Fix problem with persistent histograms getting ID of zero.

Fix problem with persistent histograms getting ID of zero.

1) Create persistent histogram records in an atomic manner
so there are no half-filled entries left around should the
process crash before it completes.

2) Validate the ID is non-zero before trying to access a
histogram from persistent storage.

BUG=649371
Committed: https://crrev.com/3027c932f1a8f52dfbc23b9a86ebfb1714aa8c87
Cr-Commit-Position: refs/heads/master@{#420654}

[bcwhite, 2016-09-22 16:26:45]

The CQ bit was checked by bcwhite@chromium.org

[commit-bot: I haz the power, 2016-09-22 16:27:02]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-22 16:27:02]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-09-22 16:27:09]

No L-G-T-M from a valid reviewer yet. 
CQ run can only be started by full committers or once the patch has
received an L-G-T-M from a full committer.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[bcwhite, 2016-09-22 16:50:36]

The CQ bit was checked by bcwhite@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-09-22 16:50:46]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-22 16:59:22]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-09-22 16:59:24]

Dry run: Try jobs failed on following builders:
  cast_shell_linux on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/cast_shell_linu...)

[bcwhite, 2016-09-22 17:28:43]

The CQ bit was checked by bcwhite@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-09-22 17:29:50]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-22 18:29:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-09-22 18:29:59]

Dry run: This issue passed the CQ dry run.

[bcwhite, 2016-09-22 18:30:40]

bcwhite@chromium.org changed reviewers:
+ asvitkine@chromium.org

[Alexei Svitkine (slow), 2016-09-23 14:03:06]

LGTM % comment

https://codereview.chromium.org/2367503002/diff/20001/base/metrics/persistent...
File base/metrics/persistent_histogram_allocator.cc (right):

https://codereview.chromium.org/2367503002/diff/20001/base/metrics/persistent...
base/metrics/persistent_histogram_allocator.cc:282:
histogram_data->samples_metadata.id == 0 ||
Can you expand comment for |id| field in Metadata to mention what special case
of 0 means?

[bcwhite, 2016-09-23 14:24:22]

https://codereview.chromium.org/2367503002/diff/20001/base/metrics/persistent...
File base/metrics/persistent_histogram_allocator.cc (right):

https://codereview.chromium.org/2367503002/diff/20001/base/metrics/persistent...
base/metrics/persistent_histogram_allocator.cc:282:
histogram_data->samples_metadata.id == 0 ||
On 2016/09/23 14:03:06, Alexei Svitkine (very slow) wrote:
> Can you expand comment for |id| field in Metadata to mention what special case
> of 0 means?

Done.

[bcwhite, 2016-09-23 14:24:29]

The CQ bit was checked by bcwhite@chromium.org

[bcwhite, 2016-09-23 14:24:29]

The patchset sent to the CQ was uploaded after l-g-t-m from
asvitkine@chromium.org
Link to the patchset: https://codereview.chromium.org/2367503002/#ps40001
(title: "added comment about metadata check")

[commit-bot: I haz the power, 2016-09-23 14:24:38]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-23 14:49:20]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-09-23 14:49:21]

Try jobs failed on following builders:
  win_chromium_compile_dbg_ng on master.tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)

[bcwhite, 2016-09-23 16:42:51]

The CQ bit was checked by bcwhite@chromium.org

[commit-bot: I haz the power, 2016-09-23 16:43:27]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-23 17:52:56]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2016-09-23 17:54:34]

Description was changed from

==========
Fix problem with persistent histograms getting ID of zero.

1) Create persistent histogram records in an atomic manner
so there are no half-filled entries left around should the
process crash before it completes.

2) Validate the ID is non-zero before trying to access a
histogram from persistent storage.

BUG=649371
==========

to

==========
Fix problem with persistent histograms getting ID of zero.

1) Create persistent histogram records in an atomic manner
so there are no half-filled entries left around should the
process crash before it completes.

2) Validate the ID is non-zero before trying to access a
histogram from persistent storage.

BUG=649371

Committed: https://crrev.com/3027c932f1a8f52dfbc23b9a86ebfb1714aa8c87
Cr-Commit-Position: refs/heads/master@{#420654}
==========

[commit-bot: I haz the power, 2016-09-23 17:54:35]

Patchset 3 (id:??) landed as
https://crrev.com/3027c932f1a8f52dfbc23b9a86ebfb1714aa8c87
Cr-Commit-Position: refs/heads/master@{#420654}