ENG-54618 Fix concurrent map write bug

view/state/kind/locator/form.go

@@ -97,19 +97,18 @@ func NewForm(opts ...Option) (kind.Locator, error) {
 	return ret, nil
 }
 
-// seedFormFromMultipart parses multipart/form-data (if needed) and copies textual values to the shared form
+// seedFormFromMultipart parses multipart/form-data and copies values into shared maps.
+// Mutex is required because multiple Form locators (one per parameter) can call this
+// concurrently on the same request. Uses form.Values directly instead of form.Set to
+// avoid deadlock (form.Set locks the same mutex).
 func (r *Form) seedFormFromMultipart() {
 	if r.request == nil || r.form == nil {
 		return
 	}
 	if r.request.MultipartForm == nil && len(r.form.Values) == 0 {
-		// Only ParseMultipartForm for form-data; other multipart types aren't
-		// supported by ParseMultipartForm. If the shared form already has
-		// values, treat it as authoritative and avoid parsing.
 		ct := r.request.Header.Get("Content-Type")
 		if ct != "" {
 			if mediaType, _, err := mime.ParseMediaType(ct); err == nil && shared.IsFormData(mediaType) {
-				// Use the same default memory threshold as Body locator
 				const maxMultipartMemory = 32 << 20 // 32 MiB
 				_ = r.request.ParseMultipartForm(maxMultipartMemory)
 			}
@@ -118,14 +117,18 @@ func (r *Form) seedFormFromMultipart() {
 	if r.request.MultipartForm == nil {
 		return
 	}
-	if len(r.request.Form) == 0 {
+	// BUG FIX (concurrent map writes):
+	mu := r.form.Mutex()
+	mu.Lock()
+	defer mu.Unlock()
+	if r.request.Form == nil {
 		r.request.Form = url.Values{}
 	}
 	for k, vs := range r.request.MultipartForm.Value {
 		if len(vs) == 0 {
 			continue
 		}
-		r.form.Set(k, vs...)
+		r.form.Values[k] = vs
 		r.request.Form[k] = vs
 	}
 }