuid THEN sqlc.arg(survivor_id)::uuid ELSE track_b_id END,
+       score, source, fetched_at
+  FROM track_similarity
+ WHERE (track_a_id = sqlc.arg(loser_id)::uuid OR track_b_id = sqlc.arg(loser_id)::uuid)
+   AND (CASE WHEN track_a_id = sqlc.arg(loser_id)::uuid THEN sqlc.arg(survivor_id)::uuid ELSE track_a_id END)
+    <> (CASE WHEN track_b_id = sqlc.arg(loser_id)::uuid THEN sqlc.arg(survivor_id)::uuid ELSE track_b_id END)
+ON CONFLICT (track_a_id, track_b_id, source) DO NOTHING;
+
+-- name: MergeInheritTrackMbid :exec
+-- A recording MBID is what the similarity pipeline keys on. If only the removed
+-- copy carried one, the survivor takes it rather than going dark to similarity.
+UPDATE tracks AS survivor
+   SET mbid = loser.mbid
+  FROM tracks AS loser
+ WHERE survivor.id = sqlc.arg(survivor_id)::uuid
+   AND loser.id = sqlc.arg(loser_id)::uuid
+   AND survivor.mbid IS NULL
+   AND loser.mbid IS NOT NULL;
+
+-- name: MarkDuplicateGroupMerged :execrows
+UPDATE duplicate_groups
+   SET status = 'merged', resolved_at = now()
+ WHERE id = sqlc.arg(id) AND status = 'pending';

internal/library/delete.go

@@ -114,10 +114,8 @@ func DeleteTrackFile(
 		return DeletedTrack{}, fmt.Errorf("get track: %w", err)
 	}
 
-	if err := removeFile(track.FilePath); err != nil && !errors.Is(err, fs.ErrNotExist) {
-		return DeletedTrack{}, &FileRemoveError{
-			Path: track.FilePath, UID: os.Getuid(), GID: os.Getgid(), Err: err,
-		}
+	if err := removeTrackFileOnDisk(track.FilePath); err != nil {
+		return DeletedTrack{}, err
 	}
 
 	// The row and any album or artist it empties go together, so a failure
@@ -138,25 +136,9 @@ func DeleteTrackFile(
 		return DeletedTrack{}, fmt.Errorf("delete track: %w", err)
 	}
 
-	var out DeletedTrack
-	album, err := tq.DeleteAlbumIfEmpty(ctx, deleted.AlbumID)
-	switch {
-	case err == nil:
-		albumID := album.ID
-		out.AlbumID = &albumID
-		artistID, aerr := tq.DeleteArtistIfEmpty(ctx, album.ArtistID)
-		switch {
-		case aerr == nil:
-			out.ArtistID = &artistID
-		case errors.Is(aerr, pgx.ErrNoRows):
-			// The artist still has other albums or stray tracks.
-		default:
-			return DeletedTrack{}, fmt.Errorf("delete artist if empty: %w", aerr)
-		}
-	case errors.Is(err, pgx.ErrNoRows):
-		// The album still has other tracks.
-	default:
-		return DeletedTrack{}, fmt.Errorf("delete album if empty: %w", err)
+	out, err := tidyEmptiedAlbum(ctx, tq, deleted.AlbumID)
+	if err != nil {
+		return DeletedTrack{}, err
 	}
 
 	if err := tx.Commit(ctx); err != nil {
@@ -179,3 +161,41 @@ func DeleteTrackFile(
 	}
 	return out, nil
 }
+
+// removeTrackFileOnDisk is the one rule for removing a track's file, shared by
+// DeleteTrackFile and the duplicate merge. A file already gone is fine; anything
+// else comes back as a *FileRemoveError, and the caller must then change nothing
+// in the database (#3918).
+func removeTrackFileOnDisk(path string) error {
+	if err := removeFile(path); err != nil && !errors.Is(err, fs.ErrNotExist) {
+		return &FileRemoveError{Path: path, UID: os.Getuid(), GID: os.Getgid(), Err: err}
+	}
+	return nil
+}
+
+// tidyEmptiedAlbum removes an album a track delete left with no tracks, and its
+// artist if that album was the artist's last. It runs on the caller's
+// transaction, so the tidy-up commits or rolls back with the delete itself.
+func tidyEmptiedAlbum(ctx context.Context, tq *dbq.Queries, albumID pgtype.UUID) (DeletedTrack, error) {
+	var out DeletedTrack
+	album, err := tq.DeleteAlbumIfEmpty(ctx, albumID)
+	switch {
+	case err == nil:
+		id := album.ID
+		out.AlbumID = &id
+		artistID, aerr := tq.DeleteArtistIfEmpty(ctx, album.ArtistID)
+		switch {
+		case aerr == nil:
+			out.ArtistID = &artistID
+		case errors.Is(aerr, pgx.ErrNoRows):
+			// The artist still has other albums or stray tracks.
+		default:
+			return DeletedTrack{}, fmt.Errorf("delete artist if empty: %w", aerr)
+		}
+	case errors.Is(err, pgx.ErrNoRows):
+		// The album still has other tracks.
+	default:
+		return DeletedTrack{}, fmt.Errorf("delete album if empty: %w", err)
+	}
+	return out, nil
+}

