add periodic delay in analysis to reduce request processing latency

pkg/analysis_server/lib/src/analysis_server.dart

 // Copyright (c) 2014, 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.
 
 library analysis.server;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/analysis_logger.dart';
@@ skipping 51 matching lines @@
  * [CommunicationChannel] for analysis requests and process them.
  */
 class AnalysisServer {
   /**
    * The version of the analysis server. The value should be replaced
    * automatically during the build.
    */
   static final String VERSION = '0.0.1';
 
   /**
+   * The number of milliseconds to perform operations before inserting
+   * a 1 millisecond delay so that the VM and dart:io can deliver content
+   * to stdin. This should be removed once the underlying problem is fixed.
+   */
+  static int performOperationDelayFreqency = 25;
+
+  /**
    * The channel from which requests are received and to which responses should
    * be sent.
    */
   final ServerCommunicationChannel channel;
 
   /**
    * The [ResourceProvider] using which paths are converted into [Resource]s.
    */
   final ResourceProvider resourceProvider;
 
@@ skipping 102 matching lines @@
    */
   StreamController<PriorityChangeEvent> _onPriorityChangeController;
 
   /**
    * True if any exceptions thrown by analysis should be propagated up the call
    * stack.
    */
   bool rethrowExceptions;
 
   /**
+   * The next time (milliseconds since epoch) after which the analysis server
+   * should pause so that pending requests can be fetched by the system.
+   */
+  // Add 1 sec to prevent delay from impacting short running tests
+  int _nextPerformOperationDelayTime =
+      new DateTime.now().millisecondsSinceEpoch +
+      1000;
+
+  /**
    * Initialize a newly created server to receive requests from and send
    * responses to the given [channel].
    *
    * If [rethrowExceptions] is true, then any exceptions thrown by analysis are
    * propagated up the call stack.  The default is true to allow analysis
    * exceptions to show up in unit tests, but it should be set to false when
    * running a full analysis server.
    */
   AnalysisServer(this.channel, this.resourceProvider,
       PackageMapProvider packageMapProvider, this.index,
@@ skipping 722 matching lines @@
     optionUpdaters.forEach((OptionUpdater optionUpdater) {
       optionUpdater(options);
     });
   }
 
   /**
    * Schedules [performOperation] exection.
    */
   void _schedulePerformOperation() {
     assert(!performOperationPending);
-    new Future(performOperation);
+    /*
+     * TODO (danrubel) Rip out this workaround once the underlying problem
+     * is fixed. Currently, the VM and dart:io do not deliver content
+     * on stdin in a timely manner if the event loop is busy.
+     * To work around this problem, we delay for 1 millisecond
+     * every 25 milliseconds.
+     *
+     * To disable this workaround and see the underlying problem,
+     * set performOperationDelayFreqency to zero
+     */
+    int now = new DateTime.now().millisecondsSinceEpoch;
+    if (now > _nextPerformOperationDelayTime &&
+        performOperationDelayFreqency > 0) {
+      _nextPerformOperationDelayTime = now + performOperationDelayFreqency;
+      new Future.delayed(new Duration(milliseconds: 1), performOperation);
+    } else {
+      new Future(performOperation);
+    }
     performOperationPending = true;
   }
 }
 
 
 class AnalysisServerOptions {
   bool enableIncrementalResolutionApi = false;
   bool enableIncrementalResolutionValidation = false;
   bool noErrorNotification = false;
   String fileReadMode = 'as-is';
@@ skipping 129 matching lines @@
    * [packageUriResolver].
    */
   SourceFactory _createSourceFactory(UriResolver packageUriResolver) {
     List<UriResolver> resolvers = <UriResolver>[
         new DartUriResolver(analysisServer.defaultSdk),
         new ResourceUriResolver(resourceProvider),
         packageUriResolver];
     return new SourceFactory(resolvers);
   }
 }