ITS#7515 Fix tracking of parent txn's cursors.

Restore mc_flags and xcursors, they were tracked but not merged. Simplify: Track parent txn's original cursors after backing them up, instead of tracking copies and merging them back at commit.

Author: hfuru

libraries/liblmdb/mdb.c

@@ -879,8 +879,8 @@ struct MDB_xcursor;
 struct MDB_cursor {
 /** Next cursor on this DB in this txn */
 MDB_cursor*mc_next;
-/** Original cursor if this is a shadow */
-MDB_cursor*mc_orig;
+/** Backup of the original cursor if this cursor is a shadow */
+MDB_cursor*mc_backup;
 /** Context used for databases with #MDB_DUPSORT, otherwise NULL */
 struct MDB_xcursor*mc_xcursor;
 /** The transaction that owns this cursor */
@@ -1633,57 +1633,35 @@ mdb_env_sync(MDB_env *env, int force)
 return rc;
 }
 
-/** Make shadow copies of all of parent txn's cursors */
+/** Back up parent txn's cursors, then grab the originals for tracking */
 static int
 mdb_cursor_shadow(MDB_txn *src, MDB_txn *dst)
 {
-MDB_cursor *mc, *m2;
-unsigned int i, j, size;
+MDB_cursor *mc, *bk;
+MDB_xcursor *mx;
+size_t size;
+int i;
 
-for (i=0;i<src->mt_numdbs; i++) {
-if (src->mt_cursors[i]) {
+for (i = src->mt_numdbs; --i >= 0; ) {
+if ((mc = src->mt_cursors[i]) != NULL) {
 size = sizeof(MDB_cursor);
-if (src->mt_cursors[i]->mc_xcursor)
+if (mc->mc_xcursor)
 size += sizeof(MDB_xcursor);
-for (m2 = src->mt_cursors[i]; m2; m2=m2->mc_next) {
-mc = malloc(size);
-if (!mc)
+for (; mc; mc = bk->mc_next) {
+bk = malloc(size);
+if (!bk)
 return ENOMEM;
-mc->mc_orig = m2;
-mc->mc_txn = dst;
-mc->mc_dbi = i;
+*bk = *mc;
+mc->mc_backup = bk;
 mc->mc_db = &dst->mt_dbs[i];
-mc->mc_dbx = m2->mc_dbx;
-mc->mc_dbflag = &dst->mt_dbflags[i];
-mc->mc_snum = m2->mc_snum;
-mc->mc_top = m2->mc_top;
-mc->mc_flags = m2->mc_flags;
-for (j=0; j<mc->mc_snum; j++) {
-mc->mc_pg[j] = m2->mc_pg[j];
-mc->mc_ki[j] = m2->mc_ki[j];
-}
-if (m2->mc_xcursor) {
-MDB_xcursor *mx, *mx2;
-mx = (MDB_xcursor *)(mc+1);
-mc->mc_xcursor = mx;
-mx2 = m2->mc_xcursor;
-mx->mx_db = mx2->mx_db;
-mx->mx_dbx = mx2->mx_dbx;
-mx->mx_dbflag = mx2->mx_dbflag;
-mx->mx_cursor.mc_txn = dst;
-mx->mx_cursor.mc_dbi = mx2->mx_cursor.mc_dbi;
-mx->mx_cursor.mc_db = &mx->mx_db;
-mx->mx_cursor.mc_dbx = &mx->mx_dbx;
-mx->mx_cursor.mc_dbflag = &mx->mx_dbflag;
-mx->mx_cursor.mc_snum = mx2->mx_cursor.mc_snum;
-mx->mx_cursor.mc_top = mx2->mx_cursor.mc_top;
-mx->mx_cursor.mc_flags = mx2->mx_cursor.mc_flags;
-for (j=0; j<mx2->mx_cursor.mc_snum; j++) {
-mx->mx_cursor.mc_pg[j] = mx2->mx_cursor.mc_pg[j];
-mx->mx_cursor.mc_ki[j] = mx2->mx_cursor.mc_ki[j];
-}
-} else {
-mc->mc_xcursor = NULL;
+/* Kill pointers into src - and dst to reduce abuse: The
+ * user may not use mc until dst ends. Otherwise we'd...
+ */
+mc->mc_txn = NULL;/* ...set this to dst */
+mc->mc_dbflag = NULL;/* ...and &dst->mt_dbflags[i] */
+if ((mx = mc->mc_xcursor) != NULL) {
+*(MDB_xcursor *)(bk+1) = *mx;
+mx->mx_cursor.mc_txn = NULL; /* ...and dst. */
 }
 mc->mc_next = dst->mt_cursors[i];
 dst->mt_cursors[i] = mc;
@@ -1693,32 +1671,40 @@ mdb_cursor_shadow(MDB_txn *src, MDB_txn *dst)
 return MDB_SUCCESS;
 }
 
