43 lines
3.9 KiB
Markdown
43 lines
3.9 KiB
Markdown
---
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id: "bug-1450"
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status: "fixed"
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status_updated: "2026-09-27"
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status_note: "7aac018c: demotion-convergence offload ON by default; inline fallback when slow lane not started"
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component: "controller"
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severity: "unknown"
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timestamp: "2026-07-18"
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last_seen: "2026-09-19T09:00:00Z"
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occurrences: 1
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files: ["src/controller/ha_convergence.c", "src/controller/ha.h"]
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tags: ["async-io", "thread-role-violation", "delaymgr", "slowlane", "ha_convergence.c", "demotion", "raft-drive-async-pattern", "xce-audit-invariants", "controller-tests"]
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related: ["bug-1429", "bug-1451"]
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fix_commits: ["7aac018c"]
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legacy: true
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gitea_issue: 25
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gitea_synced_status: "fixed"
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gitea_synced_hash: "69e69f68fbf7"
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---
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# bug-1450: xce-audit-invariants controller pass (dim: async I/O thread-role) found ha_demotion_convergence_fn…
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## Description
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xce-audit-invariants controller pass (dim: async I/O thread-role) found ha_demotion_convergence_fn scheduled directly as a delaymgr task body with no vtask/slow-lane hop: it truncates the WAL (persist_truncate_wal_above, file I/O under p->lock) then calls ha_self_restart (tlog_exit synchronous log flush + execv) -- all inline on the delaymgr thread, the SAME single thread that drives heartbeats and election timeouts cluster-wide for the whole cluster.
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## Environment
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_Not recorded (migrated from buglog.json)._
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## Reproduction
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_Not recorded (migrated from buglog.json)._
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## Root cause
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ha_demotion_convergence_fn (ha_convergence.c) ran its full body -- truncate + self-restart -- synchronously on whatever thread invoked it (the delaymgr worker in production). A prior fix attempt (2026-07-10, see bug-1429) tried moving this to a VTASK_LIVE_BLOCK vtask via c->ha.pool and deadlocked controller_unit_test, because c->ha.pool is never initialized/started by this file's lightweight unit-test fixtures (dc_setup), so a vtask enqueued VTASK_LIVE_BLOCK blocked forever waiting for a worker that never existed; that attempt was reverted.
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## Fix
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Split ha_demotion_convergence_fn into a dispatcher (fast admin work: single-flight CAS on new ha_state_t field demotion_convergence_inflight, then slowlane_submit) and a new demotion_lane_fn slow-lane worker body running the actual truncate+self-restart (extracted into ha_demotion_convergence_do_run) -- the SAME established slow-lane pattern already used for snapshot send / config-push / peer connect in ha.c (ha_snapshot_send_dispatch, ha_config_push_offloadable, ha_connect_to_peer), not the vtask pool that deadlocked before. New feature flag g_drive_async_demotion_offload (ha_set_demotion_offload_for_test / ha_demotion_offload_enabled, ha.h/ha_convergence.c) gates the offload. IMPORTANT: defaulting this flag to ON (1) was tried first and reproduced a NEW real hang -- raft_cb_become_follower (which schedules this task) is exercised by a very broad set of pre-existing controller_unit_test fixtures across many files that never call slowlane_init, so the offloaded work was silently queued on an uninitialized c->slowlane (queued, never run -- no live workers) and downstream code waiting on the now-never-happening completion hung. Reverted the default to OFF (0), matching the ORIGINAL rollout discipline of the three reference offloads (all shipped OFF-by-default, flipped ON only in a later dedicated compatibility-audit pass -- raft-drive-async task 3.4). The mechanism is correct and covered by dedicated tests (test_demotion_offload_task_returns_promptly / test_demotion_inline_task_blocks_RED in test_demotion_convergence.c) that explicitly flip the flag ON against a fixture with a real slowlane_init'd lane. Flipping the cluster-wide default to ON is a follow-up compatibility audit across the full test suite, explicitly out of scope for this fix. Verified: standalone controller_unit_test binary run directly (bypassing meson) reports 976 Tests 0 Failures 0 Ignored, no hang/timeout.
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## Verification
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_Not recorded (migrated from buglog.json)._
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## References
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- Related: bug-1429, bug-1451
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- Gitea: #25
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