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fix(playlists): one YouTube sync of a list at a time, across tabs and the scheduler

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The client's in-flight flag covers the menus of one page only. A push reads YouTube, then writes
both sides for seconds and commits step by step, so a row lock ends at its first commit: two tabs,
or a push and the scheduled mirror refresh, could interleave into a doubled YouTube item or a
baseline recorded over the other's writes. `sync_claim` takes a session-level advisory lock on a
connection of its own (it survives those commits; a crashed worker's lock dies with its
connection, so no claim column is left stuck). Push and revert answer 409 `sync_busy`; the
scheduled refresh skips a list being synced and picks it up next run.
This commit is contained in:
peter
2026-09-27 17:47:01 +02:00
parent 2525a12264
commit e0453fa07d
5 changed files with 193 additions and 35 deletions
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backend/app/routes/playlists.py
+35 -26
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@@ -19,6 +19,7 @@ from app.sync.playlists import (
plan_merge,
recompute_dirty,
repull_playlist,
sync_claim,
sync_user_playlists,
)
from app.ratelimit import RateLimiter
@@ -347,13 +348,16 @@ def revert_youtube(
raise _refused(400, "not_on_youtube")
if not has_read_scope(user):
raise _refused(403, "youtube_read_scope")
_check_pull_cooldown(user)
try:
with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
return {"ok": True, **repull_playlist(db, user, pl)}
except YouTubeError as exc:
db.rollback()
raise youtube_refusal(exc)
with sync_claim(db, pl.id) as free:
if not free:
raise _refused(409, "sync_busy")
_check_pull_cooldown(user)
try:
with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
return {"ok": True, **repull_playlist(db, user, pl)}
except YouTubeError as exc:
db.rollback()
raise youtube_refusal(exc)
def _pushable(db: Session, user: User, playlist_id: int) -> Playlist:
@@ -395,25 +399,30 @@ def push(
neither side loses an item). Refuses if the estimated quota exceeds what's left today, so a
big sync can't strand half-applied. Write scope required (require_write_scope)."""
pl = _pushable(db, user, playlist_id)
try:
with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
# Read the live playlist ONCE (linked playlists only) and share it with both the
# plan and the merge, halving read-quota and closing the second read→write TOCTOU
# window. A fresh export (no yt_playlist_id) reads nothing.
live = None
if pl.yt_playlist_id:
with YouTubeClient(db, user) as yt:
live = (
list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id)),
yt.get_playlist_snippet(pl.yt_playlist_id),
)
plan = plan_merge(db, user, pl, live=live)
if plan["units_estimate"] > quota.remaining_today(db):
raise _refused(429, "quota_exceeded")
return {"ok": True, **merge_playlist(db, user, pl, live=live)}
except YouTubeError as exc:
db.rollback()
raise youtube_refusal(exc)
# One sync of a list at a time, across tabs, devices and the scheduler (`sync_claim`): the
# client's own in-flight flag only covers the menus of one page.
with sync_claim(db, pl.id) as free:
if not free:
raise _refused(409, "sync_busy")
try:
with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
# Read the live playlist ONCE (linked playlists only) and share it with both the
# plan and the merge, halving read-quota and closing the second read→write TOCTOU
# window. A fresh export (no yt_playlist_id) reads nothing.
live = None
if pl.yt_playlist_id:
with YouTubeClient(db, user) as yt:
live = (
list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id)),
yt.get_playlist_snippet(pl.yt_playlist_id),
)
plan = plan_merge(db, user, pl, live=live)
if plan["units_estimate"] > quota.remaining_today(db):
raise _refused(429, "quota_exceeded")
return {"ok": True, **merge_playlist(db, user, pl, live=live)}
except YouTubeError as exc:
db.rollback()
raise youtube_refusal(exc)
@router.get("/{playlist_id}")