Env to add a function to allow users to query waiting queue length

Summary: Add a function to Env so that users can query the waiting queue length of each thread pool

Test Plan: add a test in env_test

Reviewers: haobo

Reviewed By: haobo

CC: dhruba, igor, yhchiang, ljin, nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D16755
Add a comment after SignalAll()

Summary: Having code after SignalAll has already caused 2 bugs. Let's make sure this doesn't happen again.

Test Plan: no test

Reviewers: sdong, dhruba, haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16785
Temp Fix of LogBuffer flushing

Summary: To temp fix the log buffer flushing. Flush the buffer inside the lock. Clean the trunk before we find an eventual fix.

Test Plan: make all check

Reviewers: haobo, igor

Reviewed By: igor

CC: ljin, leveldb, yhchiang

Differential Revision: https://reviews.facebook.net/D16791
fix db_stress test

Summary: Fix the db_stress test, let is run with HashSkipList for real

Test Plan:
python tools/db_crashtest.py
python tools/db_crashtest2.py

Reviewers: igor, haobo

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16773
Fix data race against logging data structure because of LogBuffer

Summary:
@igor pointed out that there is a potential data race because of the way we use the newly introduced LogBuffer. After "bg_compaction_scheduled_--" or "bg_flush_scheduled_--", they can both become 0. As soon as the lock is released after that, DBImpl's deconstructor can go ahead and deconstruct all the states inside DB, including the info_log object hold in a shared pointer of the options object it keeps. At that point it is not safe anymore to continue using the info logger to write the delayed logs.

With the patch, lock is released temporarily for log buffer to be flushed before "bg_compaction_scheduled_--" or "bg_flush_scheduled_--". In order to make sure we don't miss any pending flush or compaction, a new flag bg_schedule_needed_ is added, which is set to be true if there is a pending flush or compaction but not scheduled because of the max thread limit. If the flag is set to be true, the scheduling function will be called before compaction or flush thread finishes.

Thanks @igor for this finding!

Test Plan: make all check

Reviewers: haobo, igor

Reviewed By: haobo

CC: dhruba, ljin, yhchiang, igor, leveldb

Differential Revision: https://reviews.facebook.net/D16767
make assert based on FLAGS_prefix_size

Summary: as title

Test Plan: running python tools/db_crashtest.py

Reviewers: igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16803
Fix bad merge of D16791 and D16767

Summary: A bad Auto-Merge caused log buffer is flushed twice. Remove the unintended one.

Test Plan: Should already be tested (the code looks the same as when I ran unit tests).

Reviewers: haobo, igor

Reviewed By: haobo

CC: ljin, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D16821
DBStress cleanup

Summary:
*) fixed the comment
*) constant 1 was not casted to 64-bit, which (I think) might cause overflow if we shift it too much
*) default prefix size to be 7, like it was before

Test Plan: compiled

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16827
Revert "Fix bad merge of D16791 and D16767"

This reverts commit 839c8ecfcd486f0db82ecb755a137ad95909966f.
Correct version of D16821
Merge branch 'master' into columnfamilies

Conflicts:
	db/db_impl.cc
	tools/db_stress.cc