refactor(init): format the REPORT line in one place

PrintEndReports took pre-formatted "Init Duration: ..." and
"Status: ...\tError Type: ..." string fragments, splitting knowledge of
the REPORT line format between custom_interop.go and awsutil.go. Pass
the structured values (duration, status, error type) instead and render
the line in PrintEndReports only. Output is byte-identical.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: joe4dev

cmd/localstack/awsutil.go

@@ -92,25 +92,38 @@ func getBootstrap(args []string) (interop.Bootstrap, string) {
 	return NewSimpleBootstrap(bootstrapLookupCmd, currentWorkingDir), handler
 }
 
-func PrintEndReports(invokeId string, initDuration string, status string, memorySize string, invokeStart time.Time, timeoutDuration time.Duration, w io.Writer) {
+// PrintEndReports emits the END and REPORT lines that close an invocation's log envelope.
+// initDurationMS is rendered when hasInitDuration is set (the successful on-demand cold-start
+// init duration reported via the first invocation, see TakeColdStartInitDuration). status is
+// "" for a plain successful invocation, or AWS's "timeout" / "error"; errorType (a scrubbed
+// fatal error type, e.g. Runtime.ExitError) is rendered only alongside status "error".
+func PrintEndReports(invokeId string, initDurationMS float64, hasInitDuration bool, status string, errorType string, memorySize string, invokeStart time.Time, timeoutDuration time.Duration, w io.Writer) {
 	// Calculate invoke duration
 	invokeDuration := math.Min(float64(time.Now().Sub(invokeStart).Nanoseconds()),
 		float64(timeoutDuration.Nanoseconds())) / float64(time.Millisecond)
 
 	_, _ = fmt.Fprintln(w, "END RequestId: "+invokeId)
 	// We set the Max Memory Used and Memory Size to be the same (whatever it is set to) since there is
 	// not a clean way to get this information from rapidcore
-	// initDuration and status are pre-formatted fragments: pass them as %s arguments, not as
-	// part of the format string — status may embed a runtime-supplied error type, and a stray
-	// formatting verb in it would corrupt the REPORT line.
-	_, _ = fmt.Fprintf(w,
+	report := fmt.Sprintf(
 		"REPORT RequestId: %s\t"+
 			"Duration: %.2f ms\t"+
 			"Billed Duration: %.f ms\t"+
 			"Memory Size: %s MB\t"+
-			"Max Memory Used: %s MB\t"+
-			"%s%s\n",
-		invokeId, invokeDuration, math.Ceil(invokeDuration), memorySize, memorySize, initDuration, status)
+			"Max Memory Used: %s MB\t",
+		invokeId, invokeDuration, math.Ceil(invokeDuration), memorySize, memorySize)
+	if hasInitDuration {
+		report += fmt.Sprintf("Init Duration: %.2f ms\t", initDurationMS)
+	}
+	if status != "" {
+		// Concatenated, not formatted: errorType is runtime-supplied, and a stray formatting
+		// verb in it would corrupt the REPORT line.
+		report += "Status: " + status
+		if errorType != "" {
+			report += "\tError Type: " + errorType
+		}
+	}
+	_, _ = fmt.Fprintln(w, report)
 }
 
 type Sandbox interface {

cmd/localstack/custom_interop.go

@@ -133,12 +133,9 @@ func NewCustomInteropServer(lsOpts *LsOpts, delegate interop.Server, logCollecto
 				// after an inline suppressed init's own logs) — see events.go.
 
 				// First invocation into a successfully initialized on-demand environment: REPORT
-				// carries the Init phase duration as measured by rapid (take-once; empty on warm
+				// carries the Init phase duration as measured by rapid (take-once; absent on warm
 				// starts, failed/timed-out inits, and non-on-demand environments).
-				initDuration := ""
-				if initTimeMS, ok := server.eventsAPI.TakeColdStartInitDuration(); ok {
-					initDuration = fmt.Sprintf("Init Duration: %.2f ms\t", initTimeMS)
-				}
+				initDurationMS, hasInitDuration := server.eventsAPI.TakeColdStartInitDuration()
 
 				invokeStart := time.Now()
 				err = server.Invoke(invokeResp, &interop.Invoke{
@@ -162,12 +159,13 @@ func NewCustomInteropServer(lsOpts *LsOpts, delegate interop.Server, logCollecto
 				timeout := int(server.delegate.GetInvokeTimeout().Seconds())
 				isErr := false
 				status := ""
+				errorType := ""
 				if err != nil {
 					switch {
 					case errors.Is(err, rapidcore.ErrInvokeTimeout):
 						log.Debugf("Got invoke timeout")
 						isErr = true
-						status = "Status: timeout"
+						status = "timeout"
 						errorResponse := lsapi.ErrorResponse{
 							ErrorType: "Sandbox.Timedout",
 							ErrorMessage: fmt.Sprintf(
@@ -191,7 +189,8 @@ func NewCustomInteropServer(lsOpts *LsOpts, delegate interop.Server, logCollecto
 						// scrubbed fatal error type (e.g. Runtime.Unknown).
 						if errType := server.eventsAPI.InitErrorType(); errType != "" {
 							isErr = true
-							status = "Status: error\tError Type: " + errType
+							status = "error"
+							errorType = errType
 						}
 					default:
 						log.Fatalln(err)
@@ -207,7 +206,7 @@ func NewCustomInteropServer(lsOpts *LsOpts, delegate interop.Server, logCollecto
 				}
 				timeoutDuration := time.Duration(timeout) * time.Second
 				memorySize := GetEnvOrDie("AWS_LAMBDA_FUNCTION_MEMORY_SIZE")
-				PrintEndReports(invokeR.InvokeId, initDuration, status, memorySize, invokeStart, timeoutDuration, logCollector)
+				PrintEndReports(invokeR.InvokeId, initDurationMS, hasInitDuration, status, errorType, memorySize, invokeStart, timeoutDuration, logCollector)
 
 				if err2 := server.localStackAdapter.SendLogs(invokeR.InvokeId, logCollector.getLogs()); err2 != nil {
 					log.Error("failed to send logs to LocalStack: ", err2)