Limit the number of outstanding file reads in the VM loader

runtime/bin/vmservice/loader.dart

 // Copyright (c) 2015, 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.
 
 part of vmservice_io;
 
 _sanitizeWindowsPath(path) {
   // For Windows we need to massage the paths a bit according to
   // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
   //
@@ skipping 35 matching lines @@
 }
 
 // Ensure we have a trailing slash character.
 _enforceTrailingSlash(uri) {
   if (!uri.endsWith('/')) {
     return '$uri/';
   }
   return uri;
 }
 
+class FileRequest {
+  final SendPort sp;
+  final int tag;
+  final Uri uri;
+  final Uri resolvedUri;
+  final String libraryUrl;
+  FileRequest(this.sp, this.tag, this.uri, this.resolvedUri, this.libraryUrl);
+}
+
 // State associated with the isolate that is used for loading.
 class IsolateLoaderState extends IsolateEmbedderData {
   IsolateLoaderState(this.isolateId);
 
   final int isolateId;
 
   SendPort sp;
 
   void init(String packageRootFlag,
             String packagesConfigFlag,
             String workingDirectory,
             String rootScript) {
     // _workingDirectory must be set first.
     _workingDirectory = new Uri.directory(workingDirectory);
     if (rootScript != null) {
       _rootScript = Uri.parse(rootScript);
     }
     // If the --package-root flag was passed.
     if (packageRootFlag != null) {
       _setPackageRoot(packageRootFlag);
     }
     // If the --packages flag was passed.
     if (packagesConfigFlag != null) {
       _setPackagesConfig(packagesConfigFlag);
     }
+    _fileRequestQueue = new List<FileRequest>();
   }
 
   void cleanup() {
     if (_packagesPort != null) {
       _packagesPort.close();
       _packagesPort = null;
     }
   }
 
   // The working directory when the embedder started.
@@ skipping 16 matching lines @@
   String _packageError = null;
 
   // The directory to look in to resolve "package:" scheme URIs. By default it
   // is the 'packages' directory right next to the script.
   Uri _packageRoot = null;
 
   // The map describing how certain package names are mapped to Uris.
   Uri _packageConfig = null;
   Map<String, Uri> _packageMap = null;
 
+  // We issue only 16 concurrent calls to File.readAsBytes() to stay within
+  // platform-specific resource limits (e.g. max open files). The rest go on
+  // _fileRequestQueue and are processed when we can safely issue them.
+  static const int _maxFileRequests = 16;
+  int currentFileRequests = 0;
+  List<FileRequest> _fileRequestQueue;
+
+  bool get shouldIssueFileRequest => currentFileRequests < _maxFileRequests;
+  void enqueueFileRequest(FileRequest fr) {
+    _fileRequestQueue.add(fr);
+  }
+  FileRequest dequeueFileRequest() {
+    if (_fileRequestQueue.length == 0) {
+      return null;
+    }
+    return _fileRequestQueue.removeAt(0);
+  }
+
   _setPackageRoot(String packageRoot) {
     packageRoot = _sanitizeWindowsPath(packageRoot);
     if (packageRoot.startsWith('file:') ||
         packageRoot.startsWith('http:') ||
         packageRoot.startsWith('https:')) {
       packageRoot = _enforceTrailingSlash(packageRoot);
       _packageRoot = _workingDirectory.resolve(packageRoot);
     } else {
       packageRoot = _sanitizeWindowsPath(packageRoot);
       packageRoot = _trimWindowsPath(packageRoot);
@@ skipping 265 matching lines @@
     })
     .catchError((e) {
       _sendResourceResponse(
         sp, tag, uri, resolvedUri, libraryUrl, e.toString());
     });
   // It's just here to push an event on the event loop so that we invoke the
   // scheduled microtasks.
   Timer.run(() {});
 }
 
-void _loadFile(SendPort sp,
+void _loadFile(IsolateLoaderState loaderState,
+               SendPort sp,
                int tag,
                Uri uri,
                Uri resolvedUri,
                String libraryUrl) {
   var path = resolvedUri.toFilePath();
   var sourceFile = new File(path);
   sourceFile.readAsBytes().then((data) {
     _sendResourceResponse(sp, tag, uri, resolvedUri, libraryUrl, data);
   },
   onError: (e) {
     _sendResourceResponse(sp, tag, uri, resolvedUri, libraryUrl, e.toString());
+  }).whenComplete(() {
+    loaderState.currentFileRequests--;
+    while (loaderState.shouldIssueFileRequest) {
+      FileRequest fr = loaderState.dequeueFileRequest();
+      if (fr == null) {
+        break;
+      }
+      _loadFile(
+          loaderState, fr.sp, fr.tag, fr.uri, fr.resolvedUri, fr.libraryUrl);
+      loaderState.currentFileRequests++;
+    }
   });
 }
 
 void _loadDataUri(SendPort sp,
                   int tag,
                   Uri uri,
                   Uri resolvedUri,
                   String libraryUrl) {
   try {
     var mime = uri.data.mimeType;
@@ skipping 76 matching lines @@
 // TODO(johnmccutchan): This and most other top level functions in this file
 // should be turned into methods on the IsolateLoaderState class.
 _handleResourceRequest(IsolateLoaderState loaderState,
                        SendPort sp,
                        bool traceLoading,
                        int tag,
                        Uri uri,
                        Uri resolvedUri,
                        String libraryUrl) {
   if (resolvedUri.scheme == '' || resolvedUri.scheme == 'file') {
-    _loadFile(sp, tag, uri, resolvedUri, libraryUrl);
+    if (loaderState.shouldIssueFileRequest) {
+      _loadFile(loaderState, sp, tag, uri, resolvedUri, libraryUrl);
+      loaderState.currentFileRequests++;
+    } else {
+      FileRequest fr = new FileRequest(sp, tag, uri, resolvedUri, libraryUrl);
+      loaderState.enqueueFileRequest(fr);
+    }
   } else if ((resolvedUri.scheme == 'http') ||
              (resolvedUri.scheme == 'https')) {
     _loadHttp(sp, tag, uri, resolvedUri, libraryUrl);
   } else if ((resolvedUri.scheme == 'data')) {
     _loadDataUri(sp, tag, uri, resolvedUri, libraryUrl);
   } else if ((resolvedUri.scheme == 'package')) {
     _loadPackage(loaderState,
                  sp,
                  traceLoading,
                  tag,
@@ skipping 545 matching lines @@
             _log('Unknown scheme (${pathUri.scheme}) in $pathUri.');
           }
           _sendExtensionImportResponse(sp, uri, libraryUri, null);
         break;
       }
     break;
     default:
       _log('Unknown loader request tag=$tag from $isolateId');
   }
 }