[Mac] Add locking as speculative fix for X509Certificate crashes.

[Mac] Add locking as speculative fix for X509Certificate crashes.
BUG=30001
TEST=none

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=45728

[Jens Alfke, 2010-04-26 22:26:03]

Minimal tweak to prevent multiple threads from verifying the same cert at the
same time.

I looked into caching the result so the subsequent threads don't have to do the
work again, but this would be trickier since a cert can be verified against
multiple domains, with different results for each.

[wtc, 2010-04-27 00:06:43]

LGTM.

http://codereview.chromium.org/1769010/diff/1/2
File net/base/x509_certificate.h (right):

http://codereview.chromium.org/1769010/diff/1/2#newcode298
net/base/x509_certificate.h:298: mutable Lock lock_;
Nit: perhaps rename this lock "verify_lock_" to stress the
fact that it applies to the Verify() method only.

Nit: should we explain why this lock is mutable? (because
Verify is a const method)

http://codereview.chromium.org/1769010/diff/1/3
File net/base/x509_certificate_mac.cc (right):

http://codereview.chromium.org/1769010/diff/1/3#newcode450
net/base/x509_certificate_mac.cc:450: // From here on, only one thread can be
active at a time.
It would be nice to explain why you break the Verify method
into two parts at this point.

I guess the code before this point can clearly be executed
by multiple threads.  After this point, it's less clear.
And we know for sure that we do not want multiple threads
to call SecTrustEvaluate (line 503) simultaneously.