fix(init): emit INIT_REPORT(phase=init) for failed on-demand cold-start init

AWS performs a suppressed double init when an on-demand function's
cold-start init fails: it emits INIT_REPORT(phase=init, status=error)
for the failed cold-start init and INIT_REPORT(phase=invoke, status=error)
for the retried init folded into the first invocation. The RIE only
emitted the phase=invoke line.

Add ReportInitPhaseError() to emit the missing phase=init line and call
it on the on-demand init-failure path (onDemand && ErrInitDoneFailed),
mirroring the existing ReportInitTimeout() / phase=invoke emission. This
makes test_lambda_runtime_exit_segfault pass against LocalStack.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: joe4dev

cmd/localstack/custom_interop.go

@@ -428,6 +428,25 @@ func (c *CustomInteropServer) ReportInitTimeout() {
 	fprintInitReport(c.logCollector, millisSince(c.initStart), "init", "timeout", "")
 }
 
+// ReportInitPhaseError emits the AWS-style INIT_REPORT(phase=init, status=error) line for an
+// on-demand cold-start init that failed (e.g. a runtime crash or exit during module load).
+// AWS performs a suppressed double init: the failed cold-start init reports Phase: init here,
+// and the retried init folded into the first invocation reports Phase: invoke (see the invoke
+// handler). It is a no-op when no init error was recorded. The duration is rapid's measurement
+// of the Init phase when available, falling back to wall-clock for inits that died before
+// emitting their INIT_REPORT lifecycle event.
+func (c *CustomInteropServer) ReportInitPhaseError() {
+	errType, _ := c.initErrorType.Load().(string)
+	if errType == "" {
+		return
+	}
+	initTimeMS, ok := c.eventsAPI.InitDurationMS()
+	if !ok {
+		initTimeMS = millisSince(c.initStart)
+	}
+	fprintInitReport(c.logCollector, initTimeMS, "init", "error", errType)
+}
+
 // millisSince returns the wall-clock milliseconds elapsed since start.
 func millisSince(start time.Time) float64 {
 	return float64(time.Since(start).Nanoseconds()) / float64(time.Millisecond)

cmd/localstack/main.go

@@ -336,6 +336,10 @@ func main() {
 		// recorded the runtime-reported type).
 		log.Debugln("Init failed; deferring to first invocation (on-demand suppressed init).")
 		interopServer.RecordInitError(initResp.InitErrorType)
+		// Emit the failed cold-start init's INIT_REPORT(phase=init) line. AWS performs a
+		// suppressed double init, so the first invocation later emits a second
+		// INIT_REPORT(phase=invoke) line for the retried (folded-in) init.
+		interopServer.ReportInitPhaseError()
 	case err != nil:
 		// PC/SnapStart/MI, or an init-phase reset: report the failure now and exit. When the
 		// runtime reported its own error via /init/error, SendInitErrorResponse already