feat(component-failover-adoption): trigger apiserver's rejoin from the idle-period heartbeat path

xcecli_heartbeat() is XCECLI_FF_CALL-wrapped -- a deliberately
non-retrying fire-and-forget path that does its own best-effort
one-shot reconnect but never calls xcecli_invoke_rejoin() (that only
fires from the write-call retry path). Left as-is, a controller
failover with no in-flight write request would silently repair the
transport connection without ever re-joining, permanently starving
push notifications.

runHeartbeat() gains two callback hooks fired at the right points --
onFailureToSuccess on a genuine transition, onSuccessTick on every
successful tick thereafter -- wired directly to task 4.3's existing
setNeedsRejoin()/checkAndPerformRejoin() from StartHeartbeat(). No
second rejoin mechanism: checkAndPerformRejoin() is a cheap no-op when
nothing is pending, so calling it unconditionally on every successful
tick is safe and matches the requirement that retry isn't gated on the
transition edge alone.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NNB4zGPCsHyDUCDJyMbiQN

.kiro/specs/component-failover-adoption/tasks.md

@@ -109,7 +109,7 @@
   - _Requirements: 2.1, 2.2, 2.3, 2.4, 2.5_
   - _Depends: 4.2_
 
-- [ ] 4.4 Trigger apiserver's rejoin from the idle-period heartbeat path
+- [x] 4.4 Trigger apiserver's rejoin from the idle-period heartbeat path
   - On a transport failure-to-success transition detected by the heartbeat prober, set the same pending-rejoin flag the write path uses (the heartbeat's own probe mechanism never sets it itself)
   - On every subsequent heartbeat tick where the probe succeeds and a rejoin is still pending, retry the rejoin — checked every tick, not only on the transition edge, so a rejoin attempt that itself fails still gets retried on a later tick
   - Do not gate this retry on the separate backend-degraded circuit-breaker state, since that state clears itself on its own timer independent of whether a rejoin actually succeeded
@@ -171,5 +171,6 @@
 - Task 4.1: first apiserver-group (Go+CGO) task. `xce_parse_peers()` alone (`src/lib/cluster.c`) does syntax parsing only — no semantic checks (accepts port 0, non-numeric port via silent `atoi()`-to-0, empty host). Worker/scheduler always pair it with a second `xce_cluster_config_validate()` call in their `main.c`s; round-1 review caught `parseCandidates()` (`candidates.go`) skipping that second call, silently accepting malformed candidates worker/scheduler reject — a reminder that "the C function I called is correct" isn't the same as "I called every C function the precedent calls." Fix: call `xce_cluster_config_validate(&cCfg)` after `xce_parse_peers()`, stamping a local placeholder into `cCfg.identity` first since that function also checks a non-empty top-level `identity` (unrelated to per-candidate validation, never propagated to the real connection identity elsewhere in `clib.go`). `xce_cluster_config_validate()` checks `hostname`/`port`/top-level `identity` only, never per-candidate `name` — so the `name=` prefix stays legitimately optional. Established convention confirmed pre-existing (not invented this task): a `//go:build cgo` test file with no `import "C"` of its own, pairing with a plain-Go-signature function in a separate cgo-preamble file (`_test.go` files can never `import "C"` themselves — a Go-toolchain restriction, not just a project style choice) — see `wire_dispatch_test.go`/`wire_dispatch_support.go` for the pre-existing precedent this task's `candidates_test.go`/`candidates.go` split follows. Task 4.1 is parsing-only by design — `RegisterToBackend()`'s connect loop and `tryConnect()` stay untouched; task 4.2 is the one that collapses them into a single connect call, so don't expect this diff to touch that territory. Behavior change (deliberate, disclosed, not a regression): a malformed entry now rejects the WHOLE candidate list (matches worker/scheduler's all-or-nothing precedent, `xce_parse_peers()` has no partial-success mode by construction) instead of the old Go parser's skip-with-warning leniency — surfaces via `RegisterToBackend`'s error → `cmd/main.go`'s `logger.Error`+`os.Exit(1)`, loud not silent. Tooling gap for future apiserver tasks (4.2-4.6): `make test-backend`'s documented CGO env-var setup doesn't actually export `CGO_CFLAGS`/`CGO_LDFLAGS` to a general shell (they only exist as meson `custom_target` env vars) — reconstruct via `pkg-config` against the deps `apiserver/meson.build` lists, or run the full `xce-build.sh apiserver asan` (rebuilds `libxce.a` fresh too) rather than a bare `go test`.
