Experiment with a files-changed.json per PR (#161788)

This is an experiment to determine what the wall time of a (minimal)
"upload a JSON file describing files changed".

We are looking for this to take (low) single digit minutes, because if
we use this file (and expand it to become a build plan), it will block
almost _all_ other actions, and will likely get longer as we add more
complex logic and checks. Here's hoping!

.github/workflows/files-changed.yml

@@ -0,0 +1,37 @@
+name: Generate Changed Files JSON
+
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+
+jobs:
+  generate-json:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Fetch base commit and origin/master
+        # Fetch what to compare the commit against
+        run: |
+          git fetch --no-tags --prune --depth=1 origin ${{ github.event.pull_request.base.sha }}
+          git fetch --no-tags --prune --depth=1 origin master
+          echo "FLUTTER_ENGINE_VERSION=${{ github.event.pull_request.base.sha }}" >> "$GITHUB_ENV"
+
+      - name: Initialize Dart SDK
+        # This downloads the version of the Dart SDK for the current platform.
+        run: |
+          ./bin/dart --version
+          cd dev/tools && ../../bin/dart pub get
+
+      - name: Write changed files to a JSON file
+        run: |
+          ./bin/dart run dev/tools/bin/get_files_changed.dart --since="${{ github.event.pull_request.base.sha }}" > changed_files.json
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: changed-files
+          path: changed_files.json

bin/internal/update_engine_version.sh

@@ -14,10 +14,24 @@
 
 set -e
 
+# Allow overriding the intended engine version via FLUTTER_ENGINE_VERSION.
+#
+# This is for systems, such as Github Actions, where we know ahead of time the
+# base-ref we want to use (to download the engine binaries and avoid trying
+# to compute one below).
+#
+# This environment variable is EXPERIMENTAL. If you are not on the Flutter infra
+# team, this code path might be removed at anytime and cease functioning. Please
+# file an issue if you have workflow needs.
+if [ -n "${FLUTTER_ENGINE_VERSION}" ]; then
+  ENGINE_VERSION="${FLUTTER_ENGINE_VERSION}"
+  echo "[Unstable] Override: Setting engine SHA to $ENGINE_VERSION" 1>&2
+fi
+
 FLUTTER_ROOT="$(dirname "$(dirname "$(dirname "${BASH_SOURCE[0]}")")")"
 
-# Test for fusion repository
-if [ -f "$FLUTTER_ROOT/DEPS" ] && [ -f "$FLUTTER_ROOT/engine/src/.gn" ]; then
+# Test for fusion repository and no environment variable override.
+if [ -z "$ENGINE_VERSION" ] && [ -f "$FLUTTER_ROOT/DEPS" ] && [ -f "$FLUTTER_ROOT/engine/src/.gn" ]; then
   BRANCH=$(git -C "$FLUTTER_ROOT" rev-parse --abbrev-ref HEAD)
   # In a fusion repository; the engine.version comes from the git hashes.
   if [ -z "${LUCI_CONTEXT}" ]; then
@@ -35,8 +49,9 @@ if [ -f "$FLUTTER_ROOT/DEPS" ] && [ -f "$FLUTTER_ROOT/engine/src/.gn" ]; then
   else
     ENGINE_VERSION=$(git -C "$FLUTTER_ROOT" rev-parse HEAD)
   fi
+fi
 
-  if [[ "$BRANCH" != "stable" && "$BRANCH" != "beta" ]]; then
+if [[ "$BRANCH" != "stable" && "$BRANCH" != "beta" ]]; then
   # Write the engine version out so downstream tools know what to look for.
   echo $ENGINE_VERSION > "$FLUTTER_ROOT/bin/internal/engine.version"
 
@@ -44,5 +59,4 @@ if [ -f "$FLUTTER_ROOT/DEPS" ] && [ -f "$FLUTTER_ROOT/engine/src/.gn" ]; then
   if [ -n "${FLUTTER_REALM}" ]; then
     echo $FLUTTER_REALM > "$FLUTTER_ROOT/bin/internal/engine.realm"
   fi
-  fi
 fi

dev/tools/bin/GITHUB_ACTIONS.md

@@ -0,0 +1 @@
+This is a stub-file that can be deleted.

dev/tools/bin/get_files_changed.dart

@@ -0,0 +1,57 @@
+// 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.
+
+import 'dart:convert';
+import 'dart:io' as io;
+
+import 'package:args/args.dart';
+
+final ArgParser _argParser =
+    ArgParser()
+      ..addOption(
+        'since',
+        help: 'What previous SHA to compare the current git state to.',
+        defaultsTo: 'HEAD^',
+      )
+      ..addOption(
+        'output',
+        help: 'What format to output in.',
+        defaultsTo: io.stdout.hasTerminal ? 'text' : 'json',
+        allowed: <String>['text', 'json'],
+      );
+
+void main(List<String> args) async {
+  final ArgResults argResults = _argParser.parse(args);
+
+  // Get a list of files changed between this commit and the base SHA.
+  final List<String> filesChanged;
+  {
+    final List<String> args = <String>[
+      'diff',
+      '--name-only',
+      '--full-index',
+      argResults.option('since')!,
+    ];
+    final io.ProcessResult git = await io.Process.run('git', args);
+    if (git.exitCode != 0) {
+      io.stderr.writeln('$args failed (exit code: ${git.exitCode}):');
+      io.stderr.writeln(git.stdout);
+      io.stderr.writeln(git.stderr);
+      io.exitCode = 1;
+      return;
+    }
+
+    final String stdout = git.stdout as String;
+    filesChanged = const LineSplitter().convert(stdout);
+  }
+
+  // Output to stdout.
+  final _Output output = _Output.values.byName(argResults.option('output')!);
+  io.stdout.writeln(switch (output) {
+    _Output.text => filesChanged.join('\n'),
+    _Output.json => jsonEncode(filesChanged),
+  });
+}
+
+enum _Output { text, json }