Avoid assert

Due to underfilled branch page. We're in the process of merging/moving
nodes to it because we already know it's underfilled. Took this approach
rather than just removing the assert in mdb_page_search_root, because
that assert may yet catch other situations we don't know about.

(Although, it has been there since the original commit of mdb.c and
has never triggered any other times...)

libraries/liblmdb/mdb.c

@@ -4128,6 +4128,28 @@ mdb_page_search_root(MDB_cursor *mc, MDB_val *key, int modify)
 	return MDB_SUCCESS;
 }
 
+/** Search for the lowest key under the current branch page.
+ * This just bypasses a NUMKEYS check in the current page
+ * before calling mdb_page_search_root(), because the callers
+ * are all in situations where the current page is known to
+ * be underfilled.
+ */
+static int
+mdb_page_search_lowest(MDB_cursor *mc)
+{
+	MDB_page	*mp = mc->mc_pg[mc->mc_top];
+	MDB_node	*node = NODEPTR(mp, 0);
+	int rc;
+
+	if ((rc = mdb_page_get(mc->mc_txn, NODEPGNO(node), &mp)))
+		return rc;
+
+	mc->mc_ki[mc->mc_top] = 0;
+	if ((rc = mdb_cursor_push(mc, mp)))
+		return rc;
+	return mdb_page_search_root(mc, NULL, 0);
+}
+
 /** Search for the page a given key should be in.
  * Pushes parent pages on the cursor stack. This function just sets up
  * the search; it finds the root page for \b mc's database and sets this
@@ -6024,7 +6046,7 @@ mdb_node_move(MDB_cursor *csrc, MDB_cursor *cdst)
 			unsigned int snum = csrc->mc_snum;
 			MDB_node *s2;
 			/* must find the lowest key below src */
-			mdb_page_search_root(csrc, NULL, 0);
+			mdb_page_search_lowest(csrc);
 			if (IS_LEAF2(csrc->mc_pg[csrc->mc_top])) {
 				key.mv_size = csrc->mc_db->md_pad;
 				key.mv_data = LEAF2KEY(csrc->mc_pg[csrc->mc_top], 0, key.mv_size);
@@ -6047,7 +6069,7 @@ mdb_node_move(MDB_cursor *csrc, MDB_cursor *cdst)
 		MDB_node *s2;
 		MDB_val bkey;
 		/* must find the lowest key below dst */
-		mdb_page_search_root(cdst, NULL, 0);
+		mdb_page_search_lowest(cdst);
 		if (IS_LEAF2(cdst->mc_pg[cdst->mc_top])) {
 			bkey.mv_size = cdst->mc_db->md_pad;
 			bkey.mv_data = LEAF2KEY(cdst->mc_pg[cdst->mc_top], 0, bkey.mv_size);
@@ -6211,7 +6233,7 @@ mdb_page_merge(MDB_cursor *csrc, MDB_cursor *cdst)
 				unsigned int snum = csrc->mc_snum;
 				MDB_node *s2;
 				/* must find the lowest key below src */
-				mdb_page_search_root(csrc, NULL, 0);
+				mdb_page_search_lowest(csrc);
 				if (IS_LEAF2(csrc->mc_pg[csrc->mc_top])) {
 					key.mv_size = csrc->mc_db->md_pad;
 					key.mv_data = LEAF2KEY(csrc->mc_pg[csrc->mc_top], 0, key.mv_size);