feat(cli): Add support for auxiliary commands to celest start (#255)

When `--` is passed to `celest start`, run the following command after starting the local environment. For example, you can do
`celest start -- flutter test -d macos` to run tests which depend on Celest running.

Author: dnys1

apps/cli/lib/src/commands/start_command.dart

@@ -3,6 +3,7 @@ import 'package:celest_cli/src/commands/project_init.dart';
 import 'package:celest_cli/src/commands/project_migrate.dart';
 import 'package:celest_cli/src/context.dart';
 import 'package:celest_cli/src/frontend/celest_frontend.dart';
+import 'package:celest_cli/src/frontend/child_process.dart';
 import 'package:mason_logger/mason_logger.dart';
 
 final class StartCommand extends CelestCommand
@@ -30,10 +31,16 @@ final class StartCommand extends CelestCommand
 
     currentProgress ??= cliLogger.progress('Starting local environment');
 
+    ChildProcess? childProcess;
+    if (argResults!.rest case final command when command.isNotEmpty) {
+      childProcess = ChildProcess(command);
+    }
+
     // Start the Celest Frontend Loop
     return CelestFrontend().run(
       migrateProject: needsMigration,
       currentProgress: currentProgress,
+      childProcess: childProcess,
     );
   }
 }

apps/cli/lib/src/frontend/celest_frontend.dart

@@ -17,6 +17,7 @@ import 'package:celest_cli/src/compiler/api/local_api_runner.dart';
 import 'package:celest_cli/src/context.dart';
 import 'package:celest_cli/src/env/config_value_solver.dart';
 import 'package:celest_cli/src/exceptions.dart';
+import 'package:celest_cli/src/frontend/child_process.dart';
 import 'package:celest_cli/src/project/celest_project.dart';
 import 'package:celest_cli/src/project/project_resolver.dart';
 import 'package:celest_cli/src/utils/json.dart';
@@ -242,6 +243,7 @@ final class CelestFrontend {
   Future<int> run({
     required bool migrateProject,
     required Progress? currentProgress,
+    ChildProcess? childProcess,
   }) async {
     try {
       while (!stopped) {
@@ -352,9 +354,26 @@ final class CelestFrontend {
             }
 
             currentProgress = null;
+
+            if (childProcess case final childProcess?
+                when !childProcess.isStarted) {
+              logger.info('Running command: ${childProcess.command.join(' ')}');
+              await childProcess.start();
+              unawaited(
+                _stopSignal.future.then(childProcess.stop),
+              );
+            }
         }
 
-        await _nextChangeSet();
+        // Wait for the next changeset or for the child process to exit, if
+        // there is one.
+        final exitCode = await Future.any([
+          _nextChangeSet().then((_) => null),
+          Future.value(childProcess?.exitCode),
+        ]);
+        if (exitCode != null) {
+          return exitCode;
+        }
       }
       return 0;
     } on CancellationException {

apps/cli/lib/src/frontend/child_process.dart

@@ -0,0 +1,57 @@
+import 'dart:io';
+
+import 'package:celest_cli/src/context.dart';
+import 'package:celest_cli/src/utils/cli.dart';
+
+/// A process which is a child of a command like `celest start`.
+///
+/// The parent command is responsible for managing the lifecycle of the child
+/// process via this class.
+final class ChildProcess {
+  ChildProcess(this.command);
+
+  /// The command to run as a child process.
+  final List<String> command;
+  Process? _process;
+
+  /// Whether [start] has been called.
+  bool get isStarted => _process != null;
+
+  /// The process's exit code.
+  Future<int> get exitCode {
+    if (_process case final process?) {
+      return process.exitCode;
+    }
+    throw StateError('Must call start first');
+  }
+
+  /// Starts the process and streams stdout/stderr to the console.
+  Future<void> start({
+    Map<String, String>? environment,
+  }) async {
+    if (_process != null) {
+      return;
+    }
+    final process = _process = await processManager.start(
+      command,
+      environment: environment,
+    );
+
+    // Capture stdout/stderr
+    final commandName = command.first;
+    process.captureStdout(prefix: '[$commandName] ');
+    process.captureStderr(prefix: '[$commandName] ');
+
+    // TODO(dnys1): Handle stdin?
+  }
+
+  /// Kills the process with the given [signal] and waits for the process to
+  /// exit.
+  Future<void> stop([ProcessSignal signal = ProcessSignal.sigterm]) async {
+    if (_process case final process?) {
+      process.kill(signal);
+      await process.exitCode;
+    }
+    _process = null;
+  }
+}