ITS#8557 fix mdb_cursor_last

Optimize mdb_page_search_root(PS_LAST) when cursor is already near
last position, ignoring C_EOF flag for now.

libraries/liblmdb/mdb.c

@@ -5459,8 +5459,17 @@ mdb_page_search_root(MDB_cursor *mc, MDB_val *key, int flags)
 
 		if (flags & (MDB_PS_FIRST|MDB_PS_LAST)) {
 			i = 0;
-			if (flags & MDB_PS_LAST)
+			if (flags & MDB_PS_LAST) {
 				i = NUMKEYS(mp) - 1;
+				/* if already init'd, see if we're already in right place */
+				if (mc->mc_flags & C_INITIALIZED) {
+					if (mc->mc_ki[mc->mc_top] == i) {
+						mc->mc_top = mc->mc_snum++;
+						mp = mc->mc_pg[mc->mc_top];
+						goto ready;
+					}
+				}
+			}
 		} else {
 			int	 exact;
 			node = mdb_node_search(mc, key, &exact);
@@ -5486,6 +5495,7 @@ mdb_page_search_root(MDB_cursor *mc, MDB_val *key, int flags)
 		if ((rc = mdb_cursor_push(mc, mp)))
 			return rc;
 
+ready:
 		if (flags & MDB_PS_MODIFY) {
 			if ((rc = mdb_page_touch(mc)) != 0)
 				return rc;
@@ -6224,16 +6234,13 @@ mdb_cursor_last(MDB_cursor *mc, MDB_val *key, MDB_val *data)
 	if (mc->mc_xcursor)
 		mc->mc_xcursor->mx_cursor.mc_flags &= ~(C_INITIALIZED|C_EOF);
 
-	if (!(mc->mc_flags & C_EOF)) {
-
-		if (!(mc->mc_flags & C_INITIALIZED) || mc->mc_top) {
-			rc = mdb_page_search(mc, NULL, MDB_PS_LAST);
-			if (rc != MDB_SUCCESS)
-				return rc;
-		}
-		mdb_cassert(mc, IS_LEAF(mc->mc_pg[mc->mc_top]));
-
+	if (!(mc->mc_flags & C_INITIALIZED) || mc->mc_top) {
+		rc = mdb_page_search(mc, NULL, MDB_PS_LAST);
+		if (rc != MDB_SUCCESS)
+			return rc;
 	}
+	mdb_cassert(mc, IS_LEAF(mc->mc_pg[mc->mc_top]));
+
 	mc->mc_ki[mc->mc_top] = NUMKEYS(mc->mc_pg[mc->mc_top]) - 1;
 	mc->mc_flags |= C_INITIALIZED|C_EOF;
 	leaf = NODEPTR(mc->mc_pg[mc->mc_top], mc->mc_ki[mc->mc_top]);