-/** Close this write txn's cursors, after optionally merging its shadow
- * cursors back into parent's.
+/** Close this write txn's cursors, give parent txn's cursors back to parent.
  * @param[in] txn the transaction handle.
- * @param[in] merge zero to not merge cursors, non-zero to merge.
+ * @param[in] merge true to keep changes to parent cursors, false to revert.
  * @return 0 on success, non-zero on failure.
  */
 static void
 mdb_cursors_close(MDB_txn *txn, unsigned merge)
 {
-MDB_cursor **cursors = txn->mt_cursors, *mc, *next;
-int i, j;
+MDB_cursor **cursors = txn->mt_cursors, *mc, *next, *bk;
+MDB_xcursor *mx;
+int i;
 
 for (i = txn->mt_numdbs; --i >= 0; ) {
 for (mc = cursors[i]; mc; mc = next) {
-next = mc->mc_next;
-if (merge && mc->mc_orig) {
-MDB_cursor *m2 = mc->mc_orig;
-m2->mc_snum = mc->mc_snum;
-m2->mc_top = mc->mc_top;
-for (j = mc->mc_snum; --j >= 0; ) {
-m2->mc_pg[j] = mc->mc_pg[j];
-m2->mc_ki[j] = mc->mc_ki[j];
-}
+next = mc->mc_next;
+if ((bk = mc->mc_backup) != NULL) {
+if (merge) {
+/* Commit changes to parent txn */
+mc->mc_next = bk->mc_next;
+mc->mc_backup = bk->mc_backup;
+mc->mc_txn = bk->mc_txn;
+mc->mc_db = bk->mc_db;
+mc->mc_dbflag = bk->mc_dbflag;
+if ((mx = mc->mc_xcursor) != NULL)
+mx->mx_cursor.mc_txn = bk->mc_txn;
+} else {
+/* Abort nested txn */
+*mc = *bk;
+if ((mx = mc->mc_xcursor) != NULL)
+*mx = *(MDB_xcursor *)(bk+1);
 }
-/* Only malloced cursors are permanently tracked. */
-free(mc);
+mc = bk;
+}
+free(mc);
 }
 cursors[i] = NULL;
 }
@@ -5867,7 +5853,7 @@ mdb_xcursor_init1(MDB_cursor *mc, MDB_node *node)
 static void
 mdb_cursor_init(MDB_cursor *mc, MDB_txn *txn, MDB_dbi dbi, MDB_xcursor *mx)
 {
-mc->mc_orig = NULL;
+mc->mc_backup = NULL;
 mc->mc_dbi = dbi;
 mc->mc_txn = txn;
 mc->mc_db = &txn->mt_dbs[dbi];
@@ -5961,7 +5947,7 @@ mdb_cursor_count(MDB_cursor *mc, size_t *countp)
 void
 mdb_cursor_close(MDB_cursor *mc)
 {
-if (mc != NULL) {
+if (mc && !mc->mc_backup) {
 /* remove from txn, if tracked */
 if ((mc->mc_flags & C_UNTRACK) && mc->mc_txn->mt_cursors) {
 MDB_cursor **prev = &mc->mc_txn->mt_cursors[mc->mc_dbi];