Do not run real processes in clang_tidy_test.dart (flutter#45748)

Partial work towards flutter/flutter#133190.

Some context, from @zanderso on chat:

@matanlurey:

> For
[`clang_tidy_test.dart`](https://github.com/flutter/engine/blob/c020936303cb0646cd10593e6e427d8c5aa6ce52/tools/clang_tidy/test/clang_tidy_test.dart),
what is your vision there? I remember we ran into problems with it
actually executing `clang.run()` in too many of the test cases, but I
don't remember (or seem to have written down) what we decided to do 🙂

@zanderso:

> The existing tests are calling `computeFilesOfInterest` and
`getLintCommandsForFiles` and inspecting the results for positive tests
instead of hitting a real clang-tidy invocation. Tests of command line
flags that call `clangTidy.run()` are also testing cases that
short-circuit or fail before making a real invocation. Making it harder
to do a real invocation from a unit test probably requires plumbing a
fake-able ProcessRunner or ProcessManager object through to the
ClangTidy constructor.

---

All this PR does is allow providing a `ProcessManager` (defaults to
`LocalProcessManager`, i.e. `dart:io`) throughout the tool. Then, for
tests, I've implemented a _very_ minimal fake of both `ProcessManager`
and `Process` (it would be nice to share code w/ say, `flutter_tool`,
but lots of work/overhead for very little surface area).

After this CL, no tests in `clang_tidy_test.dart` actually run a
process. This should unblock adding new features (i.e. original intent
of flutter/flutter#133190) without causing
headaches for others/CI?

tools/clang_tidy/lib/clang_tidy.dart

@@ -7,6 +7,7 @@ import 'dart:io' as io show File, stderr, stdout;
 
 import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
+import 'package:process/process.dart';
 import 'package:process_runner/process_runner.dart';
 
 import 'src/command.dart';
@@ -43,17 +44,34 @@ class _SetStatusCommand {
 /// A class that runs clang-tidy on all or only the changed files in a git
 /// repo.
 class ClangTidy {
-  /// Given the path to the build commands for a repo and its root, builds
-  /// an instance of [ClangTidy].
+  /// Builds an instance of [ClangTidy] using a repo's [buildCommandPath].
   ///
-  /// `buildCommandsPath` is the path to the build_commands.json file.
-  /// `repoPath` is the path to the Engine repo.
-  /// `checksArg` are specific checks for clang-tidy to do.
-  /// `lintAll` when true indicates that all files should be linted.
-  /// `outSink` when provided is the destination for normal log messages, which
-  /// will otherwise go to stdout.
-  /// `errSink` when provided is the destination for error messages, which
-  /// will otherwise go to stderr.
+  /// ## Required
+  /// - [buildCommandsPath] is the path to the build_commands.json file.
+  ///
+  /// ## Optional
+  /// - [checksArg] are specific checks for clang-tidy to do.
+  ///
+  ///   If omitted, checks will be determined by the `.clang-tidy` file in the
+  ///   repo.
+  /// - [lintAll] when true indicates that all files should be linted.
+  ///
+  /// ## Optional (Test Overrides)
+  ///
+  /// _Most usages of this class will not need to override the following, which
+  /// are primarily used for testing (i.e. to avoid real interaction with I/O)._
+  ///
+  /// - [outSink] when provided is the destination for normal log messages.
+  ///
+  ///   If omitted, [io.stdout] will be used.
+  ///
+  /// - [errSink] when provided is the destination for error messages.
+  ///
+  ///   If omitted, [io.stderr] will be used.
+  ///
+  /// - [processManager] when provided is delegated to for running processes.
+  ///
+  ///   If omitted, [LocalProcessManager] will be used.
   ClangTidy({
     required io.File buildCommandsPath,
     String checksArg = '',
@@ -62,6 +80,7 @@ class ClangTidy {
     bool fix = false,
     StringSink? outSink,
     StringSink? errSink,
+    ProcessManager processManager = const LocalProcessManager(),
   }) :
     options = Options(
       buildCommandsPath: buildCommandsPath,
@@ -72,22 +91,26 @@ class ClangTidy {
       errSink: errSink,
     ),
     _outSink = outSink ?? io.stdout,
-    _errSink = errSink ?? io.stderr;
+    _errSink = errSink ?? io.stderr,
+    _processManager = processManager;
 
   /// Builds an instance of [ClangTidy] from a command line.
   ClangTidy.fromCommandLine(
     List<String> args, {
     StringSink? outSink,
     StringSink? errSink,
+    ProcessManager processManager = const LocalProcessManager(),
   }) :
     options = Options.fromCommandLine(args, errSink: errSink),
     _outSink = outSink ?? io.stdout,
-    _errSink = errSink ?? io.stderr;
+    _errSink = errSink ?? io.stderr,
+    _processManager = processManager;
 
   /// The [Options] that specify how this [ClangTidy] operates.
   final Options options;
   final StringSink _outSink;
   final StringSink _errSink;
+  final ProcessManager _processManager;
 
   late final DateTime _startTime;
 
@@ -185,6 +208,7 @@ class ClangTidy {
 
     final GitRepo repo = GitRepo(
       options.repoPath,
+      processManager: _processManager,
       verbose: options.verbose,
     );
     if (options.lintHead) {
@@ -351,7 +375,10 @@ class ClangTidy {
         totalJobs, completed, inProgress, pending, failed));
     }
 
-    final ProcessPool pool = ProcessPool(printReport: reporter);
+    final ProcessPool pool = ProcessPool(
+      printReport: reporter,
+      processRunner: ProcessRunner(processManager: _processManager),
+    );
     await for (final WorkerJob job in pool.startWorkers(jobs)) {
       pendingJobs.remove(job.name);
       if (pendingJobs.isNotEmpty && pendingJobs.length <= 3) {

tools/clang_tidy/lib/src/git_repo.dart

@@ -5,21 +5,26 @@
 import 'dart:io' as io show Directory, File;
 
 import 'package:path/path.dart' as path;
+import 'package:process/process.dart';
 import 'package:process_runner/process_runner.dart';
 
 /// Utility methods for working with a git repo.
 class GitRepo {
   /// The git repository rooted at `root`.
   GitRepo(this.root, {
     this.verbose = false,
-  });
+    ProcessManager processManager = const LocalProcessManager(),
+  }) : _processManager = processManager;
 
   /// Whether to produce verbose log output.
   final bool verbose;
 
   /// The root of the git repo.
   final io.Directory root;
 
+  /// The delegate to use for running processes.
+  final ProcessManager _processManager;
+
   List<io.File>? _changedFiles;
 
   /// Returns a list of all non-deleted files which differ from the nearest
@@ -41,6 +46,7 @@ class GitRepo {
   Future<List<io.File>> _getChangedFiles() async {
     final ProcessRunner processRunner = ProcessRunner(
       defaultWorkingDirectory: root,
+      processManager: _processManager,
     );
     await _fetch(processRunner);
     ProcessRunnerResult mergeBaseResult = await processRunner.runProcess(

tools/clang_tidy/pubspec.yaml

@@ -20,6 +20,7 @@ dependencies:
   args: any
   meta: any
   path: any
+  process: any
   process_runner: any
 
 dev_dependencies: