[frontend-server] flutter frontend try bot don't save to actual file

The test compiles a file, serializes it to disk, loads it from disk and
verifies it. There's more than 1500 compiles and they (at least some)
are up to something like 40 MB each meaning why might write something
like 60 GB (!) running this test.
As the data is only used in-process we can use `IOOverrides` to bybass
it actually saving to a file.

Change-Id: I4130d02c227ba6769d63482a499846a485aa5a3a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/322702
Commit-Queue: Jens Johansen <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: jensjoha

pkg/frontend_server/test/frontend_server_flutter.dart

@@ -2,9 +2,18 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
-import 'dart:async' show StreamController;
-import 'dart:convert' show utf8, LineSplitter;
-import 'dart:io' show Directory, File, FileSystemEntity, IOSink, exitCode;
+import 'dart:async' show StreamController, Zone;
+import 'dart:convert' show Encoding, LineSplitter, utf8;
+import 'dart:io'
+    show
+        Directory,
+        File,
+        FileMode,
+        FileSystemEntity,
+        IOOverrides,
+        IOSink,
+        exitCode;
+import 'dart:typed_data' show BytesBuilder, Uint8List;
 
 import 'package:front_end/src/api_prototype/language_version.dart'
     show uriUsesLegacyLanguageVersion;
@@ -277,8 +286,27 @@ Future<List<String>> attemptStuff(
   Iterator<File> testFileIterator = testFiles.iterator;
   testFileIterator.moveNext();
 
-  final Future<int> result =
-      starter(args, input: inputStreamController.stream, output: ioSink);
+  Zone parentZone = Zone.current;
+  Map<String, _MockFile> files = {};
+
+  /// Each test writes a complete (modulo platform) dill, often taking around
+  /// 40 MB. With 1,500+ tests thats easily 50+ GB data to write. We don't
+  /// really need it as an actual file though, we just need to get a hold on it
+  /// below so we can run verification on it. We thus use [IOOverrides] to
+  /// "catch" dill files (`new File("whatnot.dill")`) so we can write those to
+  /// memory instead of to actual files.
+  /// This is specialized for how it's actually written in the production code
+  /// (via `openWrite`) and will fail if that changes (at which point it will
+  /// have to be updated).
+  final Future<int> result = IOOverrides.runZoned(() {
+    return starter(args, input: inputStreamController.stream, output: ioSink);
+  }, createFile: (String path) {
+    if (files[path] != null) return files[path]!;
+    File f = parentZone.run(() => File(path));
+    if (path.endsWith(".dill")) return files[path] = _MockFile(f);
+    return f;
+  });
+
   String testName =
       testFileIterator.current.path.substring(flutterDirectory.path.length);
 
@@ -304,7 +332,7 @@ Future<List<String>> attemptStuff(
       logger.logUnexpectedResult(testName);
     }
     if (!error) {
-      List<int> resultBytes = dillFile.readAsBytesSync();
+      Uint8List resultBytes = files[dillFile.path]!.writeSink!.bb.takeBytes();
       Component component = loadComponentFromBytes(platformData);
       component = loadComponentFromBytes(resultBytes, component);
       verifyComponent(
@@ -318,6 +346,8 @@ Future<List<String>> attemptStuff(
       logger
           .log("        => verified in ${stopwatch2.elapsedMilliseconds} ms.");
     }
+
+    files.clear();
     stopwatch2.reset();
 
     inputStreamController.add('accept\n'.codeUnits);
@@ -483,3 +513,52 @@ class StdoutLogger extends Logger {
     print(s);
   }
 }
+
+class _MockFile implements File {
+  final File _f;
+  _MockIOSink? writeSink;
+
+  _MockFile(this._f);
+
+  @override
+  bool existsSync() {
+    return _f.existsSync();
+  }
+
+  @override
+  Uint8List readAsBytesSync() {
+    return _f.readAsBytesSync();
+  }
+
+  @override
+  IOSink openWrite({FileMode mode = FileMode.write, Encoding encoding = utf8}) {
+    return writeSink = _MockIOSink();
+  }
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) {
+    return super.noSuchMethod(invocation);
+  }
+}
+
+class _MockIOSink implements IOSink {
+  BytesBuilder bb = BytesBuilder();
+  bool _closed = false;
+
+  @override
+  void add(List<int> data) {
+    if (_closed) throw "Adding to closed";
+    bb.add(data);
+  }
+
+  @override
+  Future close() {
+    _closed = true;
+    return Future.value();
+  }
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) {
+    return super.noSuchMethod(invocation);
+  }
+}