Add mdb_env_get_fd()

Return the environment's filedescriptor. Useful when the caller
is doing their own locking.

libraries/liblmdb/lmdb.h

@@ -679,6 +679,18 @@ int  mdb_env_get_flags(MDB_env *env, unsigned int *flags);
 	 */
 int  mdb_env_get_path(MDB_env *env, const char **path);
 
+	/** @brief Return the filedescriptor for the given environment.
+	 *
+	 * @param[in] env An environment handle returned by #mdb_env_create()
+	 * @param[out] fd Address of a mdb_filehandle_t to contain the descriptor.
+	 * @return A non-zero error value on failure and 0 on success. Some possible
+	 * errors are:
+	 * <ul>
+	 *	<li>EINVAL - an invalid parameter was specified.
+	 * </ul>
+	 */
+int  mdb_env_get_fd(MDB_env *env, mdb_filehandle_t *fd);
+
 	/** @brief Set the size of the memory map to use for this environment.
 	 *
 	 * The size should be a multiple of the OS page size. The default is

libraries/liblmdb/mdb.c

@@ -7817,6 +7817,16 @@ mdb_env_get_path(MDB_env *env, const char **arg)
 	return MDB_SUCCESS;
 }
 
+int
+mdb_env_get_fd(MDB_env *env, mdb_filehandle_t *arg)
+{
+	if (!env || !arg)
+		return EINVAL;
+
+	*arg = env->me_fd;
+	return MDB_SUCCESS;
+}
+
 /** Common code for #mdb_stat() and #mdb_env_stat().
  * @param[in] env the environment to operate in.
  * @param[in] db the #MDB_db record containing the stats to return.