Supporting START TRANSACTION WITH CONSISTENT [ROCKSDB] SNAPSHOT

Summary:
This adds two features in RocksDB.
1. Supporting START TRANSACTION WITH CONSISTENT SNAPSHOT
2. Getting current binlog position in addition to #1.
With these features, mysqldump can take consistent logical backup.

The second feature is done by START TRANSACTION WITH
CONSISTENT ROCKSDB SNAPSHOT. This is Facebook's extension, and
it works like existing START TRANSACTION WITH CONSISTENT INNODB SNAPSHOT.

This diff changed some existing codebase/behaviors.

- Original Facebook-MySQL always started InnoDB transaction
regardless of engine clause. For example, START TRANSACTION WITH
CONSISTENT MYISAM SNAPSHOT was accepted but it actually started
InnoDB transaction, not MyISAM. This patch does not allow
setting engine that does not support consistent snapshot.

mysql> start transaction with consistent myisam snapshot;
ERROR 1105 (HY000): Consistent Snapshot is not supported for this engine

Currently only InnoDB and RocksDB support consistent snapshot.
To check engines, I modified sql/sql_yacc.yy, trans_begin()
and ha_start_consistent_snapshot() to pass handlerton.

- Changed constant name from
  MYSQL_START_TRANS_OPT_WITH_CONS_INNODB_SNAPSHOT to
MYSQL_START_TRANS_OPT_WITH_CONS_ENGINE_SNAPSHOT, because it's no longer
InnoDB dependent.

- When not setting engine, START TRANSACTION WITH CONSISTENT SNAPSHOT
takes both InnoDB and RocksDB snapshots, and both InnoDB and RocksDB
participate in transaction. When executing COMMIT, both InnoDB and
RocksDB modifications are committed. Remember that XA is not supported yet,
so mixing engines is not recommended anyway.

- When setting engine, START TRANSACTION WITH CONSISTENT.. takes
snapshot for the specified engine only. But it starts both
InnoDB and RocksDB transactions.

Test Plan: mtr --suite=rocksdb,rocksdb_rpl, --repeat=3

Reviewers: hermanlee4, jonahcohen, jtolmer, tian.xia, maykov, spetrunia

Reviewed By: spetrunia

Subscribers: steaphan

Differential Revision: https://reviews.facebook.net/D32355

storage/rocksdb/ha_rocksdb.cc

@@ -1077,6 +1077,76 @@ static bool rocksdb_show_status(handlerton*		hton,
   return res;
 }
 
+/*
+    Supporting START TRANSACTION WITH CONSISTENT [ROCKSDB] SNAPSHOT
+
+    Features:
+    1. Supporting START TRANSACTION WITH CONSISTENT SNAPSHOT
+    2. Getting current binlog position in addition to #1.
+
+    The second feature is done by START TRANSACTION WITH
+    CONSISTENT ROCKSDB SNAPSHOT. This is Facebook's extension, and
+    it works like existing START TRANSACTION WITH CONSISTENT INNODB SNAPSHOT.
+
+    - When not setting engine, START TRANSACTION WITH CONSISTENT SNAPSHOT
+    takes both InnoDB and RocksDB snapshots, and both InnoDB and RocksDB
+    participate in transaction. When executing COMMIT, both InnoDB and
+    RocksDB modifications are committed. Remember that XA is not supported yet,
+    so mixing engines is not recommended anyway.
+
+    - When setting engine, START TRANSACTION WITH CONSISTENT.. takes
+    snapshot for the specified engine only. But it starts both
+    InnoDB and RocksDB transactions.
+*/
+static int rocksdb_start_trx_and_assign_read_view(
+        handlerton*     hton,           /*!< in: RocksDB handlerton */
+        THD*            thd,            /*!< in: MySQL thread handle of the
+                                        user for whom the transaction should
+                                        be committed */
+        char*           binlog_file,    /* out: binlog file for last commit */
+        ulonglong*      binlog_pos,     /* out: binlog pos for last commit */
+        char**  gtid_executed,  /* out: Gtids logged until last commit */
+        int*    gtid_executed_length)   /*out: Length of gtid_executed string */
+{
+  ulong const tx_isolation = thd_tx_isolation(thd);
+
+  if (tx_isolation != ISO_REPEATABLE_READ)
+  {
+    my_printf_error(ER_UNKNOWN_ERROR,
+                    "Only REPEATABLE READ isolation level is supported "
+                    "for START TRANSACTION WITH CONSISTENT SNAPSHOT "
+                    "in RocksDB Storage Engine.", MYF(0));
+    return 1;
+  }
+
+  Rdb_transaction*& trx= get_trx_from_thd(thd);
+  if (trx == NULL)
+  {
+    if (!(trx= new Rdb_transaction))
+      return 1;
+    trx->pins= row_locks.get_pins();
+  }
+
+  if (trx->snapshot)
+    rdb->ReleaseSnapshot(trx->snapshot);
+
+  if (binlog_file)
+  {
+    if (binlog_pos && mysql_bin_log_is_open())
+      mysql_bin_log_lock_commits();
+    else
+      return 1;
+  }
+
+  trx->snapshot= rdb->GetSnapshot();
+
+  if (binlog_file)
+    mysql_bin_log_unlock_commits(binlog_file, binlog_pos, gtid_executed,
+                                 gtid_executed_length);
+
+  return 0;
+}
+
 
 void get_cf_options(const std::string &cf_name, rocksdb::ColumnFamilyOptions *opts)
 {
@@ -1120,6 +1190,8 @@ static int rocksdb_init_func(void *p)
   rocksdb_hton->rollback= rocksdb_rollback;
   rocksdb_hton->db_type=  DB_TYPE_ROCKSDB;
   rocksdb_hton->show_status= rocksdb_show_status;
+  rocksdb_hton->start_consistent_snapshot=
+    rocksdb_start_trx_and_assign_read_view;
 
   /*
     Don't specify HTON_CAN_RECREATE in flags. re-create is used by TRUNCATE