Add code coverage presubmit test

.ci.yaml

@@ -133,6 +133,15 @@ targets:
           {"dependency": "open_jdk", "version": "version:21"}
         ]
 
+  - name: Linux coverage_checks
+    recipe: packages/packages
+    bringup: true
+    timeout: 60
+    properties:
+      target_file: coverage_checks.yaml
+      channel: master
+      version_file: flutter_master.version
+
   - name: Linux dart_unit_test_shard_1 master
     recipe: packages/packages
     timeout: 60

.ci/targets/coverage_checks.yaml

@@ -0,0 +1,7 @@
+tasks:
+  - name: prepare tool
+    script: .ci/scripts/prepare_tool.sh
+    infra_step: true
+  - name: code coverage check
+    script: .ci/scripts/tool_runner.sh
+    args: ["coverage-check", "--base-branch=origin/main"]

script/configs/custom_coverage_minimums.yaml

@@ -0,0 +1,7 @@
+# A list of packages and their minimum required code coverage.
+# Only packages listed in this file will be checked for code coverage
+# when running the script/tool/lib/src/coverage_check_command.dart
+# command. If a package drops below its listed threshold, the check
+# will fail.
+custom_coverage_minimums:
+  camera_android_camerax: 18.3

script/tool/lib/src/coverage_check_command.dart

@@ -0,0 +1,173 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io' as io;
+
+import 'package:file/file.dart';
+import 'package:meta/meta.dart';
+import 'package:yaml/yaml.dart';
+
+import 'common/core.dart';
+import 'common/output_utils.dart';
+import 'common/package_looping_command.dart';
+import 'common/repository_package.dart';
+
+/// A command to run code coverage checks on changed packages.
+class CoverageCheckCommand extends PackageLoopingCommand {
+  /// Creates a coverage check command instance.
+  CoverageCheckCommand(super.packagesDir, {super.processRunner, super.platform, super.gitDir});
+
+  @override
+  final String name = 'coverage-check';
+
+  @override
+  final String description =
+      'Checks that code coverage meets the specified minimums '
+      'for opted-in packages as specified in `script/configs/custom_coverage_minimums.yaml`.';
+
+  // TODO(camsim99): Determine how to ensure test/ subdirectories are covered by this check:
+  // https://github.com/flutter/flutter/issues/188880.
+  @override
+  PackageLoopingType get packageLoopingType => PackageLoopingType.includeAllSubpackages;
+
+  final Map<String, double> _customMinimums = <String, double>{};
+
+  @override
+  Future<void> initializeRun() async {
+    final File minimumsFile = packagesDir.parent
+        .childDirectory('script')
+        .childDirectory('configs')
+        .childFile('custom_coverage_minimums.yaml');
+    if (!minimumsFile.existsSync()) {
+      printError('The custom_coverage_minimums.yaml file is missing.');
+      throw ToolExit(1);
+    }
+
+    final Object? yaml = loadYaml(minimumsFile.readAsStringSync());
+    if (yaml is YamlMap) {
+      final Object? packageMap = yaml['custom_coverage_minimums'];
+      if (packageMap is YamlMap) {
+        for (final MapEntry<dynamic, dynamic> entry in packageMap.entries) {
+          final Object? key = entry.key;
+          final Object? value = entry.value;
+          if (key is String && value is num) {
+            _customMinimums[key] = value.toDouble();
+          }
+        }
+      }
+    }
+  }
+
+  @override
+  Future<PackageResult> runForPackage(RepositoryPackage package) async {
+    // Only run for first-party packages.
+    final List<String> pathComponents = package.directory.fileSystem.path.split(package.path);
+    if (pathComponents.contains('third_party')) {
+      return PackageResult.skip('Not a first-party package.');
+    }
+
+    if (!package.testDirectory.existsSync()) {
+      return PackageResult.skip('No test/ directory.');
+    }
+
+    final String packageName = package.directory.basename;
+
+    if (!_customMinimums.containsKey(packageName)) {
+      return PackageResult.skip('Package not opted into coverage checks.');
+    }
+
