fix: the duplicate resolver waits for a running scan, so a copy it removes is not marked present again (#6169)

At boot the scan lists the library, the resolver removes wrong-slot copies
and marks their tracks missing, then the scan reconciles from its list and
marks them present: 7 of the 13 removed on 2026-10-20 15:00 read as
present with no file, and would not be wanted back until a later scan. A
pass now waits while a scan younger than StuckScanThreshold is in flight
and tries again a minute later; a stuck run does not hold it up.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/duplicate_resolve.go

@@ -46,6 +46,9 @@ const (
 	resolveMergeCap = 200
 	// duplicateResolveTick is how often the worker runs a pass.
 	duplicateResolveTick = time.Hour
+	// scanWaitRetry is how soon a pass put off for a running scan is tried
+	// again: a boot scan takes under a minute on the operator's library.
+	scanWaitRetry = time.Minute
 )
 
 // DuplicateResolveResult tallies one pass.
@@ -420,38 +423,58 @@ func NewDuplicateResolveWorker(
 	}
 }
 
-// Run blocks until ctx is cancelled, running a pass at start and then each tick.
+// Run blocks until ctx is cancelled, running a pass at start and then each
+// tick. A pass put off for a running scan is tried again after scanWaitRetry.
 func (w *DuplicateResolveWorker) Run(ctx context.Context) {
-	w.tickOnce(ctx)
 	t := time.NewTicker(w.tick)
 	defer t.Stop()
+	soon := time.NewTimer(0)
+	defer soon.Stop()
 	for {
 		select {
 		case <-ctx.Done():
 			return
 		case <-t.C:
-			w.tickOnce(ctx)
+		case <-soon.C:
+		}
+		if w.tickOnce(ctx) {
+			soon.Reset(scanWaitRetry)
 		}
 	}
 }
 
-// tickOnce contains one pass so nothing it does can stop the next tick (rule 157).
-func (w *DuplicateResolveWorker) tickOnce(ctx context.Context) {
+// tickOnce contains one pass so nothing it does can stop the next tick (rule
+// 157). It reports whether the pass was put off for a running scan.
+func (w *DuplicateResolveWorker) tickOnce(ctx context.Context) (waitForScan bool) {
 	defer func() {
 		if r := recover(); r != nil {
 			w.logger.Error("duplicate resolve: tick panicked", "panic", r)
 		}
 	}()
+	q := dbq.New(w.pool)
 	// A sweep in flight is rewriting the groups; the next tick sees its result.
-	_, err := dbq.New(w.pool).GetInFlightDuplicateSweep(ctx)
+	_, err := q.GetInFlightDuplicateSweep(ctx)
 	switch {
 	case err == nil:
-		return
+		return false
 	case !errors.Is(err, pgx.ErrNoRows):
 		if ctx.Err() == nil {
 			w.logger.Warn("duplicate resolve: sweep check failed", "err", err)
 		}
-		return
+		return false
+	}
+	// A scan in flight has listed the files it will reconcile: a copy removed
+	// now is marked present again when it does (7 of the 13 removed at boot
+	// on 2026-10-20, #6169). A run past StuckScanThreshold is a dead one.
+	run, err := q.GetInFlightScanRun(ctx)
+	switch {
+	case err == nil && time.Since(run.StartedAt.Time) <= StuckScanThreshold:
+		return true
+	case err != nil && !errors.Is(err, pgx.ErrNoRows):
+		if ctx.Err() == nil {
+			w.logger.Warn("duplicate resolve: scan check failed", "err", err)
+		}
+		return false
 	}
 	res, err := ResolveDuplicatesWith(ctx, w.pool, w.logger, w.dataDir, w.settings.Get().AutoResolve, w.src)
 	if err != nil && ctx.Err() == nil {
@@ -461,4 +484,5 @@ func (w *DuplicateResolveWorker) tickOnce(ctx context.Context) {
 		"groups", res.Groups, "merged", res.Merged, "merge_failed", res.MergeFailed,
 		"songs_linked", res.SongsLinked, "dismissed", res.Dismissed, "wrong_files", res.WrongFiles,
 		"held_back", res.HeldBack, "released", res.Released)
+	return false
 }

internal/library/duplicate_resolve_test.go

@@ -2,6 +2,8 @@ package library
 
 import (
 	"context"
+	"io"
+	"log/slog"
 	"os"
 	"path/filepath"
 	"testing"
@@ -549,6 +551,30 @@ func TestResolveDuplicates_AgreeingLookupsDecideAnAcousticGroup_Integration(t *t
 	})
 }
 
+// A pass while a scan is walking the library waits for it: a copy removed
+// under the walk is marked present again when the scan reconciles. A run
+// stuck past StuckScanThreshold does not hold the resolver up.
+func TestDuplicateResolveWorker_WaitsForARunningScan_Integration(t *testing.T) {
+	f := seedTrimmedAlbum(t)
+	ctx := context.Background()
+	w := NewDuplicateResolveWorker(f.pool, slog.New(slog.NewTextHandler(io.Discard, nil)), "", nil, ResolveSources{})
+	if _, err := f.pool.Exec(ctx, `INSERT INTO scan_runs DEFAULT VALUES`); err != nil {
+		t.Fatal(err)
+	}
+	if !w.tickOnce(ctx) {
+		t.Error("a pass ran under a running scan")
+	}
+	if !exists(f.aPath) || !exists(f.bPath) {
+		t.Error("a copy was removed while the scan was walking the library")
+	}
+	if _, err := f.pool.Exec(ctx, `UPDATE scan_runs SET started_at = now() - interval '2 hours'`); err != nil {
+		t.Fatal(err)
+	}
+	if w.tickOnce(ctx) {
+		t.Error("a scan stuck for two hours held the resolver up")
+	}
+}
+
 // AcoustID knows the recording as no song of the album: every copy goes. With
 // AcoustID unreachable, nothing is decided and the group waits.
 func TestResolveDuplicates_TrimmedCopiesAcoustIDCannotName_Integration(t *testing.T) {