database: flip to recursive_mutex to allow nested locks (#1937)

* Fix crash from db locking

* Change db lock to use recursive_mutex

extensions/mysql/mysql/MyDatabase.h

@@ -70,7 +70,7 @@ public:
 	const DatabaseInfo &GetInfo();
 private:
 	MYSQL *m_mysql;
-	std::mutex m_FullLock;
+	std::recursive_mutex m_FullLock;
 
 	/* ---------- */
 	DatabaseInfo m_Info;

extensions/pgsql/pgsql/PgDatabase.h

@@ -71,7 +71,7 @@ public:
 	void SetLastIDAndRows(unsigned int insertID, unsigned int affectedRows);
 private:
 	PGconn *m_pgsql;
-	std::mutex m_FullLock;
+	std::recursive_mutex m_FullLock;
 
 	unsigned int m_lastInsertID;
 	unsigned int m_lastAffectedRows;

extensions/sqlite/driver/SqDatabase.h

@@ -70,7 +70,7 @@ public:
 	}
 private:
 	sqlite3 *m_sq3;
-	std::mutex m_FullLock;
+	std::recursive_mutex m_FullLock;
 	bool m_Persistent;
 	String m_LastError;
 	int m_LastErrorCode;

plugins/include/dbi.inc

@@ -959,6 +959,10 @@ native bool SQL_Execute(Handle statement);
  * If the lock cannot be acquired, the main thread will pause until the 
  * threaded operation has concluded.
  *
+ * Care should be taken to not lock an already-locked database. Internally,
+ * lock calls are nested recursively and must be paired with an equal amount
+ * of unlocks to be undone. This behaviour should not be relied on.
+ *
  * @param database      A database Handle.
  * @error               Invalid database Handle.
  */