 - Task 4.2: found and fixed a genuine, independently-verified use-after-free while collapsing apiserver's two connect mechanisms into one — the deleted `tryConnect()` did `C.CString(host)` then `defer C.free(cAddr)` right after its single `xce_backend_init()` call returned, but `xcecli_ctx_init()` stores `ctx->conf = cfg` as a raw pointer (`src/lib/client.c:38`, never copied), and `client_failover.c`'s rotation path (`xcecli_retry_next`) re-dereferences `ac->conf->masters[cur]` on *every* failover attempt, not just at init — so any reconnect after the first would read freed heap memory. (`from`/`key` were NOT part of this bug — `xce_ctx_init()` `strdup()`s those synchronously before the defer ever fires.) Fix mirrors worker's/scheduler's own file-scope-`static` pattern for the Go/CGO side: two package-level `var`s (`gClusterCfg C.xce_cluster_config_t`, `gClientCfg C.xce_client_config_t`, alongside the pre-existing `gCtx`/`gIDCtx`) give the CGO structs a stable, process-lifetime address, and the connect path now writes host/from/key into fixed-size C arrays (`writeCCharArray()`) instead of `C.CString`/`C.free`-ing heap pointers the C library retains forever. General lesson for any future CGO code in this codebase that hands a struct to a C API storing a bare pointer to it (check the C function's own source, not just its header, for whether it copies or aliases): a Go function-local var is NOT safe once the call returns unless the C side genuinely copies everything reachable from it — verify via source read, the same standard task 2.1's original aliasing-hazard note set for the C-side daemons. `parseCandidates()` (task 4.1's deliverable) is now production-dead code — `buildClusterConfig()` calls `xce_parse_peers()` directly into the package-level `gClusterCfg` instead, since routing through `parseCandidates()`'s Go-slice return would mean re-parsing/re-copying into a fresh C struct that doesn't have the required stable address; `parseCandidates()` is exercised only by its own tests now, worth a follow-up cleanup task if it stays unused.
 - Task 4.3: rejoin flag-setter (`go_backend_rejoin_handler`), its C shim, `extern` declaration, and `xcecli_set_reconnect_handler()` registration call all live together in `clib.go`'s `xce_backend_init()` — confirms design.md's "cgo symbol resolution is per-file" constraint (see `wire_dispatch_support.go`'s own documented rationale): a `C.<exported-Go-fn>` reference only resolves against an `extern` declared in THAT file's own cgo preamble, even though the Go `//export` mechanism makes the function callable package-wide — splitting the shim into a separate `reconnect.go` preamble while the extern lived in `clib.go` (or vice versa) would silently fail to link. Registration placed strictly AFTER `xcecli_start()`'s success check (task 2.3's ordering lesson ported to Go: `xcecli_start()` unconditionally wipes `rejoin_cb` to NULL as part of its own init). `needsRejoin` is a proper `sync/atomic` flag, not a plain bool — apiserver serves concurrent HTTP requests (unlike worker's/scheduler's single-threaded daemons), so this is a real correctness requirement here, not just style. `checkAndPerformRejoin()` wired into all 5 `gCtx.ctx`-based backend-call wrappers (SubmitJob/ControlJob/ControlQueue/ControlHost/FetchFile) — note `xcecli_fetchfile()` itself isn't `XCECLI_FAILOVER_CALL`-wrapped in libxce (never triggers the C-side rejoin invoke), so wrapping it here is safe but less load-bearing than the other four; that's a libxce gap, out of this task's `ApiserverBackendBridge` boundary to fix. Reviewer found (non-blocking, correctness-safe): concurrent goroutines can each observe `needsRejoin==1` after a failover and each call the real `xcecli_join()`, serialized by the C library's own per-ctx lock (no corruption, no incorrect flag state) but each can block up to the ~100s retry budget on a redundant join — a latency/traffic concern, not a Requirement-2 violation; a CAS-based single-flight guard is a reasonable fast-follow, not required. Scope reminder for task 4.4/4.5: `ApiserverBackendHealth`/`IsDegraded()`/`MarkHealthy()` deliberately do not exist yet — `checkAndPerformRejoin()` is the POST-call half only; task 4.5 adds a pre-call check to the same helper, not a new one.
+- Task 4.4: thin adapter, not a second rejoin mechanism — `runHeartbeat()`'s new `onFailureToSuccess`/`onSuccessTick` callback params are wired directly to task 4.3's real `setNeedsRejoin`/`checkAndPerformRejoin` (`reconnect.go`) from `StartHeartbeat()`, zero duplicated flag-check logic added in `heartbeat.go` itself. Necessary because `xcecli_heartbeat()` is `XCECLI_FF_CALL`-wrapped (`client_internal.h`), a deliberately non-retrying fire-and-forget path that does its own best-effort one-shot reconnect but NEVER calls `xcecli_invoke_rejoin()` (that only fires from `xcecli_retry_next`, the write-call retry path) — so nothing sets `needsRejoin` on a pure idle-period recovery unless the heartbeat loop does it itself. `rc >= 0` success classification for `xcecli_heartbeat` confirmed against `xceapi.c`/`request.c`: it only ever returns 0 or a negative errno, never a positive value, so this is equivalent to (not just consistent with) the `rc<0`-only-failure convention used elsewhere. `onSuccessTick` fires unconditionally on every successful tick (no pending-check duplicated in `heartbeat.go`) — safe because `checkAndPerformRejoin()` is itself a cheap single-atomic-load no-op when nothing is pending. `prevFailed` (transition-tracking state) is correctly a plain non-atomic local, not `sync/atomic` — verified `runHeartbeat()` has exactly one call site (`StartHeartbeat()`, called once from `cmd/main.go`), one goroutine for the process lifetime; contrast with `needsRejoin` itself, which stays atomic since concurrent HTTP-handler goroutines reach it via the write-path wrappers. Known, accepted, already-documented (task 4.3) tradeoff carried forward: a rejoin attempt from the heartbeat goroutine shares `ac->failover.lock` with concurrent HTTP-handler backend calls, so one can block the other for up to the ~1-2 minute retry budget — not a new issue this task introduces, same mechanism task 4.3 already reviewed and accepted, task 4.4 just adds one more caller of it.
