This update introduces atomic mutations for active_streams using Redis Lua scripts, ensuring that concurrent byte-range requests do not leave stale session metadata. The changes also include improvements to the session state saving mechanism, preventing the recreation of zombie sessions after idle cleanup. Additionally, tests have been added to cover the new atomic behavior and ensure reliability across various scenarios.
Three race conditions in multi_worker_connection_manager could leave
the Redis profile_connections:<id> counter permanently elevated with no
active streams, causing all VOD requests to 503 "All profiles at capacity".
Bug 1 — decrement_active_streams() return value was ignored
All three generator exit paths (normal, GeneratorExit, Exception) called
decrement_active_streams() and unconditionally set decremented = True
regardless of whether the lock was acquired. On lock contention the decrement
was silently skipped, active_streams remained > 0, the subsequent
has_active_streams() check returned True, and _decrement_profile_connections()
was never called. The counter was then stuck until manual DEL.
Fix: add decrement_active_streams_and_check() which performs the decrement and
the "are there remaining streams?" check atomically under a single lock,
eliminating the race window. All three exit paths and the finally block now
use this method and propagate its success/remaining return values.
Bug 2 — non-atomic GET-then-DECR in _decrement_profile_connections()
The previous implementation read the counter with GET then conditionally
called DECR. Two concurrent decrements could both pass the > 0 guard and
both fire, driving the counter to -1. A subsequent _check_and_reserve_profile_slot()
INCR would then produce 0 which passes the <= max_streams check, allowing
an extra stream to bypass the limit on the next request.
Fix: replace GET-then-DECR with a direct DECR (matching the INCR-first
pattern already used by _check_and_reserve_profile_slot) and clamp the
result to 0 if it goes negative.
Bug 3 — has_active_streams() read state without holding the lock
The separate has_active_streams() call after decrement_active_streams()
released its lock left a window where another concurrent stream could
increment active_streams back to 1, causing the profile decrement to be
skipped. This is resolved as a consequence of Bug 1's fix: the new
decrement_active_streams_and_check() method reads active_streams while
the lock is still held, eliminating the window entirely.
Adds tests covering all three scenarios in
apps/proxy/vod_proxy/tests/test_profile_connections.py.
Fixes#1121.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