internal/library/duplicate_merge.go

@@ -0,0 +1,309 @@
+package library
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"sort"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
+)
+
+// Duplicate merge (M400 #3911).
+
+// ErrDuplicateGroupNotPending means the group was already merged or dismissed,
+// no longer exists, or no longer has two members to merge.
+var ErrDuplicateGroupNotPending = errors.New("library: duplicate group is not pending")
+
+// ErrSurvivorNotInGroup means the copy chosen to keep is not a member of the group.
+var ErrSurvivorNotInGroup = errors.New("library: survivor is not a member of the group")
+
+// MergedCopy is one copy a merge kept or removed.
+type MergedCopy struct {
+	TrackID   pgtype.UUID
+	FilePath  string
+	TrackMbid *string
+	AlbumMbid *string
+}
+
+// MergeResult says what a merge did.
+type MergeResult struct {
+	Tier     string
+	Survivor MergedCopy
+	Removed  []MergedCopy
+
+	// What moved onto the survivor — reported so the operator, and the audit
+	// log, can see that the history was kept rather than take it on trust.
+	PlayEvents      int64
+	SkipEvents      int64
+	Likes           int // users whose like now sits on the survivor
+	PlaylistEntries int
+
+	DeletedAlbumIDs  []pgtype.UUID
+	DeletedArtistIDs []pgtype.UUID
+}
+
+// MergeDuplicateGroup keeps one copy of a duplicate group and removes the rest,
+// carrying everything the removed copies held onto the one kept.
+//
+// survivorID chooses the copy to keep; an invalid (zero) id takes the proposal
+// from ProposeSurvivor.
+//
+// The danger this is built around: every table referencing tracks does so ON
+// DELETE CASCADE, so deleting a duplicate's row outright silently destroys its
+// likes, plays, playlist entries and tags. The merge moves all of that onto the
+// survivor first, and only then deletes the now-empty row.
+//
+// It deletes the removed copies' FILES too, and first, before any row changes
+// (#3918, note #3926). A merge that left the file behind would be undone by the
+// next scan, which re-imports it as a new track with no history. If a file cannot
+// be removed, the *FileRemoveError comes back and nothing in the database changes.
+// With several copies to remove, one file may already be gone when a later one
+// fails; that copy's row keeps all its history and is marked missing by the next
+// scan, and retrying the merge picks up where it stopped.
+//
+// Everything else happens in one transaction, which holds a lock on the group so
+// two merges of it cannot run at once. Sync changes for clients' caches are logged
+// inside it, the way the playlists service logs its own.
+func MergeDuplicateGroup(
+	ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, dataDir string,
+	groupID, survivorID pgtype.UUID,
+) (MergeResult, error) {
+	if logger == nil {
+		logger = slog.Default()
+	}
+	tx, err := pool.Begin(ctx)
+	if err != nil {
+		return MergeResult{}, fmt.Errorf("begin merge: %w", err)
+	}
+	defer func() { _ = tx.Rollback(ctx) }()
+	tq := dbq.New(tx)
+
+	group, err := tq.LockDuplicateGroupForMerge(ctx, groupID)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return MergeResult{}, ErrDuplicateGroupNotPending
+	}
+	if err != nil {
+		return MergeResult{}, fmt.Errorf("lock duplicate group: %w", err)
+	}
+	if group.Status != "pending" {
+		return MergeResult{}, ErrDuplicateGroupNotPending
+	}
+
+	members, err := tq.ListDuplicateGroupMergeMembers(ctx, groupID)
+	if err != nil {
+		return MergeResult{}, fmt.Errorf("list group members: %w", err)
+	}
+	if len(members) < 2 {
+		return MergeResult{}, ErrDuplicateGroupNotPending
+	}
+	survivor, losers, err := splitSurvivor(members, survivorID)
+	if err != nil {
+		return MergeResult{}, err
+	}
+
+	for _, l := range losers {
+		if err := removeTrackFileOnDisk(l.FilePath); err != nil {
+			return MergeResult{}, err
+		}
+	}
+
+	res := MergeResult{Tier: group.Tier, Survivor: mergedCopyOf(survivor)}
+	likers := map[string]struct{}{}
+	changes := mergeChanges{}
+	survivorKey := syncpkg.FormatUUID(survivor.ID)
+
+	for _, l := range losers {
+		ids := struct{ survivor, loser pgtype.UUID }{survivor.ID, l.ID}
+		loserKey := syncpkg.FormatUUID(l.ID)
+
+		n, err := tq.MergeRepointPlayEvents(ctx, dbq.MergeRepointPlayEventsParams{SurvivorID: ids.survivor, LoserID: ids.loser})
+		if err != nil {
+			return MergeResult{}, fmt.Errorf("move play events: %w", err)
+		}
+		res.PlayEvents += n
+		n, err = tq.MergeRepointSkipEvents(ctx, dbq.MergeRepointSkipEventsParams{SurvivorID: ids.survivor, LoserID: ids.loser})
+		if err != nil {
+			return MergeResult{}, fmt.Errorf("move skip events: %w", err)
+		}
+		res.SkipEvents += n
+		if _, err := tq.MergeRepointContextualLikes(ctx, dbq.MergeRepointContextualLikesParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
+			return MergeResult{}, fmt.Errorf("move contextual likes: %w", err)
+		}
+		if _, err := tq.MergeRepointPlaybackErrors(ctx, dbq.MergeRepointPlaybackErrorsParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
+			return MergeResult{}, fmt.Errorf("move playback errors: %w", err)
+		}
+		if _, err := tq.MergeRepointLidarrRequests(ctx, dbq.MergeRepointLidarrRequestsParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
+			return MergeResult{}, fmt.Errorf("move lidarr request matches: %w", err)
+		}
+
+		playlists, err := tq.MergeRepointPlaylistTracks(ctx, dbq.MergeRepointPlaylistTracksParams{SurvivorID: ids.survivor, LoserID: ids.loser})
+		if err != nil {
+			return MergeResult{}, fmt.Errorf("move playlist entries: %w", err)
+		}
+		res.PlaylistEntries += len(playlists)
+		for _, pl := range playlists {
+			plKey := syncpkg.FormatUUID(pl)
