Re: apr_dbm and concurrency

On Mon, Sep 25, 2023 at 03:58:18PM +0100, Joe Orton wrote:
> It is unspecified whether the apr_dbm.h interface is safe to use for 
> multiple processes/threads having r/w access to a single database. 
> Results appear to be:
> 
> - sdbm, gdbm are safe
> - bdb, ndbm are not safe

I developed a better test case (event MPM + mod_dav + apr_dbm with 
parallel clients doing cycles of LOCK/UNLOCK)

sdbm, gdbm and bdb all failed.

gdbm and sdbm both use either fcntl- or flock-based locking, which 
doesn't work between threads. (apr_file_lock for sdbm)

Unfortunately lmdb is also unsafe at least because *opening* database 
files has races. This is documented under the Caveats section of 
http://www.lmdb.tech/doc/ (which also says it relies on flock-based 
locking, though I only hit issues with it failing during open in 
testing)

The only ways forward I see are either to give up (declare that apr_dbm 
is not safe to use from threads) which passes the buck to httpd, or 
implement a locking layer inside apr_dbm() and don't assume the 
underlying drivers do thread-safe locking at all.