Enforce single computeDelta() invocation.

pkg/front_end/test/incremental_kernel_generator_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. 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:async';
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/byte_store/byte_store.dart';
@@ skipping 42 matching lines @@
       compilerOptions.packagesFileUri = Uri.parse('file:///test/.packages');
     }
     incrementalKernelGenerator = await IncrementalKernelGenerator
         .newInstance(compilerOptions, entryPoint, watch: watchFn);
     return (await incrementalKernelGenerator.computeDelta()).newProgram;
   }
 
   test_acceptLastDelta() async {
     writeFile('/test/.packages', 'test:lib/');
     String path = '/test/lib/test.dart';
-    Uri uri = writeFile(path, 'var v = 1;');
+    Uri uri = writeFile(path, '');
 
     await getInitialState(uri);
     incrementalKernelGenerator.acceptLastDelta();
 
     // Attempt to accept the second time.
     expect(() {
       incrementalKernelGenerator.acceptLastDelta();
     }, throwsStateError);
   }
 
@@ skipping 142 matching lines @@
     expect(_getLibraryText(library), r'''library;
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
 
 static field core::int a = 1;
 static field asy::Future<core::String> b;
 ''');
   }
 
+  test_computeDelta_hasAnotherRunning() async {
+    writeFile('/test/.packages', 'test:lib/');
+    String path = '/test/lib/test.dart';
+    Uri uri = writeFile(path, '');
+
+    await getInitialState(uri);
+    incrementalKernelGenerator.acceptLastDelta();
+
+    // Run, but don't wait.
+    incrementalKernelGenerator.computeDelta();

[Siggi Cherem (dart-lang), 2017/08/10 22:43:29]

we might want to store the future returned by this method and await for it at
the end of this test, so we don't have a lingering work happening after the test
is done.

[scheglov, 2017/08/10 23:46:49]

Done.

+
+    // Run another, this causes StateError.
+    try {
+      await incrementalKernelGenerator.computeDelta();

[Siggi Cherem (dart-lang), 2017/08/10 22:43:29]

Could this be flaky?

In particular, is it at all possible for the first computeDelta to complete by
the time we reach this point? Or are we guaranteed that we have at least one
microtask pending (so it is not possible)? Right now it wouldn't be visible on
this test because if it completes early we would still throw because we haven't
`accepted` the delta, but if we address the comment below, this might be more
relevant.

[scheglov, 2017/08/10 23:46:49]

I changed computeDelta() to check _isComputeDeltaExecuting synchronously and set
it to false asynchronously, before returning the result.

+      fail('StateError expected.');
+    } on StateError {}

[Siggi Cherem (dart-lang), 2017/08/10 22:43:29]

there are two reasons why computeDelta could throw a state error, let's check
that the error you are getting is the one about computeDelta is executing

[scheglov, 2017/08/10 23:46:50]

Well, there is no great way to do this.
I had to use contain(string).

+  }
+
   test_inferPackagesFile() async {
     writeFile('/test/.packages', 'test:lib/');
     String aPath = '/test/lib/a.dart';
     String bPath = '/test/lib/b.dart';
     writeFile(aPath, 'var a = 1;');
     Uri bUri = writeFile(bPath, r'''
 import "package:test/a.dart";
 var b = a;
 ''');
 
@@ skipping 237 matching lines @@
     throw fail('No library found with URI "$uri"');
   }
 
   String _getLibraryText(Library library) {
     StringBuffer buffer = new StringBuffer();
     new Printer(buffer, syntheticNames: new NameSystem())
         .writeLibraryFile(library);
     return buffer.toString();
   }
 }