 - Task 3.4: pure test-coverage task, zero production changes to `scheduler.c`/`driver.c` (correct outcome — task 3.2 already implemented the retry/shutdown/rejoin behavior correctly; 3.4 proves it holds at multi-decision QUEUE granularity through the real drain loop, which task 3.2's own tests never exercised, all 6 of which call `sched_publish_dispatch()`/`sched_publish_pend_reason()` directly on one decision). New `test_scheduler_decision_redelivery.c` links real `scheduler_srcs` with `-Wl,--wrap=xcecli_jobdispatch` (task 3.2's established technique) and drives the real `xce_sched_driver_on_decision()` (driver.c) over 3 queued, distinct-jobid decisions. Two load-bearing tests, both RED→GREEN-verified against the real code by two independent reviewer rounds (not just the implementer's claim): ordering (decision 1 stuck-then-succeeds fully completes before decision 2's call ever fires) and bounded shutdown latency (decisions queued behind one that triggers `core.stop` mid-retry get ZERO publish calls — the concrete, fast, deterministic proof that `sched_publish_retry_loop()`'s check-`stop`-before-each-attempt placement is correct, verified by reviewers moving the check to bottom-of-loop and confirming only this one test catches it). Test-fixture gotcha for any future test building a bare `xce_sched_core_t` via `xce_sched_core_init()` alone (not full `sched_init()`): do NOT call `xce_sched_core_fini()` on it — `fini()` calls `dataq_fini()` on `pend_updates[0/1]`, which only `sched_init()` (not `xce_sched_core_init()`) ever `dataq_init()`s, so `fini()` on a bare core SEGVs on a NULL dataq; omit the `fini()` call entirely, matching `test_core_events.c`'s established "fixtures aren't torn down" convention for this exact minimal-fixture pattern.
 - Task 3.3: fixed a real production gap found along the way — `sched_init()`'s two all-candidates-unreachable/join-failure log lines logged only a candidate COUNT, never host:port names (same gap worker's task 2.3 fixed for the worker). Added `sched_format_candidate_list()`, threading `sched->cluster_cfg` through as a parameter rather than a new file-scope global (unlike worker's `g_cluster_cfg`) since `test_scheduler_connect.c` links `scheduler_srcs` as separate objects, not `#include`, so the `static` helper isn't directly callable from tests — coverage requires a real `dup2()`-based stdout capture around a live `sched_init()` call hitting the failure path, not a white-box direct call (see `capture_stdout_during_sched_init()` in `test_scheduler_connect.c` — reusable pattern for any future scheduler.c log-content assertion). Round-1 review caught this diagnostic fix shipping with zero test coverage (proven by reverting the 59-line diff and rebuilding: all existing tests still passed) — a reminder that "the fix works, I verified it manually/via live logs" is not the same as a committed regression test; always add the assertion, don't just observe the correct behavior once. The worst-case-occupancy backpressure test (`test_scheduler_backpressure.c`) only proves Req 3 AC2 (buffering/block/resume-with-no-loss) — it does not exercise task 3.2's `-E_XCE_NOT_READY` retry path, so don't cite it for Req 7 coverage (that's task 3.4's job). E2E kill-primary/reconnect and all-unreachable-at-startup were validated manually against a real 3-node `/xce-deploy-ha` cluster rather than committed as an automated test, following worker's task 2.3 precedent — this repo has no automated live-multi-process-deploy E2E harness for any C daemon (the only one, `test_e2e_live_ha_push.py`, is Python/apiserver-only); reviewer judged this acceptable both rounds. Manual validation confirmed join, primary-death survival, and lazy socket-orphaning-on-idle (matches design), but could not observe a live rejoin firing against real sockets — blocked by bug-010 (`xcecd` ASAN "already joined thread" abort under `unshare -r`, needed to bypass the worker daemon's root-only check in this sandbox; unconfirmed whether it's a genuine bug or a userns artifact, never tried under real root). The exact rejoin-after-publish-success code path already has solid white-box mock coverage from task 3.2's `test_rejoin_composition_retries_across_publish_calls`, so this residual gap is narrowly "proven against mocks, not yet against a live socket" — worth closing opportunistically if bug-010 ever gets resolved, not a blocker for this spec.