Emit INIT_REPORT for successful provisioned-concurrency init

On-demand cold-start inits fold their duration into the first invocation's
REPORT line, but provisioned-concurrency / Managed Instances environments
initialize ahead of time and their invokes omit "Init Duration". AWS instead
emits a standalone INIT_REPORT line in the invoke-serving log stream carrying
only the duration -- no Phase, Status, or Error Type (those appear only on
failed or timed-out init lines).

SendInitReport previously emitted nothing for these successful non-on-demand
inits, so LocalStack's log stream diverged from AWS. Add the bare INIT_REPORT
line for that case and update the unit test accordingly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: joe4dev

cmd/localstack/events.go

@@ -16,11 +16,13 @@ import (
 //   - START is emitted on SendInvokeStart, which rapid fires after any inline (suppressed)
 //     init and before the runtime handles the invocation — so a re-run init's logs land
 //     before START, matching AWS.
-//   - INIT_REPORT is emitted on SendInitReport for failed or timed-out inits, with rapid's
-//     authoritative duration and phase (init for the eager cold-start init, invoke for a
-//     suppressed init folded into an invocation). AWS logs no INIT_REPORT for successful
-//     inits; a successful on-demand cold-start init instead surfaces as the first
-//     invocation's REPORT "Init Duration" (see TakeColdStartInitDuration).
+//   - INIT_REPORT is emitted on SendInitReport for failed or timed-out inits (carrying a
+//     Status and, for failures, an Error Type) and for successful provisioned-concurrency /
+//     Managed Instances inits (a bare line carrying only the duration and phase). It uses
+//     rapid's authoritative duration and phase (init for the eager cold-start init, invoke for
+//     a suppressed init folded into an invocation). A successful on-demand cold-start init logs
+//     no INIT_REPORT; its duration surfaces as the first invocation's REPORT "Init Duration"
+//     instead (see TakeColdStartInitDuration).
 //   - The scrubbed fatal error type of the most recent failed init (e.g. Runtime.ExitError,
 //     Runtime.Unknown) is recorded for the invoke handler's REPORT Status/Error Type line and
 //     for the init-failure report to LocalStack (see InitErrorType).
@@ -121,6 +123,13 @@ func (e *LocalStackEventsAPI) SendInitReport(data interop.InitReportData) error
 		// REPORT "Init Duration" instead of an INIT_REPORT line.
 		e.coldStartInitDuration = data.Metrics.DurationMs
 		e.hasColdStartInitDuration = true
+	case data.Phase == lambdatelemetry.InitInsideInitPhase:
+		// Successful provisioned-concurrency / Managed Instances init (non-on-demand). Those
+		// environments initialize ahead of time, so the duration cannot fold into a first
+		// invocation's REPORT (their invokes omit Init Duration); AWS instead emits a bare
+		// INIT_REPORT carrying only the duration — no Phase, Status, or Error Type (those
+		// fields appear only on the failed/timed-out init lines above).
+		line = fmt.Sprintf("INIT_REPORT Init Duration: %.2f ms\n", data.Metrics.DurationMs)
 	}
 	e.mu.Unlock()
 	if line != "" {

cmd/localstack/events_test.go

@@ -48,15 +48,22 @@ func TestEventsAPI_SuccessfulOnDemandInit_NoInitReportLine_DurationTakenOnce(t *
 	assert.Empty(t, e.InitErrorType())
 }
 
-func TestEventsAPI_SuccessfulProvisionedInit_NoDurationRecorded(t *testing.T) {
+func TestEventsAPI_SuccessfulProvisionedInit_RendersBareInitReport(t *testing.T) {
 	logs := NewLogCollector()
 	e := NewLocalStackEventsAPI(logs, false)
 
 	sendInit(t, e, "init", "success", "", 123.45)
 
-	// AWS omits Init Duration from provisioned-concurrency invokes' REPORT lines.
+	// A successful provisioned-concurrency init emits a bare INIT_REPORT: only the duration,
+	// with no Phase/Status/Error Type (those appear for failed or timed-out inits).
+	assert.Equal(t,
+		"INIT_REPORT Init Duration: 123.45 ms\n",
+		logs.getLogs().Logs)
+	// AWS omits Init Duration from provisioned-concurrency invokes' REPORT lines, so the
+	// duration is not buffered for a first-invocation REPORT.
 	_, ok := e.TakeColdStartInitDuration()
 	assert.False(t, ok)
+	assert.Empty(t, e.InitErrorType())
 }
 
 func TestEventsAPI_FailedInit_RendersInitReportAndRecordsErrorType(t *testing.T) {