fix(extension): _find_or_create_artist + _find_or_create_source race-safe via savepoint + IntegrityError recovery — same pattern as importer's helpers. Two concurrent quick-add-source calls on the same artist/url would have 500'd on uq_artist_slug / uq_source_artist_platform_url; now the second one rolls the savepoint back and returns the existing row.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/extension_service.py

@@ -11,6 +11,7 @@ from __future__ import annotations
 import re
 
 from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from ..models import Artist, Source
@@ -97,20 +98,40 @@ class ExtensionService:
         raise UnknownPlatformError(f"no platform pattern matched {url!r}")
 
     async def _find_or_create_artist(self, raw_name: str) -> tuple[Artist, bool]:
+        """Race-safe find-or-create on Artist by slug. Mirrors the
+        savepoint + IntegrityError recovery pattern used in
+        Importer._find_or_create_source/post (see
+        reference_scalar_one_or_none_duplicates memory). Without this,
+        two concurrent quick-add-source calls hitting the same artist
+        would both miss the existence check and the second INSERT would
+        500 against uq_artist_slug.
+        """
         slug = slugify(raw_name)
         existing = (await self.session.execute(
             select(Artist).where(Artist.slug == slug)
         )).scalar_one_or_none()
         if existing is not None:
             return existing, False
-        artist = Artist(name=raw_name, slug=slug, is_subscription=True)
-        self.session.add(artist)
-        await self.session.flush()
-        return artist, True
+        sp = await self.session.begin_nested()
+        try:
+            artist = Artist(name=raw_name, slug=slug, is_subscription=True)
+            self.session.add(artist)
+            await self.session.flush()
+            await sp.commit()
+            return artist, True
+        except IntegrityError:
+            await sp.rollback()
+            recovered = (await self.session.execute(
+                select(Artist).where(Artist.slug == slug)
+            )).scalar_one()
+            return recovered, False
 
     async def _find_or_create_source(
         self, *, artist_id: int, platform: str, url: str,
     ) -> tuple[Source, bool]:
+        """Race-safe — same pattern as _find_or_create_artist above. The
+        uq_source_artist_platform_url constraint catches the duplicate
+        insert; we roll the savepoint back and re-select."""
         existing = (await self.session.execute(
             select(Source).where(
                 Source.artist_id == artist_id,
@@ -120,8 +141,24 @@ class ExtensionService:
         )).scalar_one_or_none()
         if existing is not None:
             return existing, False
-        src = Source(artist_id=artist_id, platform=platform, url=url, enabled=True)
-        self.session.add(src)
-        await self.session.flush()
+        sp = await self.session.begin_nested()
+        try:
+            src = Source(
+                artist_id=artist_id, platform=platform,
+                url=url, enabled=True,
+            )
+            self.session.add(src)
+            await self.session.flush()
+            await sp.commit()
+        except IntegrityError:
+            await sp.rollback()
+            recovered = (await self.session.execute(
+                select(Source).where(
+                    Source.artist_id == artist_id,
+                    Source.platform == platform,
+                    Source.url == url,
+                )
+            )).scalar_one()
+            return recovered, False
         await self.session.commit()
         return src, True