+			changes.playlistDelete(syncpkg.EncodePlaylistTrackID(plKey, loserKey))
+			changes.playlistUpsert(syncpkg.EncodePlaylistTrackID(plKey, survivorKey))
+		}
+
+		users, err := tq.MergeCopyGeneralLikes(ctx, dbq.MergeCopyGeneralLikesParams{SurvivorID: ids.survivor, LoserID: ids.loser})
+		if err != nil {
+			return MergeResult{}, fmt.Errorf("move likes: %w", err)
+		}
+		for _, u := range users {
+			userKey := syncpkg.FormatUUID(u)
+			likers[userKey] = struct{}{}
+			changes.likeDelete(syncpkg.EncodeLikeID(userKey, loserKey))
+			changes.likeUpsert(syncpkg.EncodeLikeID(userKey, survivorKey))
+		}
+
+		if _, err := tq.MergeCopyTrackTags(ctx, dbq.MergeCopyTrackTagsParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
+			return MergeResult{}, fmt.Errorf("merge tags: %w", err)
+		}
+		if _, err := tq.MergeCopyTrackSimilarity(ctx, dbq.MergeCopyTrackSimilarityParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
+			return MergeResult{}, fmt.Errorf("merge similarity: %w", err)
+		}
+		if err := tq.MergeInheritTrackMbid(ctx, dbq.MergeInheritTrackMbidParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
+			return MergeResult{}, fmt.Errorf("inherit recording mbid: %w", err)
+		}
+
+		// Everything the loser carried now sits on the survivor, so the CASCADE
+		// this delete sets off has nothing left to destroy.
+		deleted, err := tq.DeleteTrack(ctx, l.ID)
+		if err != nil {
+			return MergeResult{}, fmt.Errorf("delete merged copy: %w", err)
+		}
+		tidied, err := tidyEmptiedAlbum(ctx, tq, deleted.AlbumID)
+		if err != nil {
+			return MergeResult{}, err
+		}
+		if tidied.AlbumID != nil {
+			res.DeletedAlbumIDs = append(res.DeletedAlbumIDs, *tidied.AlbumID)
+		}
+		if tidied.ArtistID != nil {
+			res.DeletedArtistIDs = append(res.DeletedArtistIDs, *tidied.ArtistID)
+		}
+		res.Removed = append(res.Removed, mergedCopyOf(l))
+		changes.trackDelete(loserKey)
+	}
+	res.Likes = len(likers)
+
+	marked, err := tq.MarkDuplicateGroupMerged(ctx, groupID)
+	if err != nil {
+		return MergeResult{}, fmt.Errorf("mark group merged: %w", err)
+	}
+	if marked != 1 {
+		return MergeResult{}, ErrDuplicateGroupNotPending
+	}
+	if err := changes.log(ctx, tx); err != nil {
+		return MergeResult{}, err
+	}
+	if err := tx.Commit(ctx); err != nil {
+		return MergeResult{}, fmt.Errorf("commit merge: %w", err)
+	}
+
+	// After commit, like DeleteTrackFile: a leftover art directory is only disk.
+	if dataDir != "" {
+		for _, artistID := range res.DeletedArtistIDs {
+			if err := coverart.CleanupArtistArt(dataDir, artistID); err != nil {
+				logger.Warn("duplicate merge: artist-art cleanup failed",
+					"artist_id", syncpkg.FormatUUID(artistID), "err", err)
+			}
+		}
+	}
+	return res, nil
+}
+
+// splitSurvivor separates the copy to keep from the copies to remove. An
+// invalid survivorID takes ProposeSurvivor's choice.
+func splitSurvivor(
+	members []dbq.ListDuplicateGroupMergeMembersRow, survivorID pgtype.UUID,
+) (dbq.ListDuplicateGroupMergeMembersRow, []dbq.ListDuplicateGroupMergeMembersRow, error) {
+	want := ""
+	if survivorID.Valid {
+		want = syncpkg.FormatUUID(survivorID)
+	} else {
+		cands := make([]SurvivorCandidate, len(members))
+		for i, m := range members {
+			cands[i] = SurvivorCandidate{
+				TrackID: syncpkg.FormatUUID(m.ID), FileFormat: m.FileFormat, FileSize: m.FileSize, AddedAt: m.AddedAt.Time,
+			}
+		}
+		want, _ = ProposeSurvivor(cands)
+	}
+
+	var survivor dbq.ListDuplicateGroupMergeMembersRow
+	found := false
+	var losers []dbq.ListDuplicateGroupMergeMembersRow
+	for _, m := range members {
+		if syncpkg.FormatUUID(m.ID) == want {
+			survivor, found = m, true
+			continue
+		}
+		losers = append(losers, m)
+	}
+	if !found {
+		return dbq.ListDuplicateGroupMergeMembersRow{}, nil, ErrSurvivorNotInGroup
+	}
+	return survivor, losers, nil
+}
+
+func mergedCopyOf(m dbq.ListDuplicateGroupMergeMembersRow) MergedCopy {
+	return MergedCopy{TrackID: m.ID, FilePath: m.FilePath, TrackMbid: m.Mbid, AlbumMbid: m.AlbumMbid}
+}
+
+// mergeChanges collects the sync-log entries a merge owes clients' caches, each
+// once: a user who liked two removed copies still gets one upsert for the
+// survivor.
+type mergeChanges struct {
+	tracks, likeDeletes, likeUpserts, playlistDeletes, playlistUpserts map[string]struct{}
+}
+
+func addTo(set *map[string]struct{}, id string) {
+	if *set == nil {
+		*set = map[string]struct{}{}
+	}
+	(*set)[id] = struct{}{}
+}
+
+func (c *mergeChanges) trackDelete(id string)    { addTo(&c.tracks, id) }
+func (c *mergeChanges) likeDelete(id string)     { addTo(&c.likeDeletes, id) }
+func (c *mergeChanges) likeUpsert(id string)     { addTo(&c.likeUpserts, id) }
+func (c *mergeChanges) playlistDelete(id string) { addTo(&c.playlistDeletes, id) }
+func (c *mergeChanges) playlistUpsert(id string) { addTo(&c.playlistUpserts, id) }
+
+func (c *mergeChanges) log(ctx context.Context, tx pgx.Tx) error {
+	for _, entry := range []struct {
+		kind syncpkg.EntityType
+		ids  map[string]struct{}
+		op   syncpkg.Op
+	}{
+		{syncpkg.EntityTrack, c.tracks, syncpkg.OpDelete},
+		{syncpkg.EntityLikeTrack, c.likeDeletes, syncpkg.OpDelete},
+		{syncpkg.EntityLikeTrack, c.likeUpserts, syncpkg.OpUpsert},
+		{syncpkg.EntityPlaylistTrack, c.playlistDeletes, syncpkg.OpDelete},
+		{syncpkg.EntityPlaylistTrack, c.playlistUpserts, syncpkg.OpUpsert},
+	} {
+		if len(entry.ids) == 0 {
+			continue
+		}
+		ids := make([]string, 0, len(entry.ids))
+		for id := range entry.ids {
+			ids = append(ids, id)
+		}
+		sort.Strings(ids)
+		if err := syncpkg.LogChanges(ctx, tx, entry.kind, ids, entry.op); err != nil {
+			return fmt.Errorf("log merge changes: %w", err)
+		}
+	}
+	return nil
+}

