flutter · matanlurey · Jul 15, 2025 · Jul 15, 2025 · jtmcdole · Jul 15, 2025
diff --git a/bin/internal/last_engine_commit.ps1 b/bin/internal/last_engine_commit.ps1
@@ -24,5 +24,48 @@ $ErrorActionPreference = "Stop"
 
 $progName = Split-Path -parent $MyInvocation.MyCommand.Definition
 $flutterRoot = (Get-Item $progName).parent.parent.FullName
+$gitToplevel = (git rev-parse --show-toplevel).Trim()
+# 1. Determine when we diverged from master.
+$MERGE_BASE_COMMIT = ""
+try {
+    $MERGE_BASE_COMMIT = (git merge-base HEAD master).Trim()
+}
+catch {
+    # If git merge-base fails (e.g., master not found, no common history),
+    # $MERGE_BASE_COMMIT will remain empty.
+}
 
-cmd /c "git log -1 --pretty=format:%H -- ""$(git rev-parse --show-toplevel)/DEPS"" ""$(git rev-parse --show-toplevel)/engine"""
+# If we did not find a merge-base, fail
+if ([string]::IsNullOrEmpty($MERGE_BASE_COMMIT)) {
+    Write-Error "Error: Could not determine a suitable engine commit." -ErrorAction Stop
+    Write-Error "Current branch: $(git rev-parse --abbrev-ref HEAD).Trim()" -ErrorAction Stop
+    Write-Error "Expected a different branch, from 'master', or a 'master' branch that exists and has history." -ErrorAction Stop
+    exit 1
+}
+
+# 2. Define and search history range to search within (unique to changes on this branch).
+$HISTORY_RANGE = "$MERGE_BASE_COMMIT..HEAD"
+$DEPS_PATH = Join-Path $gitToplevel "DEPS"
+$ENGINE_PATH = Join-Path $gitToplevel "engine"
+
+$ENGINE_COMMIT = (git log -1 --pretty=format:%H --ancestry-path $HISTORY_RANGE -- "$DEPS_PATH" "$ENGINE_PATH")
+
+# 3. If no engine-related commit was found within the current branch's history, fallback to the first commit on this branch.
+if ([string]::IsNullOrEmpty($ENGINE_COMMIT)) {
+    # Find the oldest commit on HEAD that is *not* reachable from MERGE_BASE_COMMIT.
+    # This is the first commit *on this branch* after it diverged from 'master'.
+    $ENGINE_COMMIT = (git log --pretty=format:%H --reverse --ancestry-path "$MERGE_BASE_COMMIT..HEAD" | Select-Object -First 1).Trim()
+
+    # Final check: If even this fallback fails (which would be highly unusual if MERGE_BASE_COMMIT was found),
+    # then something is truly wrong.
+    if ([string]::IsNullOrEmpty($ENGINE_COMMIT)) {
+        Write-Error "Error: Unexpected state. MERGE_BASE_COMMIT was found ($MERGE_BASE_COMMIT), but no commits found on current branch after it." -ErrorAction Stop
+        Write-Error "Current branch: $((git rev-parse --abbrev-ref HEAD).Trim())" -ErrorAction Stop
+        Write-Error "History range searched for fallback: $HISTORY_RANGE" -ErrorAction Stop
+        Write-Error "All commits on current branch (for debug):" -ErrorAction Stop
+        (git log --pretty=format:%H) | Write-Error -ErrorAction Stop
+        exit 1
+    }
+}
+
+Write-Output $ENGINE_COMMIT
diff --git a/bin/internal/last_engine_commit.sh b/bin/internal/last_engine_commit.sh
@@ -26,4 +26,37 @@ set -e
 
 FLUTTER_ROOT="$(dirname "$(dirname "$(dirname "${BASH_SOURCE[0]}")")")"
 
-git log -1 --pretty=format:%H -- "$(git rev-parse --show-toplevel)/DEPS" "$(git rev-parse --show-toplevel)/engine"
+# 1. Determine when we diverged from master, and prevent set -e from exiting.
+MERGE_BASE_COMMIT="$(git merge-base HEAD master || echo "")"
+
+# If we did not find a merge-base, fail
+if [[ -z "$MERGE_BASE_COMMIT" ]]; then
+  echo >&2 "Error: Could not determine a suitable engine commit."
+  echo >&2 "Current branch: $(git rev-parse --abbrev-ref HEAD)"
+  echo >&2 "Expected a different branch, from master"
+  exit 1
+fi
+
+# 2. Define and search history range to searhc within (unique to changes on this branch).
+HISTORY_RANGE="$MERGE_BASE_COMMIT..HEAD"
+ENGINE_COMMIT="$(git log -1 --pretty=format:%H --ancestry-path "$HISTORY_RANGE" -- "$(git rev-parse --show-toplevel)/DEPS" "$(git rev-parse --show-toplevel)/engine")"
+
+# 3. If no engine-related commit was found within the current branch's history, fallback to the first commit on this branch.
+if [[ -z "$ENGINE_COMMIT" ]]; then
+  # Find the oldest commit on HEAD that is *not* reachable from MERGE_BASE_COMMIT.
+  # This is the first commit *on this branch* after it diverged from 'master'.
+  ENGINE_COMMIT="$(git log --pretty=format:%H --reverse --ancestry-path "$MERGE_BASE_COMMIT"..HEAD | head -n 1)"
+
+  # Final check: If even this fallback fails (which would be highly unusual if MERGE_BASE_COMMIT was found),
+  # then something is truly wrong.
+  if [[ -z "$ENGINE_COMMIT" ]]; then
+    echo >&2 "Error: Unexpected state. MERGE_BASE_COMMIT was found ($MERGE_BASE_COMMIT), but no commits found on current branch after it."
+    echo >&2 "Current branch: $(git rev-parse --abbrev-ref HEAD)"
+    echo >&2 "History range searched for fallback: $HISTORY_RANGE"
+    echo >&2 "All commits on current branch (for debug):"
+    git log --pretty=format:%H
+    exit 1
+  fi
+fi
+
+echo "$ENGINE_COMMIT"
diff --git a/dev/tools/test/last_engine_commit_test.dart b/dev/tools/test/last_engine_commit_test.dart
@@ -53,7 +53,6 @@ void main() {
   }
 
