First step towards asynchronous loading of sources

First step towards asynchronous loading of sources. With this change, sources are read asynchronously in dart code (builtin.dart) and when the i/o future completes, a callback in the VM loads and compiles the script or library code.

In a next step, the i/o will be moved to the service isolate so that not all isolates have to import dart:io.

Fallout from this change:
1) Loading has become a tad slower. Will have to investigate why.

2) Errors that happen during loading (file not found, compilation error in an imported library)  are no longer reported at the offending import clause. This can be fixed later if necessary by passing the import clause source location to the library tag handler.

Committed: https://code.google.com/p/dart/source/detail?r=36575

[Ivan Posva, 2014-05-21 22:02:07]

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/builtin.dart
File runtime/bin/builtin.dart (right):

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/builtin.dart...
runtime/bin/builtin.dart:333: _loadScript(uri, data);//native
The comment here is unnecessary.

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/builtin.dart...
runtime/bin/builtin.dart:341: void _loadLibrarySource(tag, uri, libraryUri,
text)
I am wondering why we do not unify the loading of the script source with the
rest of the library sources and only deal with List<int> as part of this
interface. Granted that you will need to invent a special "tag" such as -1 for
the script loading, but then you do not need to duplicate the loading code.

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/builtin.dart...
runtime/bin/builtin.dart:355: _loadLibrarySource(tag, uri, libraryUri,
text);//native
ditto.

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/dartutils.cc
File runtime/bin/dartutils.cc (right):

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/dartutils.cc...
runtime/bin/dartutils.cc:674: // Callback function that gets called from
dartutils when the script
...gets called from asynchronous script loading code...

[hausner, 2014-05-21 23:53:07]

After our chat, I realized that unifying the two code paths as you suggest may
have more drawbacks than it does good. See my explanation below.

PTAL

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/builtin.dart
File runtime/bin/builtin.dart (right):

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/builtin.dart...
runtime/bin/builtin.dart:333: _loadScript(uri, data);//native
On 2014/05/21 22:02:07, Ivan Posva wrote:
> The comment here is unnecessary.

Left-overs. Removed.

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/builtin.dart...
runtime/bin/builtin.dart:341: void _loadLibrarySource(tag, uri, libraryUri,
text)
On 2014/05/21 22:02:07, Ivan Posva wrote:
> I am wondering why we do not unify the loading of the script source with the
> rest of the library sources and only deal with List<int> as part of this
> interface. Granted that you will need to invent a special "tag" such as -1 for
> the script loading, but then you do not need to duplicate the loading code.

This can be done, but on the C++ side we save nothing. The price we pay is that
for every load, we'd copy the data two more times, from a dart list to a C
buffer, and then from that C buffer into a Dart string. That would mean, we copy
source code at least 4 times:

- from read buffers into a List<int>
- sending the data from service isolate to the loading isolate
- in the C callback code from dart list to external C buffer
- in the C callback code from external buffer to Dart string

Also, loading the script source is handled slightly differently in dartutils
because the URI is resolved differently. Some global state is set when resolving
the script URI that is then used to resolve library and part uris.

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/builtin.dart...
runtime/bin/builtin.dart:355: _loadLibrarySource(tag, uri, libraryUri,
text);//native
On 2014/05/21 22:02:07, Ivan Posva wrote:
> ditto.

Done.

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/dartutils.cc
File runtime/bin/dartutils.cc (right):

https://codereview.chromium.org/290713004/diff/20001/runtime/bin/dartutils.cc...
runtime/bin/dartutils.cc:674: // Callback function that gets called from
dartutils when the script
On 2014/05/21 22:02:07, Ivan Posva wrote:
> ...gets called from asynchronous script loading code...

Done

[hausner, 2014-05-23 17:43:22]

Committed patchset #4 manually as r36575 (presubmit successful).

[hausner, 2014-05-23 17:43:48]

Ivan's LGTM came asynchronously via chat :-)