control-connection: reconnect when down handling is skipped - #1024
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Sequence Diagram(s)sequenceDiagram
participant ControlConnection
participant Cluster
participant Host
ControlConnection->>Cluster: signal_connection_failure(host)
Cluster->>Host: signal_connection_failure()
Host-->>Cluster: conviction result
Cluster->>Cluster: on_down(host)
Cluster-->>ControlConnection: dispatch result
ControlConnection->>ControlConnection: wait or reconnect
Suggested reviewers: Priority: ➖ Normal Change: Bug fix · Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to Control-connection recovery now handles rejected dispatches and overlapping retry triggers correctly, leaving no merge-blocking risk. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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Inline comments:
In `@cassandra/cluster.py`:
- Line 2055: Update Cluster.on_down’s dispatch flow to return False when
executor.submit fails, including exceptions handled by run_in_executor, and
return True only after the DOWN callback is successfully queued so
ControlConnection._signal_error can trigger reconnect fallback.
- Around line 4597-4600: Prevent duplicate control reconnection tasks when a
host failure is signaled while an asynchronous DOWN callback is still pending.
Update ControlConnection.reconnect or the Cluster.on_down/_signal_error flow to
track and deduplicate pending reconnection submissions, ensuring each failure
schedules at most one _reconnect task.
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cassandra/cluster.pytests/unit/test_cluster.pytests/unit/test_control_connection.py
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ControlConnection._signal_error() treated a conviction-policy decision as proof that Cluster.on_down() had dispatched its asynchronous callbacks. DOWN handling can instead be suppressed by an open session pool, existing host state, disabled pool creation, shutdown, or a rejected executor submission, leaving the defunct control connection without recovery work. Report whether DOWN handling was actually queued and reconnect directly when it was not. Keep callback-driven recovery for accepted transitions so the normal host-down path does not submit a duplicate attempt, and cover TCP, Unix-socket, alternate-route, and stable host-ID connections. Fixes scylladb#847
The direct recovery path can overlap queued attempts, heartbeat returns, topology-driven replacements, and finite retry schedules. Without a single owner for pending and delayed work, later triggers can reset backoff, stale handlers can replace healthy connections, and exhausted handlers can suppress an independent failure. Serialize reconnect submissions, retain triggers until an active attempt or handler owns them, and release handler slots with identity checks. Preserve failures that arrive during proactive retries, honor empty and exhausted schedules, and keep a successfully restored control connection open while still closing host-probe connections. Make connection installation and shutdown races explicit so an overlapping failed attempt cannot park retries over a newer connection and an orphaned connection is closed.
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Fixes #847
Problem
A defunct control connection could remain disconnected when its host failure did not dispatch cluster DOWN handling.
ControlConnection._signal_error()treated the conviction policy accepting a failure as proof that a DOWN callback would reconnect it, but DOWN handling can be rejected, discounted because a session pool is still open, skipped for existing host state, or dropped during shutdown.Opening the direct fallback also exposed ownership races in the existing reconnect lifecycle. Repeated errors or heartbeat returns could reset a handler's backoff, overlapping attempts could leave stale retry work over a healthy connection, and a handler that eventually succeeded closed the control connection it had just installed.
Changes
Cluster.on_down()andCluster.signal_connection_failure()report whether asynchronous DOWN handling was actually dispatched. Executor rejection and every intentional suppression path reportFalse.Compatibility and risk
There is no CQL protocol, wire-format, or public configuration change. The behavior change is limited to internal host-down reporting and control-connection recovery. The main risk is reconnect concurrency and shutdown ordering; unit coverage exercises pending submissions, overlapping success and failure, handler replacement, finite and empty schedules, heartbeat triggers, shutdown races, and Unix/alternate endpoints.
Testing
uv run pytest -q tests/unit/test_control_connection.py tests/unit/test_cluster.py— 159 passed on the final tree.uv run pytest -q tests/unit— 1,132 passed, 24 skipped, with one unrelated pre-existing date-range rounding failure intests/unit/test_types.py::DateRangeDeserializationTests::test_deserialize_date_range_year.Pre-review checklist
CHANGELOG.rst; no public API documentation change is required.Fixesannotation.