Summary:
For GetLogFileSize() and Flush(), they previously did not follow the
synchronization pattern for accessing logger_. This meant ResetLogger() could
cause logger_ destruction while the unsynchronized functions were accessing it,
causing a segfault.
Also made the mutex instance variable mutable so we can preserve
GetLogFileSize()'s const-ness.
Test Plan:
new test case, it's quite ugly because both threads need to access
one of the functions with SyncPoints (PosixLogger::Flush()), and also special
handling is needed to prevent the mutex and sync points from conflicting.
Reviewers: kradhakrishnan, IslamAbdelRahman, sdong
Reviewed By: sdong
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D54237
