Add a script that will find the appropriate commit in another repository. (#82462)

dev/tools/bin/find_commit.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This script looks at the current commit and branch of the git repository in
+// which it was run, and finds the contemporary commit in the master branch of
+// another git repository, whose path is provided on the command line. The
+// contemporary commit is the one that was public at the time of the last commit
+// on the master branch before the current commit's branch was created.
+
+import 'dart:io';
+
+const bool debugLogging = false;
+
+void log(String message) {
+  if (debugLogging)
+    print(message);
+}
+
+class Commit {
+  Commit(this.hash, this.timestamp);
+
+  final String hash;
+  final DateTime timestamp;
+
+  static String formatArgument = '--format=%H %cI';
+
+  static Commit parse(String line) {
+    final int space = line.indexOf(' ');
+    return Commit(line.substring(0, space), DateTime.parse(line.substring(space+1, line.length).trimRight()));
+  }
+
+  static List<Commit> parseList(String lines) {
+    return lines.split('\n').where((String line) => line.isNotEmpty).map(parse).toList().reversed.toList();
+  }
+}
+
+String findCommit({
+  required String primaryRepoDirectory,
+  required String primaryBranch,
+  required String primaryTrunk,
+  required String secondaryRepoDirectory,
+  required String secondaryBranch,
+}) {
+  final Commit anchor;
+  if (primaryBranch == primaryTrunk) {
+    log('on $primaryTrunk, using last commit as anchor');
+    anchor = Commit.parse(git(primaryRepoDirectory, <String>['log', Commit.formatArgument, '--max-count=1', primaryBranch]));
+  } else {
+    final List<Commit> branchCommits = Commit.parseList(git(primaryRepoDirectory, <String>['log', Commit.formatArgument, primaryBranch]));
+    final List<Commit> trunkCommits = Commit.parseList(git(primaryRepoDirectory, <String>['log', Commit.formatArgument, primaryTrunk]));
+    if (branchCommits.isEmpty || trunkCommits.isEmpty || branchCommits.first.hash != trunkCommits.first.hash)
+      throw StateError('Branch $primaryBranch does not seem to have a common history with trunk $primaryTrunk.');
+    if (branchCommits.last.hash == trunkCommits.last.hash) {
+      log('$primaryBranch is even with $primaryTrunk, using last commit as anchor');
+      anchor = trunkCommits.last;
+    } else {
+      int index = 0;
+      while (branchCommits.length > index && trunkCommits.length > index && branchCommits[index].hash == trunkCommits[index].hash)
+        index += 1;
+      log('$primaryBranch branched from $primaryTrunk ${branchCommits.length - index} commits ago, trunk has advanced by ${trunkCommits.length - index} commits since then.');
+      anchor = trunkCommits[index - 1];
+    }
+  }
+  return git(secondaryRepoDirectory, <String>[
+    'log',
+    '--format=%H',
+    '--until=${anchor.timestamp.toIso8601String()}',
+    '--max-count=1',
+    secondaryBranch,
+  ]);
+}
+
+String git(String workingDirectory, List<String> arguments) {
+  final ProcessResult result = Process.runSync('git', arguments, workingDirectory: workingDirectory);
+  if (result.exitCode != 0 || '${result.stderr}'.isNotEmpty)
+    throw ProcessException('git', arguments, '${result.stdout}${result.stderr}', result.exitCode);
+  return '${result.stdout}';
+}
+
+void main(List<String> arguments) {
+  if (arguments.isEmpty || arguments.length > 2 || arguments.contains('--help') || arguments.contains('-h')) {
+    print(
+      'Usage: dart find_commit.dart [<path-to-primary-repo>] <path-to-secondary-repo>\n'
+      'This script will find the commit in the secondary repo that was contemporary\n'
+      'when the commit in the primary repo was created. If that commit is on a\n'
+      "branch, then the date of the branch's creation is used instead.\n"
+      'If <path-to-primary-repo> is omitted, the current directory is used for the\n'
+      'primary repo.'
+    );
+  } else {
+    final String primaryRepo = arguments.length == 1 ? '.' : arguments.first;
+    final String secondaryRepo = arguments.last;
+    print(findCommit(
+      primaryRepoDirectory: primaryRepo,
+      primaryBranch: git(primaryRepo, <String>['rev-parse', '--abbrev-ref', 'HEAD']).trim(),
+      primaryTrunk: 'master',
+      secondaryRepoDirectory: secondaryRepo,
+      secondaryBranch: 'master',
+    ).trim());
+  }
+}