internal/library/duplicate_merge_test.go

@@ -0,0 +1,281 @@
+package library
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/dbtest"
+)
+
+// mergeFixture is a library with one duplicate pair carrying history on both
+// copies, and a neighbour track for similarity edges.
+type mergeFixture struct {
+	pool                  *pgxpool.Pool
+	keep, remove, other   dbq.Track
+	keepPath, removePath  string
+	groupID               pgtype.UUID
+	alice, bob            dbq.User
+	playlistID            pgtype.UUID
+	removePlaylistPos     int32
+	aliceEarlierLikeOnRem time.Time
+}
+
+func newMergeFixture(t *testing.T) mergeFixture {
+	t.Helper()
+	pool := newPool(t)
+	ctx := context.Background()
+	q := dbq.New(pool)
+	dir := t.TempDir()
+	f := mergeFixture{pool: pool}
+
+	f.keepPath = filepath.Join(dir, "keep.flac")
+	f.removePath = filepath.Join(dir, "remove.mp3")
+	for _, p := range []string{f.keepPath, f.removePath} {
+		if err := os.WriteFile(p, []byte("audio"), 0o644); err != nil {
+			t.Fatalf("write %s: %v", p, err)
+		}
+	}
+	var album dbq.Album
+	var artist dbq.Artist
+	f.keep, album, artist = seedTrack(t, pool, f.keepPath)
+	upsert := func(title, path string) dbq.Track {
+		t.Helper()
+		tr, err := q.UpsertTrack(ctx, dbq.UpsertTrackParams{
+			Title: title, AlbumID: album.ID, ArtistID: artist.ID,
+			DurationMs: 215000, FilePath: path, FileSize: 100, FileFormat: "mp3",
+		})
+		if err != nil {
+			t.Fatalf("track %s: %v", title, err)
+		}
+		return tr
+	}
+	f.remove = upsert("WWW (copy)", f.removePath)
+	f.other = upsert("Neighbour", filepath.Join(dir, "other.mp3"))
+
+	mustExec := func(sql string, args ...any) {
+		t.Helper()
+		if _, err := pool.Exec(ctx, sql, args...); err != nil {
+			t.Fatalf("exec %q: %v", sql, err)
+		}
+	}
+	// Only the copy being removed carries a recording MBID.
+	mustExec(`UPDATE tracks SET mbid = 'rec-www' WHERE id = $1`, f.remove.ID)
+
+	user := func(name string) dbq.User {
+		t.Helper()
+		u, err := q.CreateUser(ctx, dbq.CreateUserParams{
+			Username: dbtest.TestUserPrefix + name, PasswordHash: "x", ApiToken: name + "-merge-token",
+		})
+		if err != nil {
+			t.Fatalf("user %s: %v", name, err)
+		}
+		return u
+	}
+	f.alice, f.bob = user("merge-alice"), user("merge-bob")
+
+	// Alice liked both copies, the removed one first; Bob liked only the removed one.
+	f.aliceEarlierLikeOnRem = time.Now().Add(-72 * time.Hour).UTC().Truncate(time.Microsecond)
+	mustExec(`INSERT INTO general_likes (user_id, track_id, liked_at) VALUES ($1, $2, $3), ($1, $4, now()), ($5, $2, now())`,
+		f.alice.ID, f.remove.ID, f.aliceEarlierLikeOnRem, f.keep.ID, f.bob.ID)
+
+	now := pgtype.Timestamptz{Time: time.Now(), Valid: true}
+	session, err := q.InsertPlaySession(ctx, dbq.InsertPlaySessionParams{UserID: f.alice.ID, StartedAt: now})
+	if err != nil {
+		t.Fatalf("session: %v", err)
+	}
+	for _, track := range []dbq.Track{f.remove, f.remove, f.keep} {
+		if _, err := q.InsertPlayEvent(ctx, dbq.InsertPlayEventParams{
+			UserID: f.alice.ID, TrackID: track.ID, SessionID: session.ID, StartedAt: now,
+		}); err != nil {
+			t.Fatalf("play event: %v", err)
+		}
+	}
+	if _, err := q.InsertSkipEvent(ctx, dbq.InsertSkipEventParams{
+		UserID: f.alice.ID, TrackID: f.remove.ID, SessionID: session.ID, SkippedAt: now, PositionMs: 1000,
+	}); err != nil {
+		t.Fatalf("skip event: %v", err)
+	}
+
+	pl, err := q.CreatePlaylist(ctx, dbq.CreatePlaylistParams{UserID: f.alice.ID, Name: "merge-mix"})
+	if err != nil {
+		t.Fatalf("playlist: %v", err)
+	}
+	f.playlistID = pl.ID
+	entry, err := q.AppendPlaylistTrack(ctx, dbq.AppendPlaylistTrackParams{PlaylistID: pl.ID, TrackID: f.remove.ID})
+	if err != nil {
+		t.Fatalf("playlist entry: %v", err)
+	}
+	f.removePlaylistPos = entry.Position
+
+	mustExec(`INSERT INTO track_tags (track_id, tag, weight) VALUES ($1, 'j-pop', 1), ($1, 'house', 0.5), ($2, 'house', 0.9)`,
+		f.remove.ID, f.keep.ID)
+	mustExec(`INSERT INTO track_similarity (track_a_id, track_b_id, score, source) VALUES
+		($1, $3, 0.8, 'listenbrainz'),
+		($2, $3, 0.7, 'listenbrainz'),
+		($1, $2, 0.99, 'listenbrainz'),
+		($3, $1, 0.6, 'musicbrainz_tag')`, f.remove.ID, f.keep.ID, f.other.ID)
+
+	if err := pool.QueryRow(ctx,
+		`INSERT INTO duplicate_groups (member_key, tier) VALUES ('merge-fixture', 'exact') RETURNING id`,
+	).Scan(&f.groupID); err != nil {
+		t.Fatalf("group: %v", err)
+	}
+	mustExec(`INSERT INTO duplicate_group_members (group_id, track_id) VALUES ($1, $2), ($1, $3)`,
+		f.groupID, f.keep.ID, f.remove.ID)
+	return f
+}
+
+func (f mergeFixture) count(t *testing.T, sql string, args ...any) int {
+	t.Helper()
+	var n int
+	if err := f.pool.QueryRow(context.Background(), sql, args...).Scan(&n); err != nil {
+		t.Fatalf("count %q: %v", sql, err)
+	}
+	return n
+}
+
+// The #3911 proof: after a merge, every piece of history the removed copy held
+// is on the copy kept, nothing is doubled, and the removed copy — row and file —
+// is gone.
+func TestMergeDuplicateGroup_Integration(t *testing.T) {
+	f := newMergeFixture(t)
+	ctx := context.Background()
+
+	res, err := MergeDuplicateGroup(ctx, f.pool, nil, "", f.groupID, f.keep.ID)
+	if err != nil {
+		t.Fatalf("merge: %v", err)
+	}
+	if len(res.Removed) != 1 || res.Removed[0].FilePath != f.removePath || res.Survivor.TrackID != f.keep.ID {
+		t.Fatalf("result = %+v, want the removed copy reported and the kept one as survivor", res)
+	}
+	if res.PlayEvents != 2 || res.SkipEvents != 1 || res.Likes != 2 || res.PlaylistEntries != 1 {
+		t.Errorf("moved = plays %d skips %d likes %d playlist %d, want 2, 1, 2, 1",
+			res.PlayEvents, res.SkipEvents, res.Likes, res.PlaylistEntries)
+	}
+
