Honor the latest WebRTC policy across flag merges

server/lib/chromiumflags/chromiumflags.go

@@ -204,7 +204,8 @@ func ReadOptionalFlagFile(path string) ([]string, error) {
 // both lists Chromium disables it — disable wins over enable.
 //
 // Non-feature, non-extension flags from both base and runtime are combined with deduplication
-// (first occurrence preserved).
+// (first occurrence preserved), except the singleton WebRTC policy, whose last
+// occurrence wins as it does in Chromium.
 func MergeFlags(baseTokens, runtimeTokens []string) []string {
 	// Buckets
 	var (
@@ -265,9 +266,14 @@ func MergeFlags(baseTokens, runtimeTokens []string) []string {
 	combined := append(append([]string{}, baseNonExt...), runtimeNonExt...)
 	combined = append(combined, featureFlags...)
 	combined = append(combined, extFlags...)
+	var webRTCPolicy string
 	seen := make(map[string]struct{}, len(combined))
 	final := make([]string, 0, len(combined))
 	for _, tok := range combined {
+		if name, _, _ := strings.Cut(tok, "="); name == "--webrtc-ip-handling-policy" {
+			webRTCPolicy = tok
+			continue
+		}
 		if tok == "" {
 			continue
 		}
@@ -277,6 +283,9 @@ func MergeFlags(baseTokens, runtimeTokens []string) []string {
 		seen[tok] = struct{}{}
 		final = append(final, tok)
 	}
+	if webRTCPolicy != "" {
+		final = append(final, webRTCPolicy)
+	}
 	return final
 }
 

server/lib/chromiumflags/chromiumflags_test.go

@@ -494,3 +494,28 @@ func TestMergeFeatureEntries(t *testing.T) {
 		})
 	}
 }
+
+func TestWebRTCPolicyLastValueSurvivesRuntimeMerge(t *testing.T) {
+	const prefix = "--webrtc-ip-handling-policy="
+	for _, tc := range []struct{ base, previous, want string }{
+		{"default", "emulated", "default"},
+		{"disable_non_proxied_udp", "emulated", "disable_non_proxied_udp"},
+		{"emulated", "default", "emulated"},
+		{"default", "disable_non_proxied_udp", "emulated"},
+	} {
+		t.Run(tc.base+"/"+tc.want, func(t *testing.T) {
+			// Persist a policy update, then merge that file with the original launch.
+			overlay := MergeFlags([]string{prefix + tc.base, prefix + tc.previous}, []string{prefix + tc.want})
+			flags := MergeFlagsWithRuntimeTokens(prefix+tc.base, overlay)
+			var policies []string
+			for _, flag := range flags {
+				if strings.HasPrefix(flag, prefix) {
+					policies = append(policies, flag)
+				}
+			}
+			if !reflect.DeepEqual(policies, []string{prefix + tc.want}) {
+				t.Fatalf("merged policies = %#v, want latest %q", policies, tc.want)
+			}
+		})
+	}
+}