   io.ProcessResult run(String executable, List<String> args) {
-    print('Running "$executable ${args.join(" ")}');
     final io.ProcessResult result = io.Process.runSync(
       executable,
       args,
@@ -76,7 +75,9 @@ void main() {
   setUpAll(() async {
     if (usePowershellOnPosix) {
       final io.ProcessResult result = io.Process.runSync('pwsh', <String>['--version']);
-      print('Using Powershell (${result.stdout}) on POSIX for local debugging and testing');
+      print(
+        'Using Powershell (${(result.stdout as String).trim()}) on POSIX for local debugging and testing',
+      );
     }
   });
 
@@ -104,7 +105,7 @@ void main() {
 
     if (const LocalPlatform().isWindows || usePowershellOnPosix) {
       // Copy a minimal set of environment variables needed to run the update_engine_version script in PowerShell.
-      const List<String> powerShellVariables = <String>['SystemRoot', 'Path', 'PATHEXT'];
+      const List<String> powerShellVariables = <String>['SYSTEMROOT', 'PATH', 'PATHEXT'];
       for (final String key in powerShellVariables) {
         final String? value = io.Platform.environment[key];
         if (value != null) {
@@ -143,7 +144,12 @@ void main() {
       executable = testRoot.binInternalLastEngineCommit.path;
       args = <String>[];
     }
-    return run(executable, args).stdout as String;
+    return (run(executable, args).stdout as String).trim();
+  }
+
+  /// Gets the latest commit on the current branch.
+  String getLastCommit() {
+    return (run('git', <String>['rev-parse', 'HEAD']).stdout as String).trim();
   }
 
   void writeCommit(Iterable<String> files) {
@@ -157,7 +163,12 @@ void main() {
     run('git', <String>['commit', '-m', 'Wrote ${files.length} files']);
   }
 
+  void changeBranch({required String newBranchName}) {
+    run('git', <String>['checkout', '-b', newBranchName]);
+  }
+
   test('returns the last engine commit', () {
+    changeBranch(newBranchName: 'flutter-1.2.3-candidate.0');
     writeCommit(<String>['DEPS', 'engine/README.md']);
 
     final String lastEngine = getLastEngineCommit();
@@ -168,6 +179,7 @@ void main() {
   });
 
   test('considers DEPS an engine change', () {
+    changeBranch(newBranchName: 'flutter-1.2.3-candidate.0');
     writeCommit(<String>['DEPS', 'engine/README.md']);
 
     final String lastEngineA = getLastEngineCommit();
@@ -177,6 +189,40 @@ void main() {
     final String lastEngineB = getLastEngineCommit();
     expect(lastEngineB, allOf(isNotEmpty, isNot(equals(lastEngineA))));
   });
+
+  test('if there have been no engine changes, uses the first commit since the branch point', () {
+    final String initialStartingCommit = getLastCommit();
+
+    // Make an engine change *before* the branch.
+    writeCommit(<String>['engine/README.md']);
+    final String engineCommitPreBranch = getLastCommit();
+    changeBranch(newBranchName: 'flutter-1.2.3-candidate.0');
+
+    // Make a non-engine change, but no engine changes, after branching.
+    writeCommit(<String>['bin/internal/release-candidate-branch.version']);
+    final String initialBranchCommit = getLastCommit();
+
+    // Write another commit to make sure we don't always use the latest.
+    writeCommit(<String>['CHANGELOG.md']);
+    final String latestCommitIgnore = getLastCommit();
+
+    // Get the engine commit, which should fallback to HEAD~2 (in this case).
+    final String lastCommitToEngine = getLastEngineCommit();
+    expect(
+      lastCommitToEngine,
+      initialBranchCommit,
+      reason:
+          'The git history for this simulation looks like this:\n'
+          'master (intial commit)    | $initialStartingCommit\n'
+          'master (touches engine)   | $engineCommitPreBranch\n'
+          'flutter-1.2.3-candidate.0 | $initialBranchCommit\n'
+          'flutter-1.2.3-candidate.0 | $latestCommitIgnore\n'
+          '\n'
+          'We expected our script to select HEAD~2, $initialBranchCommit, but '
+          'instead it selected $lastCommitToEngine, which is incorrect. See '
+          'the table above to help debug.',
+    );
+  });
 }
 
 extension on File {