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(ITS#8021) Wrong mdb_env_sync() with at least MDB_RDONLY

Full_Name: Hallvard B Furuseth
Version: lmdb 0.9.14
Submission from: (NULL) (
Submitted by: hallvard

mdb_env_sync() works with MDB_RDONLY at least on Unix, but
it syncs MDB_WRITEMAP changes incorrectly: fdatasync instead
of msync because mdb_env_open() cleared MDB_WRITEMAP.

Users are likely doing the same thing anyway: Using MDB_RDONLY

Finally, writers committing with and without WRITEMAP will
sync each others' commits incorrectly.  lmdb.h does warn they
will "not cooperate well", but it should warn it breaks ACID.

LMDBv2 can instead fix all this with 2 bits in MDB_meta:
"Changed with/without WRITEMAP since last sync".

In LMDBv1, I think the smallest-impact fix is for env_sync
to both msync and fdatasync when RDONLY is set and WRITEMAP
is clear.  A greedier fix would be to always sync both ways
when the user calls env_sync.

Either way, it may be best to keep WRITEMAP when RDONLY.
Only env_map() needs to deal with that.

Pushing suggestions except doc to "mdb/wmap-fix" in my repo.