chore(init): close HTTP response bodies and minor cleanups

- Close response bodies in SendStatus/SendLogs/SendResult so idle
  connections are released instead of leaked.
- Use errors.New instead of fmt.Errorf with no format arguments.
- Document the single-invoke assumption behind the unsynchronized
  initStart/warmStart fields.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: joe4dev

cmd/localstack/custom_interop.go

@@ -27,8 +27,11 @@ type CustomInteropServer struct {
 	localStackAdapter *LocalStackAdapter
 	port              string
 	upstreamEndpoint  string
-	initStart         time.Time
-	warmStart         bool
+	// initStart is set once in Init() and warmStart is flipped on the first invoke.
+	// Both are accessed only from the single sequential init -> invoke flow (the RIE
+	// processes one invocation at a time), so they need no additional synchronization.
+	initStart time.Time
+	warmStart bool
 }
 
 type LocalStackAdapter struct {
@@ -45,10 +48,11 @@ const (
 
 func (l *LocalStackAdapter) SendStatus(status LocalStackStatus, payload []byte) error {
 	statusUrl := fmt.Sprintf("%s/status/%s/%s", l.UpstreamEndpoint, l.RuntimeId, status)
-	_, err := http.Post(statusUrl, "application/json", bytes.NewReader(payload))
+	resp, err := http.Post(statusUrl, "application/json", bytes.NewReader(payload))
 	if err != nil {
 		return err
 	}
+	defer resp.Body.Close()
 	return nil
 }
 
@@ -58,8 +62,12 @@ func (l *LocalStackAdapter) SendLogs(invokeId string, logs LogResponse) error {
 	if err != nil {
 		return err
 	}
-	_, err = http.Post(l.UpstreamEndpoint+"/invocations/"+invokeId+"/logs", "application/json", bytes.NewReader(serialized))
-	return err
+	resp, err := http.Post(l.UpstreamEndpoint+"/invocations/"+invokeId+"/logs", "application/json", bytes.NewReader(serialized))
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	return nil
 }
 
 // SendResult posts the invocation result body to LocalStack.
@@ -79,8 +87,12 @@ func (l *LocalStackAdapter) SendResult(invokeId string, body []byte, isError boo
 	} else {
 		log.Infoln("Sending to /response")
 	}
-	_, err := http.Post(l.UpstreamEndpoint+endpoint, "application/json", bytes.NewReader(body))
-	return err
+	resp, err := http.Post(l.UpstreamEndpoint+endpoint, "application/json", bytes.NewReader(body))
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	return nil
 }
 
 // The InvokeRequest is sent by LocalStack to trigger an invocation
@@ -94,12 +106,12 @@ type InvokeRequest struct {
 
 // The ErrorResponse is sent TO LocalStack when encountering an error
 type ErrorResponse struct {
-	ErrorMessage string   `json:"errorMessage"`
-	ErrorType    string   `json:"errorType,omitempty"`
+	ErrorMessage string `json:"errorMessage"`
+	ErrorType    string `json:"errorType,omitempty"`
 	// RequestId uses *string so that an empty string "" is serialized (not omitted),
 	// while nil is omitted — init errors always set this field, fault events leave it nil.
-	RequestId *string  `json:"requestId,omitempty"`
-	StackTrace   []string `json:"stackTrace,omitempty"`
+	RequestId  *string  `json:"requestId,omitempty"`
+	StackTrace []string `json:"stackTrace,omitempty"`
 }
 
 func NewCustomInteropServer(lsOpts *LsOpts, adapter *LocalStackAdapter, delegate interop.Server, logCollector *LogCollector) (server *CustomInteropServer) {

cmd/localstack/supervisor.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"strings"
 	"sync/atomic"
@@ -78,7 +79,7 @@ func (ls *LocalStackSupervisor) loop(ctx context.Context) {
 
 			faultData := interop.FaultData{
 				RequestID:    interop.RequestID(uuid.NewString()),
-				ErrorMessage: fmt.Errorf("Runtime exited without providing a reason"),
+				ErrorMessage: errors.New("Runtime exited without providing a reason"),
 				ErrorType:    fatalerror.RuntimeExit,
 			}
 			if !termination.Success() {