Add SNAPSHOT sync mode to LocalFileSyncContext::PrepareForSync

Add SNAPSHOT sync mode to LocalFileSyncContext::PrepareForSync

So that local-to-remote sync can use snapshot file without
locking the file for the entire sync period.

The actual snapshotting part is not implemented yet.

This one corresponds to the first item in this design note: https://docs.google.com/a/chromium.org/document/d/1-oIGAGakMq0YS_3dfG11YHjfYRgOvP7kK7JagP39qLM/view#heading=h.rs5x2f5z1e34

BUG=175693
TEST=LocalFileSyncContextTest.PrepareSyncWhileWriting

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=223177

[kinuko, 2013-09-12 02:04:53]

ptl

[kinuko, 2013-09-13 01:59:39]

On 2013/09/12 02:04:53, kinuko wrote:
> ptl

Updated to change the local change record handling during the sync not to clear
in-memory changes but will record new changes during the sync, so that the
backend can return the correct change status for the file (also updated the
design note).

The follow-up patch that actually uses SNAPSHOT mode in LocalSync is: 
https://codereview.chromium.org/23578026/
In my local build it looks working.

[tzik, 2013-09-13 05:31:12]

lgtm

[nhiroki, 2013-09-13 06:23:08]

lgtm with some nits

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
File chrome/browser/sync_file_system/local/local_file_change_tracker.h (right):

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
chrome/browser/sync_file_system/local/local_file_change_tracker.h:82: // Removes
a mirror for |url|, and commit the change status to database.
commit -> commits

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
chrome/browser/sync_file_system/local/local_file_change_tracker.h:85: // Reset
the changes to the ones recorded in mirror for |url|, and commit the
Reset -> Resets
commit -> commits

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
File chrome/browser/sync_file_system/local/local_file_sync_context.cc (right):

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
chrome/browser/sync_file_system/local/local_file_sync_context.cc:752: if
(sync_mode != SYNC_EXCLUSIVE) {
Do you have any plans to add new SyncMode other than SYNC_EXCLUSIVE and
SYNC_SNAPSHOT? If not so, "sync_mode == SYNC_SNAPSHOT" seems more clear.

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
File chrome/browser/sync_file_system/local/local_file_sync_context.h (right):

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
chrome/browser/sync_file_system/local/local_file_sync_context.h:39: struct
LocalFileSyncInfo;
nit: can you sort this alphabetically?

[kinuko, 2013-09-13 08:21:46]

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
File chrome/browser/sync_file_system/local/local_file_change_tracker.h (right):

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
chrome/browser/sync_file_system/local/local_file_change_tracker.h:82: // Removes
a mirror for |url|, and commit the change status to database.
On 2013/09/13 06:23:08, nhiroki wrote:
> commit -> commits

Done.

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
chrome/browser/sync_file_system/local/local_file_change_tracker.h:85: // Reset
the changes to the ones recorded in mirror for |url|, and commit the
On 2013/09/13 06:23:08, nhiroki wrote:
> Reset -> Resets
> commit -> commits

Done.

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
File chrome/browser/sync_file_system/local/local_file_sync_context.cc (right):

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
chrome/browser/sync_file_system/local/local_file_sync_context.cc:752: if
(sync_mode != SYNC_EXCLUSIVE) {
On 2013/09/13 06:23:08, nhiroki wrote:
> Do you have any plans to add new SyncMode other than SYNC_EXCLUSIVE and
> SYNC_SNAPSHOT? If not so, "sync_mode == SYNC_SNAPSHOT" seems more clear.

Not for now, but "If it's not exclusive we can release lock" feels more natural
to me (as 'snapshot' doesn't usually mean sharing constraint).

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
File chrome/browser/sync_file_system/local/local_file_sync_context.h (right):

https://codereview.chromium.org/24106002/diff/13001/chrome/browser/sync_file_...
chrome/browser/sync_file_system/local/local_file_sync_context.h:39: struct
LocalFileSyncInfo;
On 2013/09/13 06:23:08, nhiroki wrote:
> nit: can you sort this alphabetically?

Done.

[commit-bot: I haz the power, 2013-09-13 08:41:03]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/kinuko@chromium.org/24106002/90001

[commit-bot: I haz the power, 2013-09-13 14:14:24]

Retried try job too often on win7_aura for step(s) browser_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win7_aura&...

[commit-bot: I haz the power, 2013-09-13 17:44:18]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/kinuko@chromium.org/24106002/90001

[commit-bot: I haz the power, 2013-09-13 23:52:10]

Change committed as 223177