(ITS#7754) Unaligned MDB_DUPSORT sub-pages
by h.b.furuseth＠usit.uio.no
Full_Name: Hallvard B Furuseth
Version: LMDB_0.9.10
OS: Linux x86_64
URL:
Submission from: (NULL) (81.191.45.35)
Submitted by: hallvard
Uneven-sized keys in MDB_DUPSORT databases get unaligned sub-pages,
and thus everything inside their sub-pages gets unaligned too.
A fix preserving the current format when !MISALIGNED_OK would be more
memcpying, e.g. in COPY_PGNO(). dcmp functions would do byte by byte
comparison for such sub-pages. MDB_page.mp_flags could be split
two uint8_t fields: mp_flags, mp_zero (zero for now anyway).
Renumber P_KEEP to 0x80 or P_META.
A fix with a format change would be to put the padding byte in front
of the sub-page instead of at the end.
One fix with a format change could be that the data item gets uneven
size too, with a padding byte in front of the sub-page.
The most general way would be to do that in all nodes with uneven-
sized key and even-sized data:
#define NODEDATA(node) ((void *) ((char *)(node)->mn_data + \
(node)->mn_ksize + ((node)->mn_ksize & ~(node)->mn_lo & 1)))
Or do it only with sub-pages. Make the data item uneven-sized so
code which is not interested in whether this is a sub-page, need
not know about the hack. Access the sub-page specially:
#define NODEPAGE(node) ((MDB_page *) \
((char *)(node)->mn_data + (((node)->mn_ksize + 1) & -2)))
That could be coded so the old format is still readable. Then a
utility can walk all databases and adjust sub-pages so they have
padding in front instead of at the end, so no dump/reload will be
needed:
/** Address of the possibly 2-byte aligned sub-page in a node.
* If the data item has uneven size, then the first byte is
* padding to re-align the sub-page after an uneven-sized key.
*/
#define NODEPAGE(node) ((MDB_page *) \
((char *)(node)->mn_data + (node)->mn_ksize + ((node)->mn_lo & 1)))
