[Engine] Make SkiaGoldClient a NOP when the branch is not main or master. (#161187)

Unblocks https://github.com/flutter/flutter/pull/160556.

Currently the merge queue runs in post-submit mode, which looks for (and
fails to find) digests for PRs that have not yet been submitted,
blocking the engine goldens from being enabled/checked in
https://github.com/flutter/flutter/pull/160556. This PR is a proposal to
fix that by skipping the tests.

We might decide instead that tests should not be running in the merge
queue at all, in which case we will _not_ merge this PR and will change
how the merge queue works instead. Otherwise this PR is a proof of
concept of [aligning implementations with the
framework](a9b3f6c042/packages/flutter_goldens/lib/flutter_goldens.dart (L338)).

engine/src/flutter/testing/skia_gold_client/lib/skia_gold_client.dart

@@ -88,6 +88,12 @@ interface class SkiaGoldClient {
       path.join(_engineRoot.flutterDir.path, '.engine-release.version'),
     );
 
+    if (!_isMainBranch) {
+      _stderr.writeln(
+        'Current git branch (${_environment['GIT_BRANCH']}) is not "main" or "master", so golden files are not checked.',
+      );
+    }
+
     // If the file is not found or cannot be read, we are in an invalid state.
     try {
       _releaseVersion = ReleaseVersion.parse(releaseVersionFile.readAsStringSync());
@@ -113,6 +119,14 @@ interface class SkiaGoldClient {
     return (environment ?? io.Platform.environment).containsKey(_kLuciEnvName);
   }
 
+  /// Whether the client is currently running on the main branch.
+  bool get _isMainBranch {
+    return switch (_environment['GIT_BRANCH']) {
+      'main' || 'master' => true,
+      _ => false,
+    };
+  }
+
   /// Whether the current environment is a presubmit job.
   bool get _isPresubmit {
     return isLuciEnv(environment: _environment) &&
@@ -319,6 +333,13 @@ interface class SkiaGoldClient {
     int pixelColorDelta = 0,
     required int screenshotSize,
   }) async {
+    if (!_isMainBranch) {
+      if (verbose) {
+        _stderr.writeln('Skipping $testName (not on git main branch)');
+      }
+      return;
+    }
+
     assert(_isPresubmit || _isPostsubmit);
 
     // Clean the test name to remove the file extension.

engine/src/flutter/testing/skia_gold_client/test/skia_gold_client_test.dart

@@ -20,6 +20,7 @@ void main() {
 
   /// Simulating what a presubmit environment would look like.
   const Map<String, String> presubmitEnv = <String, String>{
+    'GIT_BRANCH': 'master',
     'GOLDCTL': 'python tools/goldctl.py',
     'GOLD_TRYJOB': 'flutter/engine/1234567890',
     'LOGDOG_STREAM_PREFIX': 'buildbucket/cr-buildbucket.appspot.com/1234567890/+/logdog',
@@ -28,6 +29,7 @@ void main() {
 
   /// Simulating what a postsubmit environment would look like.
   const Map<String, String> postsubmitEnv = <String, String>{
+    'GIT_BRANCH': 'master',
     'GOLDCTL': 'python tools/goldctl.py',
     'LOGDOG_STREAM_PREFIX': 'buildbucket/cr-buildbucket.appspot.com/1234567890/+/logdog',
     'LUCI_CONTEXT': '{}',
@@ -87,6 +89,74 @@ void main() {
     }
   });
 
+  test('prints a warning and skips when the git branch is not master or main', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: {...presubmitEnv, 'GIT_BRANCH': 'merge-queue-foo'},
+        onRun: (List<String> command) {
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'auth',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--luci',
+          ]);
+          createAuthOptDotJson(fixture.workDirectory.path);
+          return io.ProcessResult(0, 0, '', '');
+        },
+      );
+
+      // In case we change our mind, auth is still expected to work.
+      await client.auth();
+
+      expect(
+        fixture.outputSink.toString(),
+        stringContainsInOrder([
+          'Current git branch',
+          'merge-queue-foo',
+          'is not "main" or "master"',
+        ]),
+      );
+    } finally {
+      fixture.dispose();
+    }
+  });
+
+  test('always a success when the git branch is not master or main', () async {
+    final _TestFixture fixture = _TestFixture();
+    try {
+      final SkiaGoldClient client = createClient(
+        fixture,
+        environment: {...presubmitEnv, 'GIT_BRANCH': 'merge-queue-foo'},
+        onRun: (List<String> command) {
+          expect(command, <String>[
+            'python tools/goldctl.py',
+            'auth',
+            '--work-dir',
+            p.join(fixture.workDirectory.path, 'temp'),
+            '--luci',
+          ]);
+          createAuthOptDotJson(fixture.workDirectory.path);
+          return io.ProcessResult(0, 0, '', '');
+        },
+      );
+
+      // In case we change our mind, auth is still expected to work.
+      await client.auth();
+
+      // Always completes OK.
+      await client.addImg(
+        'test-name.foo',
+        io.File(p.join(fixture.workDirectory.path, 'temp', 'golden.png')),
+        screenshotSize: 1000,
+      );
+    } finally {
+      fixture.dispose();
+    }
+  });
+
   test('auth executes successfully', () async {
     final _TestFixture fixture = _TestFixture();
     try {