Introduces the panicMu mutex

Ensures that panic failpoints and serving of the http requests won't
be able to be executed at the same time.

runtime/http.go

@@ -37,14 +37,11 @@ func serve(host string) error {
 }
 
 func (*httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	// This prevents all failpoints from being triggered. It ensures
-	// the server(runtime) doesn't panic due to any failpoints during
-	// processing the HTTP request.
-	// It may be inefficient, but correctness is more important than
-	// efficiency. Usually users will not enable too many failpoints
-	// at a time, so it (the efficiency) isn't a problem.
-	failpointsMu.Lock()
-	defer failpointsMu.Unlock()
+	// Ensures the server(runtime) doesn't panic due to the execution of
+	// panic failpoints during processing of the HTTP request
+	panicMu.Lock()
+	defer panicMu.Unlock()
+
 	// flush before unlocking so a panic failpoint won't
 	// take down the http server before it sends the response
 	defer flush(w)

runtime/runtime.go

@@ -27,7 +27,10 @@ var (
 
 	failpoints   map[string]*Failpoint
 	failpointsMu sync.RWMutex
-	envTerms     map[string]string
+
+	envTerms map[string]string
+
+	panicMu sync.Mutex
 )
 
 func init() {

runtime/terms.go

@@ -317,6 +317,9 @@ func actSleep(t *term) interface{} {
 }
 
 func actPanic(t *term) interface{} {
+	panicMu.Lock()
+	defer panicMu.Unlock()
+
 	if t.val != nil {
 		panic(fmt.Sprintf("failpoint panic: %v", t.val))
 	}