+	if _, err := os.Stat(f.removePath); !errors.Is(err, os.ErrNotExist) {
+		t.Errorf("removed copy's file still on disk: %v", err)
+	}
+	if _, err := os.Stat(f.keepPath); err != nil {
+		t.Errorf("kept copy's file is gone: %v", err)
+	}
+	if n := f.count(t, `SELECT count(*) FROM tracks WHERE id = $1`, f.remove.ID); n != 0 {
+		t.Errorf("removed copy's row still exists")
+	}
+
+	// Likes: one per user, Alice's dated to her earlier like.
+	if n := f.count(t, `SELECT count(*) FROM general_likes WHERE track_id = $1`, f.keep.ID); n != 2 {
+		t.Errorf("likes on the kept copy = %d, want 2 (Alice once, Bob)", n)
+	}
+	var aliceLiked time.Time
+	if err := f.pool.QueryRow(ctx, `SELECT liked_at FROM general_likes WHERE user_id = $1 AND track_id = $2`,
+		f.alice.ID, f.keep.ID).Scan(&aliceLiked); err != nil {
+		t.Fatalf("alice's like: %v", err)
+	}
+	if !aliceLiked.Equal(f.aliceEarlierLikeOnRem) {
+		t.Errorf("alice's like dated %v, want her earlier like %v", aliceLiked, f.aliceEarlierLikeOnRem)
+	}
+
+	// Plays and skips move exactly: none lost, none invented.
+	if n := f.count(t, `SELECT count(*) FROM play_events WHERE track_id = $1`, f.keep.ID); n != 3 {
+		t.Errorf("plays on the kept copy = %d, want 3", n)
+	}
+	if n := f.count(t, `SELECT count(*) FROM skip_events WHERE track_id = $1`, f.keep.ID); n != 1 {
+		t.Errorf("skips on the kept copy = %d, want 1", n)
+	}
+
+	// The playlist entry stays where it was and now plays the kept copy.
+	if n := f.count(t, `SELECT count(*) FROM playlist_tracks WHERE playlist_id = $1 AND position = $2 AND track_id = $3`,
+		f.playlistID, f.removePlaylistPos, f.keep.ID); n != 1 {
+		t.Errorf("playlist entry at position %d does not point at the kept copy", f.removePlaylistPos)
+	}
+
+	// Tags are a union; the kept copy's own weight wins where both had the tag.
+	if n := f.count(t, `SELECT count(*) FROM track_tags WHERE track_id = $1`, f.keep.ID); n != 2 {
+		t.Errorf("tags on the kept copy = %d, want 2 (house, j-pop)", n)
+	}
+	if n := f.count(t, `SELECT count(*) FROM track_tags WHERE track_id = $1 AND tag = 'house' AND weight = 0.9`, f.keep.ID); n != 1 {
+		t.Errorf("the kept copy's own house weight was overwritten")
+	}
+
+	// Similarity: rewritten onto the kept copy, no duplicate edge, no self-edge,
+	// nothing left pointing at the removed copy.
+	if n := f.count(t, `SELECT count(*) FROM track_similarity WHERE track_a_id = $1 AND track_b_id = $2 AND source = 'listenbrainz'`,
+		f.keep.ID, f.other.ID); n != 1 {
+		t.Errorf("listenbrainz edge keep→other = %d rows, want exactly 1", n)
+	}
+	if n := f.count(t, `SELECT count(*) FROM track_similarity WHERE track_a_id = $1 AND track_b_id = $2 AND source = 'musicbrainz_tag'`,
+		f.other.ID, f.keep.ID); n != 1 {
+		t.Errorf("musicbrainz_tag edge other→keep was not carried over")
+	}
+	if n := f.count(t, `SELECT count(*) FROM track_similarity WHERE track_a_id = track_b_id`); n != 0 {
+		t.Errorf("a self-edge was written")
+	}
+
+	// The removed copy's recording MBID is inherited; the group is closed.
+	if n := f.count(t, `SELECT count(*) FROM tracks WHERE id = $1 AND mbid = 'rec-www'`, f.keep.ID); n != 1 {
+		t.Errorf("the kept copy did not inherit the recording MBID")
+	}
+	if n := f.count(t, `SELECT count(*) FROM duplicate_groups WHERE id = $1 AND status = 'merged' AND resolved_at IS NOT NULL`, f.groupID); n != 1 {
+		t.Errorf("group was not marked merged")
+	}
+
+	// A second merge of the same group is refused rather than repeated.
+	if _, err := MergeDuplicateGroup(ctx, f.pool, nil, "", f.groupID, f.keep.ID); !errors.Is(err, ErrDuplicateGroupNotPending) {
+		t.Errorf("second merge err = %v, want ErrDuplicateGroupNotPending", err)
+	}
+}
+
+// When the removed copy's file cannot go, nothing may change: its likes, plays
+// and row stay exactly where they were, and the group stays pending.
+func TestMergeDuplicateGroup_UnremovableFileChangesNothing(t *testing.T) {
+	f := newMergeFixture(t)
+	stubRemoveFile(t, func(name string) error {
+		return &fs.PathError{Op: "remove", Path: name, Err: syscall.EROFS}
+	})
+
+	_, err := MergeDuplicateGroup(context.Background(), f.pool, nil, "", f.groupID, f.keep.ID)
+	var fre *FileRemoveError
+	if !errors.As(err, &fre) || !fre.NotWritable() {
+		t.Fatalf("err = %v, want a not-writable *FileRemoveError", err)
+	}
+	if n := f.count(t, `SELECT count(*) FROM tracks WHERE id = $1`, f.remove.ID); n != 1 {
+		t.Errorf("the copy's row was deleted although its file was not")
+	}
+	if n := f.count(t, `SELECT count(*) FROM general_likes WHERE track_id = $1`, f.remove.ID); n != 2 {
+		t.Errorf("likes on the copy = %d, want both still there", n)
+	}
+	if n := f.count(t, `SELECT count(*) FROM play_events WHERE track_id = $1`, f.remove.ID); n != 2 {
+		t.Errorf("plays on the copy = %d, want both still there", n)
+	}
+	if n := f.count(t, `SELECT count(*) FROM duplicate_groups WHERE id = $1 AND status = 'pending'`, f.groupID); n != 1 {
+		t.Errorf("group left pending = false, want it still pending")
+	}
+}
+
+func TestMergeDuplicateGroup_SurvivorMustBeAMember(t *testing.T) {
+	f := newMergeFixture(t)
+	var stranger pgtype.UUID
+	stranger.Bytes[15], stranger.Valid = 0xEE, true
+
+	_, err := MergeDuplicateGroup(context.Background(), f.pool, nil, "", f.groupID, stranger)
+	if !errors.Is(err, ErrSurvivorNotInGroup) {
+		t.Fatalf("err = %v, want ErrSurvivorNotInGroup", err)
+	}
+	if _, err := os.Stat(f.removePath); err != nil {
+		t.Errorf("a refused merge removed a file: %v", err)
+	}
+	if n := f.count(t, `SELECT count(*) FROM duplicate_groups WHERE id = $1 AND status = 'pending'`, f.groupID); n != 1 {
+		t.Errorf("a refused merge changed the group")
+	}
+}

