Fixes possible crash in SessionBackend. I believe what's happening...

Fixes possible crash in SessionBackend. I believe what's happening
here is we fail to create the file, and so current_session_file_.get()
is NULL and we crash. current_session_file_.get() is NULL if
OpenAndWriteHeader returns NULL (which is does if the full header
isn't written correctly.

Here's how I'm changing the code:
. The file is now truncated instead of closed/reopened. Hopefully this avoids the possibility of a scanner locking the file and the delete failing.
. Added a unit test for coverage of truncation.
. The file is opened in exclusive access. There is no reason why a scanner should open this file.
. Added null checks.

BUG=8476
TEST=none


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=11262

[Finnur, 2009-03-06 22:37:51]

This looks fine. I do wonder though why we don't take an exclusive lock on the
file to make sure we are never blocked from recreating the file...

[Peter Kasting, 2009-03-06 22:49:56]

Maybe you should comment in the code why this can be NULL?

[sky, 2009-03-09 16:30:31]

I've changed the code to truncate the file if already open, which should avoid
the possibility of scanners locking the file out from under us. I've also
changed the code to open the file with exclusive access. Added a test for
coverage of truncation, and of course added the null checks.

New snapshot uploaded.

[Finnur, 2009-03-09 17:36:50]

http://codereview.chromium.org/39275/diff/1002/6
File chrome/browser/sessions/session_backend.cc (right):

http://codereview.chromium.org/39275/diff/1002/6#newcode338
Line 338: base::PLATFORM_FILE_WRITE | base::PLATFORM_FILE_EXCLUSIVE_WRITE);
I would have thought that the open failed here because the AV had it open for
Read at the time you tried to recreate it. Do they really open the file for
writing as well?

http://codereview.chromium.org/39275/diff/1002/8
File chrome/browser/sessions/session_backend_unittest.cc (right):

http://codereview.chromium.org/39275/diff/1002/8#newcode212
Line 212: }
Don't need the extra line break here.

[sky, 2009-03-09 17:58:47]

Good call. Updated.

[Finnur, 2009-03-09 18:02:19]

LGTM