Lazy loading of deferred libraries

runtime/vm/parser.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/parser.h"
 
 #include "lib/invocation_mirror.h"
 #include "platform/utils.h"
 #include "vm/bootstrap.h"
 #include "vm/class_finalizer.h"
@@ skipping 5145 matching lines @@
     }
     if (hide_list.Length() > 0) {
       hide_names = Array::MakeArray(hide_list);
     }
   }
   ExpectSemicolon();
 
   // Canonicalize library URL.
   const String& canon_url = String::CheckedHandle(
       CallLibraryTagHandler(Dart_kCanonicalizeUrl, import_pos, url));
-  // Lookup the library URL.
-  Library& library = Library::Handle(isolate(),
-                                     Library::LookupLibrary(canon_url));
+
+  // Create a new library if it does not exist yet.
+  Library& library =
+      Library::Handle(isolate(), Library::LookupLibrary(canon_url));
   if (library.IsNull()) {
-    // Create an empty library to mark that we have initiated loading of this
-    // library.
     library = Library::New(canon_url);
     library.Register();
-    // Call the library tag handler to load the library.
-    // TODO(hausner): do not load eagerly if import is deferred.
+  }
+
+  // If loading hasn't been requested yet, and if this is not a deferred
+  // library import, call the library tag handler to request loading
+  // the library.
+  if (library.LoadNotStarted() && !is_deferred_import) {
+    library.SetLoadRequested();
     CallLibraryTagHandler(Dart_kImportTag, import_pos, canon_url);
   }
 
   Namespace& ns = Namespace::Handle(isolate(),
       Namespace::New(library, show_names, hide_names));
   if (metadata_pos >= 0) {
     ns.AddMetadata(metadata_pos, current_class());
   }
 
   if (is_import) {
@@ skipping 16 matching lines @@
         if (!is_deferred_import && library_prefix.is_deferred_load()) {
           ErrorMsg(prefix_pos,
                    "prefix '%s' already used in a deferred import clause",
                    prefix.ToCString());
         }
         if (is_deferred_import) {
           ErrorMsg(prefix_pos, "prefix of deferred import must be uniqe");
         }
         library_prefix.AddImport(ns);
       } else {
-        library_prefix = LibraryPrefix::New(prefix, ns, is_deferred_import);
+        library_prefix =
+            LibraryPrefix::New(prefix, ns, is_deferred_import, library_);
         library_.AddObject(library_prefix, prefix);
       }
     }
   } else {
     ASSERT(is_export);
     library_.AddExport(ns);
   }
 }
 
 
@@ skipping 5856 matching lines @@
 void Parser::SkipQualIdent() {
   ASSERT(IsIdentifier());
   ConsumeToken();
   if (CurrentToken() == Token::kPERIOD) {
     ConsumeToken();  // Consume the kPERIOD token.
     ExpectIdentifier("identifier expected after '.'");
   }
 }
 
 }  // namespace dart