internal/tracks/merge_test.go

@@ -0,0 +1,39 @@
+package tracks
+
+import (
+	"testing"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
+)
+
+func mbids(track, album string) library.MergedCopy {
+	c := library.MergedCopy{}
+	if track != "" {
+		c.TrackMbid = &track
+	}
+	if album != "" {
+		c.AlbumMbid = &album
+	}
+	return c
+}
+
+// Unmonitoring a second file of the kept copy's own album track would stop
+// Lidarr managing the kept file too; that case must be recognised.
+func TestSameLidarrTrack(t *testing.T) {
+	for _, tc := range []struct {
+		name     string
+		kept     library.MergedCopy
+		removed  library.MergedCopy
+		wantSame bool
+	}{
+		{"same recording on the same album", mbids("rec", "alb"), mbids("rec", "alb"), true},
+		{"same recording on a compilation", mbids("rec", "alb"), mbids("rec", "comp"), false},
+		{"different recordings on one album", mbids("rec", "alb"), mbids("rec-2", "alb"), false},
+		{"kept copy has no mbids", mbids("", ""), mbids("rec", "alb"), false},
+		{"removed copy has no album mbid", mbids("rec", "alb"), mbids("rec", ""), false},
+	} {
+		if got := sameLidarrTrack(tc.kept, tc.removed); got != tc.wantSame {
+			t.Errorf("%s: sameLidarrTrack = %v, want %v", tc.name, got, tc.wantSame)
+		}
+	}
+}

internal/tracks/service.go

@@ -28,8 +28,10 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/audit"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
 // ErrNotFound is returned when the track id doesn't resolve. Aliased
@@ -147,3 +149,70 @@ func (s *Service) RemoveTrack(
 
 	return deleted.AlbumID, deleted.ArtistID, lidarrUnmonitorFailed, nil
 }
+
+// MergeDuplicates merges a duplicate group into the copy to keep
+// (library.MergeDuplicateGroup), then — when asked — tells Lidarr to stop
+// monitoring the removed copies so it does not download them again, and records
+// the merge in the audit log.
+//
+// lidarrUnmonitorFailed reports that unmonitoring was asked for and at least one
+// removed copy could not be unmonitored. Like RemoveTrack, that never fails the
+// merge: the files and rows are already gone.
+func (s *Service) MergeDuplicates(
+	ctx context.Context, groupID, survivorID, actorID pgtype.UUID, unmonitor bool,
+) (res library.MergeResult, lidarrUnmonitorFailed bool, err error) {
+	res, err = library.MergeDuplicateGroup(ctx, s.pool, s.logger, s.dataDir, groupID, survivorID)
+	if err != nil {
+		return res, false, err
+	}
+
+	if unmonitor && s.lidarr != nil {
+		for _, removed := range res.Removed {
+			if sameLidarrTrack(res.Survivor, removed) {
+				continue
+			}
+			if !hasLidarrIdentity(removed) {
+				s.logger.Warn("duplicate merge: lidarr unmonitor skipped — removed copy has no mbids",
+					"track_id", syncpkg.FormatUUID(removed.TrackID))
+				lidarrUnmonitorFailed = true
+				continue
+			}
+			if uerr := s.lidarr.UnmonitorTrack(ctx, *removed.TrackMbid, *removed.AlbumMbid); uerr != nil {
+				s.logger.Warn("duplicate merge: lidarr unmonitor failed",
+					"track_id", syncpkg.FormatUUID(removed.TrackID), "err", uerr)
+				lidarrUnmonitorFailed = true
+			}
+		}
+	}
+
+	removed := make([]map[string]string, 0, len(res.Removed))
+	for _, c := range res.Removed {
+		removed = append(removed, map[string]string{"track_id": syncpkg.FormatUUID(c.TrackID), "file_path": c.FilePath})
+	}
+	audit.WriteOrLog(ctx, s.pool, s.logger, actorID, pgtype.UUID{}, audit.ActionDuplicateMerge, map[string]any{
+		"group_id":          syncpkg.FormatUUID(groupID),
+		"tier":              res.Tier,
+		"survivor_track_id": syncpkg.FormatUUID(res.Survivor.TrackID),
+		"survivor_path":     res.Survivor.FilePath,
+		"removed":           removed,
+		"moved": map[string]any{
+			"play_events": res.PlayEvents, "skip_events": res.SkipEvents,
+			"likes": res.Likes, "playlist_entries": res.PlaylistEntries,
+		},
+	})
+	return res, lidarrUnmonitorFailed, nil
+}
+
+// sameLidarrTrack reports whether two copies are the same Lidarr track: one
+// recording on one album. Lidarr monitors per album track, so when the removed
+// copy is a second file of the kept copy's own album track — the #3885 case —
+// unmonitoring it would also stop Lidarr managing the file the operator chose to
+// keep. Those are skipped, and are not a failure.
+func sameLidarrTrack(a, b library.MergedCopy) bool {
+	return hasLidarrIdentity(a) && hasLidarrIdentity(b) &&
+		*a.TrackMbid == *b.TrackMbid && *a.AlbumMbid == *b.AlbumMbid
+}
+
+func hasLidarrIdentity(c library.MergedCopy) bool {
+	return c.TrackMbid != nil && *c.TrackMbid != "" && c.AlbumMbid != nil && *c.AlbumMbid != ""
+}

web/src/lib/api/admin.duplicates.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest';
-import { dismissDuplicateGroup, listDuplicates, runDuplicateSweep } from './admin';
+import { dismissDuplicateGroup, listDuplicates, mergeDuplicateGroup, runDuplicateSweep } from './admin';
 
