Thread safety of WriteBatch object not synchronized

[braydonf]

There is a memory safety issue when making batch database calls.

Details:

Within a single process, the same leveldb::DB object may be safely shared by multiple concurrent threads. (...) However other objects (like Iterator and WriteBatch) may require external synchronization. If two threads share such an object, they must protect access to it using their own locking protocol. (...)

https://github.com/google/leveldb/blob/master/doc/index.md#concurrency

The bindings (./src/bindings.cc) methods batch_init, batch_put, batch_del, batch_write and batch_clear all share access to the WriteBatch object. The batch_write method will share the object with another thread in BatchWriteWorker. It's possible that batch_put, batch_del and batch_clear can unsafely modify the WriteBatch object that is shared with the other thread.

Reproduce:

Call batch_put, batch_del or batch_clear after a call to batch_write and before the batch_write worker thread is complete.

Some of the possible errors:

• Segmentation fault
• Abort trap 6
• Corruption: unknown WriteBatch tag
• Corruption: bad WriteBatch Put
• Corruption: WriteBatch has wrong count

[braydonf]

It can also be reproduced by calling db_close after batch_write and before the worker thread is complete. This can be seen, as with the other cases, in the pull request at #7.

[braydonf]

Also related bcoin-org/bcoin#864