2 　// -----A begin-------  3   Writer w(&mutex_);  4   w.batch = my_batch;  5   w.sync = options.sync;  6   w.done = false;  7   // -----A end  --------  8 9
10   // -----B begin-------
11   MutexLock l(&mutex_);
12   writers_.push_back(&w);
13   while (!w.done && &w != writers_.front()) {
14     w.cv.Wait();
15   }
16   if (w.done) {
17     return w.status;
18   }
19   // -----B end  -------
20
21   // May temporarily unlock and wait.
22   Status status = MakeRoomForWrite(my_batch == NULL);
23   uint64_t last_sequence = versions_->LastSequence();
24   Writer* last_writer = &w;
25   if (status.ok() && my_batch != NULL) {  // NULL batch is for compactions
26     WriteBatch* updates = BuildBatchGroup(&last_writer);
27     WriteBatchInternal::SetSequence(updates, last_sequence + 1);
28     last_sequence += WriteBatchInternal::Count(updates);
29
30     // Add to log and apply to memtable.  We can release the lock
31     // during this phase since &w is currently responsible for logging
32     // and protects against concurrent loggers and concurrent writes
33     // into mem_.
34     {
35       // -----C begin-------
36       mutex_.Unlock();
37       // -----C end  -------  

　　　　　 // add batch data to log files
38       status = log_->AddRecord(WriteBatchInternal::Contents(updates));
39       if (status.ok() && options.sync) {
40         status = logfile_->Sync();
41       }
42       if (status.ok()) {
43         status = WriteBatchInternal::InsertInto(updates, mem_);
44       }
45       // -----D begin-------
46       mutex_.Lock();
47       // -----D end  -------
48     }
49     if (updates == tmp_batch_) tmp_batch_->Clear();
50
51     versions_->SetLastSequence(last_sequence);
52   }
53
54   // -----E begin-------
55   while (true) {
56     Writer* ready = writers_.front();
57     writers_.pop_front();
58     if (ready != &w) {
59       ready->status = status;
60       ready->done = true;
61       ready->cv.Signal();
62     }
63     if (ready == last_writer) break;
64   }
65   // -----E end -------
66
67
68   // -----F begin-------
69   // Notify new head of write queue
70   if (!writers_.empty()) {
71     writers_.front()->cv.Signal();
72   }
73   // -----F end-------
74
75   return status;
76 }