+    // Collect code coverage for the package.
+    double? currentCoverage;
+    try {
+      currentCoverage = await _runCoverageAndParse(package);
+    } on NoLinesFoundException {
+      return PackageResult.fail(<String>[
+        'No instrumented Dart lines were found in the coverage report for $packageName.',
+        'If this package contains no code to cover, please remove it from ',
+            'script/configs/custom_coverage_minimums.yaml.',
+      ]);
+    }
+
+    if (currentCoverage == null) {
+      return PackageResult.fail(<String>['Failed to run tests or parse coverage']);
+    }
+    final double requiredCoverage = _customMinimums[packageName]!;
+
+    if (currentCoverage < requiredCoverage) {
+      return PackageResult.fail(<String>[
+        'Code coverage for $packageName is ${currentCoverage.toStringAsFixed(1)}%, which is below the required ${requiredCoverage.toStringAsFixed(1)}%.',
+      ]);
+    }
+
+    return PackageResult.success();
+  }
+
+  Future<double?> _runCoverageAndParse(RepositoryPackage package) async {
+    final Directory coverageDir = package.directory.childDirectory('coverage');
+    try {
+      final io.ProcessResult result = await processRunner.run('flutter', <String>[
+        'test',
+        '--coverage',
+      ], workingDir: package.directory);
+
+      if (result.exitCode != 0) {
+        print('Test failed for ${package.directory.basename}:\n${result.stdout}\n${result.stderr}');
+        return null;
+      }
+
+      final File lcovFile = coverageDir.childFile('lcov.info');
+      if (!lcovFile.existsSync()) {
+        print('Coverage file not found at ${lcovFile.path}.');
+        return null;
+      }
+
+      return calculateCoverage(lcovFile);
+    } finally {
+      if (coverageDir.existsSync()) {
+        coverageDir.deleteSync(recursive: true);
+      }
+    }
+  }
+
+  /// Calculates code coverage for non-generated code files by finding
+  /// the percentage of covered lines of code over the total lines of code.
+  @visibleForTesting
+  double calculateCoverage(File lcovFile) {
+    var linesHit = 0;
+    var linesFound = 0;
+    var skipCurrentFile = false;
+
+    final List<String> lines = lcovFile.readAsLinesSync();
+    for (final line in lines) {
+      if (line.startsWith('SF:')) {
+        final String fileName = line.substring(3);
+        skipCurrentFile = _isGeneratedFile(fileName);
+      }
+      if (!skipCurrentFile) {
+        if (line.startsWith('LH:')) {
+          linesHit += int.parse(line.substring(3));
+        } else if (line.startsWith('LF:')) {
+          linesFound += int.parse(line.substring(3));
+        }
+      }
+    }
+
+    if (linesFound == 0) {
+      throw const NoLinesFoundException();
+    }
+    return (linesHit / linesFound) * 100.0;
+  }
+}
+
+/// Exception thrown when no instrumented lines are found in the coverage report.
+class NoLinesFoundException implements Exception {
+  /// Creates a new [NoLinesFoundException].
+  const NoLinesFoundException();
+}
+
+bool _isGeneratedFile(String fileName) {
+  return fileName.endsWith('.g.dart') ||
+      fileName.endsWith('.pb.dart') ||
+      fileName.endsWith('.mocks.dart');
+}

script/tool/lib/src/main.dart

@@ -12,6 +12,7 @@ import 'analyze_command.dart';
 import 'branches_for_batch_release_command.dart';
 import 'build_examples_command.dart';
 import 'common/core.dart';
+import 'coverage_check_command.dart';
 import 'create_all_packages_app_command.dart';
 import 'custom_test_command.dart';
 import 'dart_test_command.dart';
@@ -57,6 +58,7 @@ void main(List<String> args) {
         ..addCommand(BranchesForBatchReleaseCommand(packagesDir))
         ..addCommand(BuildExamplesCommand(packagesDir))
         ..addCommand(CreateAllPackagesAppCommand(packagesDir))
+        ..addCommand(CoverageCheckCommand(packagesDir))
         ..addCommand(CustomTestCommand(packagesDir))
         ..addCommand(DartTestCommand(packagesDir))
         ..addCommand(DriveExamplesCommand(packagesDir))