Clean up and comment C_UNTRACK

Don't use it as a "cursor is tracked" hint in mdb_pages_xkeep().
It's been harmless so far, but would break after mdb_cursor_copy().
Checking m0 directly short-circuits better anyway.

libraries/liblmdb/mdb.c

@@ -2094,13 +2094,9 @@ mdb_pages_xkeep(MDB_cursor *mc, unsigned pflags, int all)
 	unsigned i, j;
 	int rc = MDB_SUCCESS, level;
 
-	/* Mark pages seen by cursors */
-	if (mc->mc_flags & C_UNTRACK)
-		mc = NULL;				/* will find mc in mt_cursors */
-	for (i = txn->mt_numdbs;; mc = txn->mt_cursors[--i]) {
-		for (; mc; mc=mc->mc_next) {
-			if (!(mc->mc_flags & C_INITIALIZED))
-				continue;
+	/* Mark pages seen by cursors: First m0, then tracked cursors */
+	for (i = txn->mt_numdbs;; ) {
+		if (mc->mc_flags & C_INITIALIZED) {
 			for (m3 = mc;; m3 = &mx->mx_cursor) {
 				mp = NULL;
 				for (j=0; j<m3->mc_snum; j++) {
@@ -2119,10 +2115,13 @@ mdb_pages_xkeep(MDB_cursor *mc, unsigned pflags, int all)
 					break;
 			}
 		}
-		if (i == 0)
-			break;
+		mc = mc->mc_next;
+		for (; !mc || mc == m0; mc = txn->mt_cursors[--i])
+			if (i == 0)
+				goto mark_done;
 	}
 
+mark_done:
 	if (all) {
 		/* Mark dirty root pages */
 		for (i=0; i<txn->mt_numdbs; i++) {
@@ -8442,7 +8441,10 @@ mdb_cursor_close(MDB_cursor *mc)
 		MDB_CURSOR_UNREF(mc, 0);
 	}
 	if (mc && !mc->mc_backup) {
-		/* remove from txn, if tracked */
+		/* Remove from txn, if tracked.
+		 * A read-only txn (!C_UNTRACK) may have been freed already,
+		 * so do not peek inside it.  Only write txns track cursors.
+		 */
 		if ((mc->mc_flags & C_UNTRACK) && mc->mc_txn->mt_cursors) {
 			MDB_cursor **prev = &mc->mc_txn->mt_cursors[mc->mc_dbi];
 			while (*prev && *prev != mc) prev = &(*prev)->mc_next;
@@ -9287,7 +9289,6 @@ mdb_del0(MDB_txn *txn, MDB_dbi dbi,
 		 * run out of space, triggering a split. We need this
 		 * cursor to be consistent until the end of the rebalance.
 		 */
-		mc.mc_flags |= C_UNTRACK;
 		mc.mc_next = txn->mt_cursors[dbi];
 		txn->mt_cursors[dbi] = &mc;
 		rc = mdb_cursor_del(&mc, flags);