Full_Name: Steven Lang
Version: LMDB 0.9.16
OS: Ubuntu 14.04
Submission from: (NULL) (18.104.22.168)
While doing some testing using randomized key and data lengths and random puts
and deletes, I found a situation in which mdb_cursor_del(...) followed by
mdb_cursor_get(..., MDB_NEXT) causes an error:
mdb.c:5726: Assertion 'IS_LEAF(mp)' failed in mdb_cursor_next()
After a little debugging I was able to isolate what the cause was and produce a
simple program to reproduce the issue; during the re-balance following a delete,
it would try to move a node from the first child of the root page to the second.
If the node it moves has a key longer than the second node in the root page, and
the new key doesn't fit, it will split the root page. As a result, by deleting a
node, the tree becomes deeper.
However, the cursor only checks and compensates for the tree becoming shallower.
So now the cursor was pointing to a branch page rather than a leaf page. Any
further cursor operations fail. (Trying to do a MDB_SET, MDB_SET_RANGE, etc will
silently fail at this point, while MDB_NEXT and possibly MDB_PREV will assert.)
The latest head with the fix for ITS#8221 changes the behavior slightly, due to
the different arrangement of the tree with the less aggressive merging. However,
it still fails.
Attached is a program which fills a DB, then deletes keys until it asserts.
Thanks for the report. Our alternate approach to solve #8221 was to check the
page fill factor including the (usually omitted) key #0 size; we didn't go
that way because it was slightly more processing. But given this bug, it seems
that's the only way to go.
-- Howard Chu
CTO, Symas Corp.