From d5459cb7ce6aa888cb53cb341641b339d545b358 Mon Sep 17 00:00:00 2001
From: Mark Daoust <markdaoust@google.com>
Date: Thu, 11 Sep 2025 11:09:20 -0700
Subject: [PATCH] Fix: "fatal error: concurrent map writes" when multiple tests
 in parallel.

---
 internal/replay/replay_http_server.go | 31 +++++++++++++++++++++------
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/internal/replay/replay_http_server.go b/internal/replay/replay_http_server.go
index 4428775..cdee8e2 100644
--- a/internal/replay/replay_http_server.go
+++ b/internal/replay/replay_http_server.go
@@ -26,6 +26,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 	"unicode"
 
 	"github.com/google/test-server/internal/config"
@@ -35,11 +36,13 @@ import (
 )
 
 type ReplayHTTPServer struct {
+	mu             sync.Mutex // Mutex to protect fields below
 	prevRequestSHA string
 	seenFiles      map[string]struct{}
-	config         *config.EndpointConfig
-	recordingDir   string
-	redactor       *redact.Redact
+
+	config       *config.EndpointConfig
+	recordingDir string
+	redactor     *redact.Redact
 }
 
 func NewReplayHTTPServer(cfg *config.EndpointConfig, recordingDir string, redactor *redact.Redact) *ReplayHTTPServer {
@@ -70,9 +73,14 @@ func (r *ReplayHTTPServer) handleRequest(w http.ResponseWriter, req *http.Reques
 		return
 	}
 
-	redactedReq, err := r.createRedactedRequest(req)
+	// Safely get the current prevRequestSHA
+	r.mu.Lock()
+	currentPrevSHA := r.prevRequestSHA
+	r.mu.Unlock()
+
+	redactedReq, err := r.createRedactedRequest(req, currentPrevSHA)
 	if err != nil {
-		fmt.Printf("Error processing request")
+		fmt.Printf("Error processing request: %v\n", err)
 		http.Error(w, fmt.Sprintf("Error processing request: %v", err), http.StatusInternalServerError)
 		return
 	}
@@ -83,10 +91,15 @@ func (r *ReplayHTTPServer) handleRequest(w http.ResponseWriter, req *http.Reques
 		http.Error(w, fmt.Sprintf("Invalid recording file name: %v", err), http.StatusInternalServerError)
 		return
 	}
+
+	// Safely check seenFiles and update redactedReq.PreviousRequest if needed
+	r.mu.Lock()
 	if _, ok := r.seenFiles[fileName]; !ok {
 		// Reset to HeadSHA when first time seen request from the given file.
 		redactedReq.PreviousRequest = store.HeadSHA
 	}
+	r.mu.Unlock()
+
 	if req.Header.Get("Upgrade") == "websocket" {
 		fmt.Printf("Upgrading connection to websocket...\n")
 
@@ -114,14 +127,18 @@ func (r *ReplayHTTPServer) handleRequest(w http.ResponseWriter, req *http.Reques
 		fmt.Printf("Error writing response: %v\n", err)
 		panic(err)
 	}
+
+	// Safely update prevRequestSHA and seenFiles
+	r.mu.Lock()
 	if fileName != shaSum {
 		r.prevRequestSHA = shaSum
 	}
 	r.seenFiles[fileName] = struct{}{}
+	r.mu.Unlock()
 }
 
-func (r *ReplayHTTPServer) createRedactedRequest(req *http.Request) (*store.RecordedRequest, error) {
-	recordedRequest, err := store.NewRecordedRequest(req, r.prevRequestSHA, *r.config)
+func (r *ReplayHTTPServer) createRedactedRequest(req *http.Request, prevSHA string) (*store.RecordedRequest, error) {
+	recordedRequest, err := store.NewRecordedRequest(req, prevSHA, *r.config)
 	if err != nil {
 		return nil, err
 	}