Adds blocking file locks.

Adds blocking file locks.

Fixes #26665

R=sgjesse@google.com

Committed: https://github.com/dart-lang/sdk/commit/678cb048ed0eaa96729be7c955c7fadd32cf7024

[zra, 2016-06-10 18:20:34]

zra@google.com changed reviewers:
+ asiva@google.com, sgjesse@google.com

[zra, 2016-06-10 18:20:35]

Soren, is there any reason why this wasn't wired up initially?

[Søren Gjesse, 2016-06-13 06:47:16]

On 2016/06/10 18:20:35, zra wrote:
> Soren, is there any reason why this wasn't wired up initially?

lgtm

Adding this was discussed in https://github.com/dart-lang/sdk/issues/22081,
where the option of adding a timeout to these operations entered the discussion.
That part was never resolved, and I just think the issue was dropped as
requester at that time ended up not needing this feature.

You can probably close https://github.com/dart-lang/sdk/issues/22081 as well
saying that timeout is not supported. If we find a solution for the timeout it
can be added as an optionally named argument.

[zra, 2016-06-13 14:58:12]

Description was changed from

==========
Adds blocking file locks.

Fixes #26665
==========

to

==========
Adds blocking file locks.

Fixes #26665

R=sgjesse@google.com

Committed:
https://github.com/dart-lang/sdk/commit/678cb048ed0eaa96729be7c955c7fadd32cf7024
==========

[zra, 2016-06-13 14:58:14]

Committed patchset #3 (id:40001) manually as
678cb048ed0eaa96729be7c955c7fadd32cf7024 (presubmit successful).

[siva, 2016-06-13 15:56:08]

lgtm with one comment about the Dart API with regards to asynchronous
operations.

https://codereview.chromium.org/2050413002/diff/40001/sdk/lib/io/file.dart
File sdk/lib/io/file.dart (right):

https://codereview.chromium.org/2050413002/diff/40001/sdk/lib/io/file.dart#ne...
sdk/lib/io/file.dart:746: *
This operation is not synchronous, should we allow the BLOCKING options to be
used in this function as a call to this function can potentially block and seems
to go against the grain of being asynchronous.

[zra, 2016-06-13 16:03:33]

https://codereview.chromium.org/2050413002/diff/40001/sdk/lib/io/file.dart
File sdk/lib/io/file.dart (right):

https://codereview.chromium.org/2050413002/diff/40001/sdk/lib/io/file.dart#ne...
sdk/lib/io/file.dart:746: *
On 2016/06/13 15:56:08, siva wrote:
> This operation is not synchronous, should we allow the BLOCKING options to be
> used in this function as a call to this function can potentially block and
seems
> to go against the grain of being asynchronous.

This call is implemented as a request sent to the IO Service thread, which then
makes the potentially blocking call. The calling thread doesn't block. The
returned Future is completed when the response is received from the IO Service
thread.

[Søren Gjesse, 2016-06-13 16:05:10]

https://codereview.chromium.org/2050413002/diff/40001/sdk/lib/io/file.dart
File sdk/lib/io/file.dart (right):

https://codereview.chromium.org/2050413002/diff/40001/sdk/lib/io/file.dart#ne...
sdk/lib/io/file.dart:746: *
On 2016/06/13 15:56:08, siva wrote:
> This operation is not synchronous, should we allow the BLOCKING options to be
> used in this function as a call to this function can potentially block and
seems
> to go against the grain of being asynchronous.

The blocking I/O call is dispatched on the "native isolate" thread-pool, so it
will not block other Dart code from running. However performing blocking lock
calls on more files than there are threads in the native isolate can end up
blocking the native isolate is neither of these calls can gain the lock, so care
should be taken in that case.

I cannot remember how many threads are allocated for the native isolate.