ITS#7594 Fix MDB cursor tracking with subDBs.

The tracking code should not change the current cursor.
It did when that was a C_SUB cursor, which should not be
checked against the tracked cursors but their xcursors.

However, do not bother to skip the tracking code for the
current cursor when it would not change that cursor anyway.

libraries/liblmdb/mdb.c

@@ -1621,8 +1621,8 @@ finish:
 			MDB_dbi dbi = mc->mc_dbi-1;
 
 			for (m2 = mc->mc_txn->mt_cursors[dbi]; m2; m2=m2->mc_next) {
-				if (m2 == mc) continue;
 				m3 = &m2->mc_xcursor->mx_cursor;
+				if (m3 == mc) continue;
 				if (m3->mc_snum < mc->mc_snum) continue;
 				if (m3->mc_pg[mc->mc_top] == mc->mc_pg[mc->mc_top]) {
 					m3->mc_pg[mc->mc_top] = mp;
@@ -6236,11 +6236,11 @@ mdb_node_move(MDB_cursor *csrc, MDB_cursor *cdst)
 			dbi--;
 
 		for (m2 = csrc->mc_txn->mt_cursors[dbi]; m2; m2=m2->mc_next) {
-			if (m2 == csrc) continue;
 			if (csrc->mc_flags & C_SUB)
 				m3 = &m2->mc_xcursor->mx_cursor;
 			else
 				m3 = m2;
+			if (m3 == csrc) continue;
 			if (m3->mc_pg[csrc->mc_top] == mp && m3->mc_ki[csrc->mc_top] ==
 				csrc->mc_ki[csrc->mc_top]) {
 				m3->mc_pg[csrc->mc_top] = cdst->mc_pg[cdst->mc_top];
@@ -6496,10 +6496,10 @@ mdb_rebalance(MDB_cursor *mc)
 			mc->mc_db->md_depth = 0;
 			mc->mc_db->md_leaf_pages = 0;
 			mdb_midl_append(&mc->mc_txn->mt_free_pgs, mp->mp_pgno);
+			/* Adjust cursors pointing to mp */
 			mc->mc_snum = 0;
 			mc->mc_top = 0;
 			{
-				/* Adjust other cursors pointing to mp */
 				MDB_cursor *m2, *m3;
 				MDB_dbi dbi = mc->mc_dbi;
 
@@ -6507,7 +6507,6 @@ mdb_rebalance(MDB_cursor *mc)
 					dbi--;
 
 				for (m2 = mc->mc_txn->mt_cursors[dbi]; m2; m2=m2->mc_next) {
-					if (m2 == mc) continue;
 					if (mc->mc_flags & C_SUB)
 						m3 = &m2->mc_xcursor->mx_cursor;
 					else
@@ -6537,12 +6536,11 @@ mdb_rebalance(MDB_cursor *mc)
 					dbi--;
 
 				for (m2 = mc->mc_txn->mt_cursors[dbi]; m2; m2=m2->mc_next) {
-					if (m2 == mc) continue;
 					if (mc->mc_flags & C_SUB)
 						m3 = &m2->mc_xcursor->mx_cursor;
 					else
 						m3 = m2;
-					if (m3->mc_snum < mc->mc_snum) continue;
+					if (m3 == mc || m3->mc_snum < mc->mc_snum) continue;
 					if (m3->mc_pg[0] == mp) {
 						m3->mc_pg[0] = mc->mc_pg[0];
 						m3->mc_snum = 1;
@@ -7069,11 +7067,12 @@ done:
 			dbi--;
 
 		for (m2 = mc->mc_txn->mt_cursors[dbi]; m2; m2=m2->mc_next) {
-			if (m2 == mc) continue;
 			if (mc->mc_flags & C_SUB)
 				m3 = &m2->mc_xcursor->mx_cursor;
 			else
 				m3 = m2;
+			if (m3 == mc)
+				continue;
 			if (!(m3->mc_flags & C_INITIALIZED))
 				continue;
 			if (m3->mc_flags & C_SPLITTING)