report invalid param if offset is out of bounds

pkg/analysis_server/lib/src/domain_completion.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 domain.completion;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/analysis_server.dart';
@@ skipping 163 matching lines @@
         performance.complete('priorityChanged caching: $success');
       });
     }
   }
 
   /**
    * Process a `completion.getSuggestions` request.
    */
   Response processRequest(Request request, [CompletionManager manager]) {
     performance = new CompletionPerformance();
-    // extract params
+
+    // extract and validate params
     CompletionGetSuggestionsParams params =
         new CompletionGetSuggestionsParams.fromRequest(request);
-    // schedule completion analysis
-    String completionId = (_nextCompletionId++).toString();
     ContextSourcePair contextSource = server.getContextSourcePair(params.file);
     AnalysisContext context = contextSource.context;
     Source source = contextSource.source;
     if (context == null || !context.exists(source)) {
       return new Response.unknownSource(request);
     }
+    TimestampedData<String> contents = context.getContents(source);
+    if (params.offset < 0 || params.offset > contents.data.length) {
+      return new Response.invalidParameter(
+          request,
+          'params.offset',
+          'Expected offset between 0 and source length inclusive,'
+          ' but found ${params.offset}');
+    }
+
+    // schedule completion analysis
     recordRequest(performance, context, source, params.offset);
     if (manager == null) {
       manager = completionManagerFor(context, source);
     }
     CompletionRequest completionRequest = new CompletionRequestImpl(context,
         server.resourceProvider, server.searchEngine, source, params.offset);
     int notificationCount = 0;
+    String completionId = (_nextCompletionId++).toString();
     manager.results(completionRequest).listen((CompletionResult result) {
       ++notificationCount;
       bool isLast = result is CompletionResultImpl ? result.isLast : true;
       performance.logElapseTime("notification $notificationCount send", () {
         sendCompletionNotification(completionId, result.replacementOffset,
             result.replacementLength, result.suggestions, isLast);
       });
       if (notificationCount == 1) {
         performance.logFirstNotificationComplete('notification 1 complete');
         performance.suggestionCountFirst = result.suggestions.length;
@@ skipping 74 matching lines @@
     if (_sourcesChangedSubscription != null) {
       _sourcesChangedSubscription.cancel();
       _sourcesChangedSubscription = null;
     }
     if (_manager != null) {
       _manager.dispose();
       _manager = null;
     }
   }
 }