[RocksDB] env_posix cleanup

Summary:
1. SetBackgroundThreads was not thread safe
2. queue_size_ does not seem necessary
3. moved condition signal after shared state change. Even though the original
   order is in practice ok (because the mutex is still held), it looks fishy
   and non-intuitive.

Test Plan: make check

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb, zshao

Differential Revision: https://reviews.facebook.net/D9825

util/env_posix.cc

@@ -880,10 +880,12 @@ class PosixEnv : public Env {
 
   // Allow increasing the number of worker threads.
   virtual void SetBackgroundThreads(int num) {
+    PthreadCall("lock", pthread_mutex_lock(&mu_));
     if (num > num_threads_) {
       num_threads_ = num;
       bgthread_.resize(num_threads_);
     }
+    PthreadCall("unlock", pthread_mutex_unlock(&mu_));
   }
 
   virtual std::string TimeToString(uint64_t secondsSince1970) {
@@ -961,7 +963,6 @@ class PosixEnv : public Env {
   // Entry per Schedule() call
   struct BGItem { void* arg; void (*function)(void*); };
   typedef std::deque<BGItem> BGQueue;
-  int queue_size_; // number of items in BGQueue
   BGQueue queue_;
 };
 
@@ -969,8 +970,7 @@ PosixEnv::PosixEnv() : checkedDiskForMmap_(false),
                        forceMmapOff(false),
                        page_size_(getpagesize()),
                        started_bgthread_(0),
-                       num_threads_(1),
-                       queue_size_(0) {
+                       num_threads_(1) {
   PthreadCall("mutex_init", pthread_mutex_init(&mu_, nullptr));
   PthreadCall("cvar_init", pthread_cond_init(&bgsignal_, nullptr));
   bgthread_.resize(num_threads_);
@@ -990,14 +990,13 @@ void PosixEnv::Schedule(void (*function)(void*), void* arg) {
     fprintf(stdout, "Created bg thread 0x%lx\n", bgthread_[started_bgthread_]);
   }
 
-  // always wake up at least one waiting thread.
-  PthreadCall("signal", pthread_cond_signal(&bgsignal_));
-
   // Add to priority queue
   queue_.push_back(BGItem());
   queue_.back().function = function;
   queue_.back().arg = arg;
-  queue_size_++;
+
+  // always wake up at least one waiting thread.
+  PthreadCall("signal", pthread_cond_signal(&bgsignal_));
 
   PthreadCall("unlock", pthread_mutex_unlock(&mu_));
 }
@@ -1013,7 +1012,6 @@ void PosixEnv::BGThread() {
     void (*function)(void*) = queue_.front().function;
     void* arg = queue_.front().arg;
     queue_.pop_front();
-    queue_size_--;
 
     PthreadCall("unlock", pthread_mutex_unlock(&mu_));
     (*function)(arg);