 vi.mock('./client', () => ({
   api: { get: vi.fn(), post: vi.fn() }
@@ -27,4 +27,13 @@ describe('admin duplicates API', () => {
     await dismissDuplicateGroup('g/1');
     expect(api.post).toHaveBeenCalledWith('/api/admin/library/duplicates/g%2F1/dismiss', {});
   });
+
+  it('mergeDuplicateGroup POSTs the chosen survivor', async () => {
+    (api.post as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({ removed_paths: [] });
+    await mergeDuplicateGroup('g-1', { survivor_track_id: 't-1', unmonitor: true });
+    expect(api.post).toHaveBeenCalledWith('/api/admin/library/duplicates/g-1/merge', {
+      survivor_track_id: 't-1',
+      unmonitor: true
+    });
+  });
 });

web/src/lib/api/admin.ts

@@ -5,6 +5,7 @@ import type {
   ActionResult,
   AdminMissingResponse,
   AdminDuplicatesResponse,
+  MergeDuplicateResult,
   AdminPlaybackError,
   AdminQuarantineRow,
   LidarrConfig,
@@ -723,6 +724,19 @@ export async function runDuplicateSweep(): Promise<{ started: boolean }> {
   return api.post<{ started: boolean }>('/api/admin/library/duplicates/sweep', {});
 }
 
+// Merges a group into the copy chosen to keep, removing the other copies' files.
+// survivor_track_id is the operator's choice; the server checks it belongs to
+// the group.
+export async function mergeDuplicateGroup(
+  id: string,
+  body: { survivor_track_id: string; unmonitor: boolean }
+): Promise<MergeDuplicateResult> {
+  return api.post<MergeDuplicateResult>(
+    `/api/admin/library/duplicates/${encodeURIComponent(id)}/merge`,
+    body
+  );
+}
+
 export async function dismissDuplicateGroup(id: string): Promise<void> {
   await api.post(`/api/admin/library/duplicates/${encodeURIComponent(id)}/dismiss`, {});
 }

web/src/lib/api/types.ts

@@ -460,6 +460,15 @@ export type AdminDuplicateSweep = {
   error_message: string | null;
 };
 
+// What a merge did (#3911). removed_paths are the files that were deleted from
+// disk; lidarr_unmonitor_failed appears only when unmonitoring was asked for and
+// failed.
+export type MergeDuplicateResult = {
+  survivor_track_id: string;
+  removed_paths: string[];
+  lidarr_unmonitor_failed?: boolean;
+};
+
 export type AdminDuplicatesResponse = {
   sweep: AdminDuplicateSweep;
   fingerprints: { total: number; fingerprinted: number; rejected: number; pending: number };

web/src/lib/styles/error-copy.json

@@ -44,6 +44,7 @@
   "file_delete_failed": "The file couldn't be deleted.",
   "sweep_in_progress": "A duplicate sweep is already running.",
   "duplicate_group_not_pending": "That group has already been resolved.",
+  "survivor_not_in_group": "That copy isn't part of this group any more.",
   "album_not_found": "That album no longer exists.",
   "artist_not_found": "That artist no longer exists.",
   "playlist_not_found": "That playlist no longer exists.",

web/src/routes/admin/duplicates/+page.svelte

@@ -4,22 +4,30 @@
   import {
     createDuplicatesQuery,
     runDuplicateSweep,
-    dismissDuplicateGroup
+    dismissDuplicateGroup,
+    mergeDuplicateGroup
   } from '$lib/api/admin';
   import { errMessage } from '$lib/api/errors';
   import { pushToast } from '$lib/stores/toast.svelte';
   import { relativeTime } from '$lib/utils/relativeTime';
   import type { AdminDuplicateGroup, AdminDuplicateMember } from '$lib/api/types';
 
-  // Tracks the duplicate sweep believes hold one recording (#3912). A group is a
-  // proposal: nothing here deletes or merges. Dismissing one says "these are not
-  // duplicates", and the sweep will not propose that set again.
+  // Tracks the duplicate sweep believes hold one recording (#3912). Dismissing a
+  // group says "these are not duplicates", and the sweep will not propose that
+  // set again. Merging (#3911) keeps one copy, moves the others' likes, plays and
+  // playlist entries onto it, and deletes their files — so it asks twice.
 
   const PAGE_SIZE = 25;
 
   let offset = $state(0);
   let sweeping = $state(false);
   let dismissing = $state<string | null>(null);
+  // Per group: which copy to keep (defaults to the proposed survivor), whether to
+  // unmonitor the removed copies in Lidarr, and the two-click confirm.
+  let keepChoice = $state<Record<string, string>>({});
+  let unmonitorChoice = $state<Record<string, boolean>>({});
+  let confirmingMerge = $state<string | null>(null);
+  let merging = $state<string | null>(null);
 
   const queryStore = $derived(createDuplicatesQuery(offset, PAGE_SIZE));
   const query = $derived($queryStore);
@@ -56,6 +64,40 @@
     }
   }
 
+  function keeperOf(group: AdminDuplicateGroup): string {
+    return keepChoice[group.id] ?? group.survivor_track_id;
+  }
+
+  function fileCountLabel(n: number): string {
+    return n === 1 ? '1 file' : `${n} files`;
+  }
+
+  async function onMerge(group: AdminDuplicateGroup) {
+    // First click arms; the second, on the button that now names how many files
+    // go, does it. A merge deletes files, and nothing brings them back.
+    if (confirmingMerge !== group.id) {
+      confirmingMerge = group.id;
+      return;
+    }
+    confirmingMerge = null;
+    merging = group.id;
+    try {
+      const result = await mergeDuplicateGroup(group.id, {
+        survivor_track_id: keeperOf(group),
+        unmonitor: unmonitorChoice[group.id] ?? false
+      });
+      pushToast(`Merged. Removed ${fileCountLabel(result.removed_paths.length)}.`);
+      if (result.lidarr_unmonitor_failed) {
+        pushToast("Merged, but Lidarr couldn't be told to stop monitoring the removed copies.", 'error');
+      }
+      query.refetch();
+    } catch (e: unknown) {
+      pushToast(errMessage(e), 'error');
+    } finally {
+      merging = null;
+    }
+  }
+
   // "Identical audio" and "same recording" are different claims, and an
   // operator deciding whether to merge needs to know which one they are
   // looking at before anything else.
@@ -177,20 +219,71 @@
               <h3 class="text-sm text-text-primary" data-testid="duplicate-tier">{tierLabel(group)}</h3>
               <p class="text-xs text-text-secondary">Found {relativeTime(group.detected_at)}</p>
             </div>
-            <button
-              type="button"
-              onclick={() => onDismiss(group)}
-              disabled={dismissing === group.id}
-              class="shrink-0 rounded-md border border-border px-3 py-1.5 text-sm text-text-secondary hover:bg-surface-hover hover:text-text-primary disabled:opacity-50"
-            >
-              Not duplicates
-            </button>
+            <div class="flex shrink-0 items-center gap-2">
+              <button
+                type="button"
+                onclick={() => onDismiss(group)}
+                disabled={dismissing === group.id || merging === group.id}
+                class="rounded-md border border-border px-3 py-1.5 text-sm text-text-secondary hover:bg-surface-hover hover:text-text-primary disabled:opacity-50"
+              >
+                Not duplicates
+              </button>
+              <button
+                type="button"
+                onclick={() => onMerge(group)}
+                disabled={merging === group.id}
+                class="rounded-md px-3 py-1.5 text-sm disabled:opacity-50 {confirmingMerge === group.id
+                  ? 'bg-action-destructive text-action-fg hover:opacity-90'
+                  : 'border border-border text-text-primary hover:bg-surface-hover'}"
+              >
+                {#if merging === group.id}
+                  Merging…
+                {:else if confirmingMerge === group.id}
+                  Remove {fileCountLabel(group.members.length - 1)} and merge
+                {:else}
+                  Merge…
+                {/if}
+              </button>
+            </div>
           </div>
 
