fix(db): re-running 0095/0096 rebuilds an invalid leftover index

A concurrent build that failed part-way leaves an INVALID index under its
name; IF NOT EXISTS kept it on the re-run, and 0095 then dropped the old
channel_id index, leaving none usable. The leftover is now dropped and
built again, and the old index goes only once the new one is valid.

backend/alembic/versions/0095_videos_channel_cover_index.py

@@ -17,10 +17,13 @@ cascade, the sync upsert, channel detail) rides this one. The old index is dropp
 tests/test_channel_list_index_only.py holds the queries and this column list together.
 
 CONCURRENTLY (autocommit_block) builds it without blocking a live prod. IF (NOT) EXISTS keeps the drift
-test's downgrade → re-upgrade round trip idempotent.
+test's downgrade → re-upgrade round trip idempotent. A re-run after a build that failed part-way rebuilds
+the INVALID leftover, and the old index is dropped only once the new one is valid: a re-run that kept the
+leftover and dropped the old one left `videos` with no usable channel_id index (E31 S5 review).
 """
 
 from alembic import op
+from sqlalchemy import text
 
 revision: str = "0095_videos_channel_cover_index"
 down_revision: str | None = "0094_plex_genres_hu_everywhere"
@@ -28,20 +31,49 @@ branch_labels: str | None = None
 depends_on: str | None = None
 
 
+def _valid(name: str) -> bool | None:
+    """The index's `indisvalid`, or None when there is no index of that name."""
+    row = op.get_bind().execute(
+        text(
+            "SELECT i.indisvalid FROM pg_index i JOIN pg_class c ON c.oid = i.indexrelid "
+            "WHERE c.relname = :name"
+        ),
+        {"name": name},
+    ).first()
+    return None if row is None else bool(row[0])
+
+
+def _build(name: str, ddl: str) -> None:
+    """CREATE INDEX CONCURRENTLY `name` (`ddl` is what follows the name), re-runnable. A build that
+    failed part-way leaves an INVALID index under the name, and `IF NOT EXISTS` would keep it: the
+    planner never uses it, and nothing says so (E31 S5 review). Such a leftover is dropped and built
+    again."""
+    if _valid(name) is False:
+        op.execute(f"DROP INDEX CONCURRENTLY IF EXISTS {name}")
+    op.execute(f"CREATE INDEX CONCURRENTLY IF NOT EXISTS {name} {ddl}")
+
+
+def _swap(build: str, ddl: str, drop: str) -> None:
+    _build(build, ddl)
+    if not _valid(build):
+        raise RuntimeError(f"{build} did not build valid; {drop} is kept")
+    op.execute(f"DROP INDEX CONCURRENTLY IF EXISTS {drop}")
+
+
 def upgrade() -> None:
     with op.get_context().autocommit_block():
-        op.execute(
-            "CREATE INDEX CONCURRENTLY IF NOT EXISTS ix_videos_channel_pub_cover "
+        _swap(
+            "ix_videos_channel_pub_cover",
             "ON videos (channel_id, published_at DESC NULLS LAST, id) "
-            "INCLUDE (duration_seconds, unavailable_since, via_explore, is_short, live_status)"
+            "INCLUDE (duration_seconds, unavailable_since, via_explore, is_short, live_status)",
+            "ix_videos_channel_pub_desc",
         )
-        op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_channel_pub_desc")
 
 
 def downgrade() -> None:
     with op.get_context().autocommit_block():
-        op.execute(
-            "CREATE INDEX CONCURRENTLY IF NOT EXISTS ix_videos_channel_pub_desc "
-            "ON videos (channel_id, published_at DESC NULLS LAST, id ASC)"
+        _swap(
+            "ix_videos_channel_pub_desc",
+            "ON videos (channel_id, published_at DESC NULLS LAST, id ASC)",
+            "ix_videos_channel_pub_cover",
         )
-        op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_channel_pub_cover")

backend/alembic/versions/0096_videos_sort_indexes.py

@@ -28,6 +28,7 @@ test's downgrade → re-upgrade round trip idempotent.
 """
 
 from alembic import op
+from sqlalchemy import text
 
 revision: str = "0096_videos_sort_indexes"
 down_revision: str | None = "0095_videos_channel_cover_index"
@@ -47,10 +48,32 @@ INDEXES = {
 }
 
 
+def _valid(name: str) -> bool | None:
+    """The index's `indisvalid`, or None when there is no index of that name."""
+    row = op.get_bind().execute(
+        text(
+            "SELECT i.indisvalid FROM pg_index i JOIN pg_class c ON c.oid = i.indexrelid "
+            "WHERE c.relname = :name"
+        ),
+        {"name": name},
+    ).first()
+    return None if row is None else bool(row[0])
+
+
+def _build(name: str, ddl: str) -> None:
+    """CREATE INDEX CONCURRENTLY `name` (`ddl` is what follows the name), re-runnable. A build that
+    failed part-way leaves an INVALID index under the name, and `IF NOT EXISTS` would keep it: the
+    planner never uses it, and nothing says so (E31 S5 review). Such a leftover is dropped and built
+    again."""
+    if _valid(name) is False:
+        op.execute(f"DROP INDEX CONCURRENTLY IF EXISTS {name}")
+    op.execute(f"CREATE INDEX CONCURRENTLY IF NOT EXISTS {name} {ddl}")
+
+
 def upgrade() -> None:
     with op.get_context().autocommit_block():
         for name, columns in INDEXES.items():
-            op.execute(f"CREATE INDEX CONCURRENTLY IF NOT EXISTS {name} ON videos ({columns})")
+            _build(name, f"ON videos ({columns})")
 
 
 def downgrade() -> None: