Add MDB_PREV_MULTIPLE (collected mdb.master changes)

Logical counterpart to GET_MULTIPLE, NEXT_MULTIPLE

libraries/liblmdb/lmdb.h

@@ -388,7 +388,9 @@ typedef enum MDB_cursor_op {
 	MDB_PREV_NODUP,			/**< Position at last data item of previous key */
 	MDB_SET,				/**< Position at specified key */
 	MDB_SET_KEY,			/**< Position at specified key, return key + data */
-	MDB_SET_RANGE			/**< Position at first key greater than or equal to specified key. */
+	MDB_SET_RANGE,			/**< Position at first key greater than or equal to specified key. */
+	MDB_PREV_MULTIPLE		/**< Position at previous page and return key and up to
+								a page of duplicate data items. Only for #MDB_DUPFIXED */
 } MDB_cursor_op;
 
 /** @defgroup  errors	Return Codes
@@ -1095,8 +1097,9 @@ int  mdb_txn_renew(MDB_txn *txn);
 	 *		This flag may only be used in combination with #MDB_DUPSORT. This option
 	 *		tells the library that the data items for this database are all the same
 	 *		size, which allows further optimizations in storage and retrieval. When
-	 *		all data items are the same size, the #MDB_GET_MULTIPLE and #MDB_NEXT_MULTIPLE
+	 *		all data items are the same size, the #MDB_GET_MULTIPLE, #MDB_NEXT_MULTIPLE
-	 *		cursor operations may be used to retrieve multiple items at once.
+	 *		and #MDB_PREV_MULTIPLE cursor operations may be used to retrieve multiple
+	 *		items at once.
 	 *	<li>#MDB_INTEGERDUP
 	 *		This option specifies that duplicate data items are binary integers,
 	 *		similar to #MDB_INTEGERKEY keys.

libraries/liblmdb/mdb.c

@@ -6215,6 +6215,30 @@ fetchm:
 			}
 		}
 		break;
+	case MDB_PREV_MULTIPLE:
+		if (data == NULL) {
+			rc = EINVAL;
+			break;
+		}
+		if (!(mc->mc_db->md_flags & MDB_DUPFIXED)) {
+			rc = MDB_INCOMPATIBLE;
+			break;
+		}
+		if (!(mc->mc_flags & C_INITIALIZED))
+			rc = mdb_cursor_last(mc, key, data);
+		else
+			rc = MDB_SUCCESS;
+		if (rc == MDB_SUCCESS) {
+			MDB_cursor *mx = &mc->mc_xcursor->mx_cursor;
+			if (mx->mc_flags & C_INITIALIZED) {
+				rc = mdb_cursor_sibling(mx, 0);
+				if (rc == MDB_SUCCESS)
+					goto fetchm;
+			} else {
+				rc = MDB_NOTFOUND;
+			}
+		}
+		break;
 	case MDB_NEXT:
 	case MDB_NEXT_DUP:
 	case MDB_NEXT_NODUP: