add TODO about serializing writes

andrewkolos · andrewkolos · commit a5d97f0422f2 · 2024-07-30T13:36:41.000-07:00
diff --git a/packages/flutter_tools/lib/src/base/process.dart b/packages/flutter_tools/lib/src/base/process.dart
@@ -228,8 +228,11 @@ abstract class ProcessUtils {
 
   /// Write [line] to [stdin] and catch any errors with [onError].
   ///
-  /// Specifically with [Process] file descriptors, an exception that is
-  /// thrown as part of a write can be most reliably caught with a
+  /// Concurrent calls to this method will result in an exception due to its
+  /// dependence on [IOSink.flush] (see https://github.com/dart-lang/sdk/issues/25277).
+  ///
+  /// Context: specifically with [Process] file descriptors, an exception that
+  /// is thrown as part of a write can be most reliably caught with a
   /// [ZoneSpecification] error handler.
   ///
   /// On some platforms, the following code appears to work:
@@ -278,6 +281,9 @@ abstract class ProcessUtils {
     );
   }
 
+  /// See [writelnToStdinGuarded].
+  ///
+  /// In the event that the write or flush fails, this will throw an Exception.
   static Future<void> writelnToStdinUnsafe({
     required IOSink stdin,
     required String line,
@@ -289,6 +295,9 @@ abstract class ProcessUtils {
     );
   }
 
+  /// See [writeToStdinGuarded].
+  ///
+  /// In the event that the write or flush fails, this will throw an Exception.
   static Future<void> writeToStdinUnsafe({
     required IOSink stdin,
     required String content,
diff --git a/packages/flutter_tools/lib/src/compile.dart b/packages/flutter_tools/lib/src/compile.dart
@@ -1067,23 +1067,30 @@ class DefaultResidentCompiler implements ResidentCompiler {
     await _writelnToServerStdinAll(<String>[line], printTrace: printTrace);
   }
 
-  // TODO dont merge; explain why this is needed
-  final Pool _serverWritePool = Pool(1);
+  // TODO(andrewkolos): Concurrent calls to ProcessUtils.writelnToStdinUnsafe
+  //  against the same stdin will result in an exception. To guard against this,
+  //  we need to force calls to run serially. Ideally, this wouldn't be
+  //  necessary since we shouldn't have multiple concurrent writes to the
+  //  compiler process. However, we do (https://github.com/flutter/flutter/issues/152577).
+  final Pool _serverStdinWritePool = Pool(1);
   Future<void> _writelnToServerStdinAll(List<String> lines, {
     bool printTrace = false,
   }) async {
     final Process? server = _server;
     if (server == null) {
       return;
     }
-    final PoolResource request = await _serverWritePool.request();
-    for (final String line in lines) {
-      await ProcessUtils.writelnToStdinUnsafe(stdin: server.stdin, line: line);
-      if (printTrace) {
-        _logger.printTrace('<- $line');
+    final PoolResource request = await _serverStdinWritePool.request();
+    try {
+      for (final String line in lines) {
+        await ProcessUtils.writelnToStdinUnsafe(stdin: server.stdin, line: line);
+        if (printTrace) {
+          _logger.printTrace('<- $line');
+        }
       }
+    } finally {
+      request.release();
     }
-    request.release();
   }
 }
 
