Add __contains__ implementation to avoid a race in __get_item__.

Add __contains__ implementation to avoid a race in __get_item__.

__get_item__ enumerates ExtensionBackendList, and when it discovers
a matching index, it access ExtensionBackendList at that index. However,
ExtensionBackendList.__get_item__ calls Update, which can invalidate index.

BUG=411612
TEST=extension_unittest

Committed: https://crrev.com/a91527a67047664d5f13bbb1ae7533b9f85e2b14
Cr-Commit-Position: refs/heads/master@{#293944}

[achuithb, 2014-09-09 11:44:38]

achuith@chromium.org changed reviewers:
+ nednguyen@google.com, tbarzic@chromium.org

[achuithb, 2014-09-09 11:44:38]

Please review, Toni and Ned

[tonyg, 2014-09-09 14:40:39]

tonyg@chromium.org changed reviewers:
+ tonyg@chromium.org

[tonyg, 2014-09-09 14:40:39]

The CQ bit was checked by tonyg@chromium.org

[tonyg, 2014-09-09 14:40:39]

lgtm

[commit-bot: I haz the power, 2014-09-09 14:41:29]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/achuith@chromium.org/555823002/1

[nednguyen, 2014-09-09 15:29:17]

Sorry, can you explain how adding __contains__ help avoid the race condition
mentioned? I don't see where it's used in this patch.

[commit-bot: I haz the power, 2014-09-09 15:41:59]

Committed patchset #1 (id:1) as 9a6b0abb16d995ff17e533874e6787b788cc7129

[tbarzic, 2014-09-09 16:52:28]

On 2014/09/09 15:41:59, I haz the power (commit-bot) wrote:
> Committed patchset #1 (id:1) as 9a6b0abb16d995ff17e533874e6787b788cc7129

hm, I don't think this will work. In AllExtensionsLoaded, right after calling
"if not e.extension_id in self.extension_backend:" (what currently breaks),
self.extension_backend[e.extension_id] is called, which will end up calling
__get_items__ and encounter the same race condition.
(https://code.google.com/p/chromium/codesearch#chromium/src/tools/telemetry/te...)

[achuithb, 2014-09-09 23:09:29]

On 2014/09/09 16:52:28, tbarzic wrote:
> On 2014/09/09 15:41:59, I haz the power (commit-bot) wrote:
> > Committed patchset #1 (id:1) as 9a6b0abb16d995ff17e533874e6787b788cc7129
> 
> hm, I don't think this will work. In AllExtensionsLoaded, right after calling
> "if not e.extension_id in self.extension_backend:" (what currently breaks),
> self.extension_backend[e.extension_id] is called, which will end up calling
> __get_items__ and encounter the same race condition.
>
(https://code.google.com/p/chromium/codesearch#chromium/src/tools/telemetry/te...)

You're right :(

[commit-bot: I haz the power, 2014-09-10 03:53:32]

Patchset 1 (id:??) landed as
https://crrev.com/a91527a67047664d5f13bbb1ae7533b9f85e2b14
Cr-Commit-Position: refs/heads/master@{#293944}