+          {#if confirmingMerge === group.id}
+            <!-- What the second click will do, said plainly before it is done. -->
+            <div class="space-y-2 border-b border-border bg-surface-hover px-4 py-3 text-sm" data-testid="merge-confirm">
+              <p class="text-text-primary">
+                The copy marked Keep stays. The other {fileCountLabel(group.members.length - 1)} will be
+                deleted from disk, and their likes, plays and playlist entries move to the copy kept.
+              </p>
+              <label class="flex items-center gap-2 text-text-secondary">
+                <input
+                  type="checkbox"
+                  checked={unmonitorChoice[group.id] ?? false}
+                  onchange={(e) => (unmonitorChoice[group.id] = e.currentTarget.checked)}
+                />
+                Tell Lidarr to stop monitoring the removed copies, so it doesn't download them again
+              </label>
+              <button
+                type="button"
+                class="text-xs text-text-secondary underline hover:text-text-primary"
+                onclick={() => (confirmingMerge = null)}
+              >
+                Cancel
+              </button>
+            </div>
+          {/if}
+
           <ul class="divide-y divide-border">
             {#each group.members as m (m.track_id)}
               {@const keep = m.track_id === group.survivor_track_id}
               <li class="flex items-start gap-3 px-4 py-3" data-testid="duplicate-member">
+                <input
+                  type="radio"
+                  name="keep-{group.id}"
+                  class="mt-1"
+                  checked={keeperOf(group) === m.track_id}
+                  onchange={() => (keepChoice[group.id] = m.track_id)}
+                  aria-label="Keep {m.title}, {m.file_format.toUpperCase()}, {m.file_path}"
+                />
                 <div class="min-w-0 flex-1 space-y-0.5">
                   <div class="flex items-center gap-2">
                     <span class="truncate text-sm text-text-primary">{m.title}</span>

web/src/routes/admin/duplicates/duplicates.test.ts

@@ -6,11 +6,20 @@ import type { AdminDuplicatesResponse } from '$lib/api/types';
 vi.mock('$lib/api/admin', () => ({
   createDuplicatesQuery: vi.fn(),
   runDuplicateSweep: vi.fn().mockResolvedValue({ started: true }),
-  dismissDuplicateGroup: vi.fn().mockResolvedValue(undefined)
+  dismissDuplicateGroup: vi.fn().mockResolvedValue(undefined),
+  mergeDuplicateGroup: vi.fn().mockResolvedValue({
+    survivor_track_id: 'www-01',
+    removed_paths: ['/music/Moe Shop/WWW (2020)/www-02.mp3']
+  })
 }));
 
 import AdminDuplicatesPage from './+page.svelte';
-import { createDuplicatesQuery, dismissDuplicateGroup, runDuplicateSweep } from '$lib/api/admin';
+import {
+  createDuplicatesQuery,
+  dismissDuplicateGroup,
+  mergeDuplicateGroup,
+  runDuplicateSweep
+} from '$lib/api/admin';
 
 const HOUR = 3_600_000;
 const ago = (ms: number) => new Date(Date.now() - ms).toISOString();
@@ -144,4 +153,40 @@ describe('admin duplicates', () => {
     await fireEvent.click(screen.getByRole('button', { name: 'Sweep now' }));
     expect(runDuplicateSweep).toHaveBeenCalledTimes(1);
   });
+
+  // A merge deletes files. The first click must only arm it.
+  test('Merge needs a second click, and keeps the proposed copy by default', async () => {
+    renderWith(response());
+    await fireEvent.click(screen.getByRole('button', { name: 'Merge…' }));
+    expect(mergeDuplicateGroup).not.toHaveBeenCalled();
+    expect(text(screen.getByTestId('merge-confirm'))).toContain('The other 1 file will be deleted from disk');
+
+    await fireEvent.click(screen.getByRole('button', { name: 'Remove 1 file and merge' }));
+    expect(mergeDuplicateGroup).toHaveBeenCalledWith('g-1', {
+      survivor_track_id: 'www-01',
+      unmonitor: false
+    });
+  });
+
+  test('choosing another copy to keep sends that copy', async () => {
+    renderWith(response());
+    const radios = screen.getAllByRole('radio');
+    expect((radios[0] as HTMLInputElement).checked).toBe(true);
+    await fireEvent.click(radios[1]);
+
+    await fireEvent.click(screen.getByRole('button', { name: 'Merge…' }));
+    await fireEvent.click(screen.getByRole('button', { name: 'Remove 1 file and merge' }));
+    expect(mergeDuplicateGroup).toHaveBeenCalledWith('g-1', {
+      survivor_track_id: 'www-02',
+      unmonitor: false
+    });
+  });
+
+  test('Cancel disarms the merge', async () => {
+    renderWith(response());
+    await fireEvent.click(screen.getByRole('button', { name: 'Merge…' }));
+    await fireEvent.click(screen.getByRole('button', { name: 'Cancel' }));
+    expect(screen.queryByTestId('merge-confirm')).toBeNull();
+    expect(screen.getByRole('button', { name: 'Merge…' })).toBeTruthy();
+  });
 });