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Re: (ITS#8258) LMDB cursor_del followed by MDB_NEXT can trigger assert crash

steven.lang@hgst.com wrote:
> Full_Name: Steven Lang
> Version: LMDB 0.9.16
> OS: Ubuntu 14.04
> URL: ftp://ftp.openldap.org/incoming/steven-lang-150929.c
> Submission from: (NULL) (
> 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()
> Aborted
> 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.

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