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Re: Using LMDB safely with fork() and exec()

On 22/08/16 12:47, Lorenz Bauer wrote:

The LMDB documentation says the following in its section on caveats:


* Use an MDB_env* in the process which opened it, without fork()ing.
* Do not have open an LMDB database twice in the same process at the same time. Not even from a plain open() call - close()ing it breaks flock() advisory locking.


This seems contrary to an earlier thread on this list (1), which
suggests that fork/execing a process using LMDB is OK so long as the
MDB_env is not used in the forked process.


It's just poorly worded.  The point of the first sentence is,
"do not use the MDB_env after forking".  It's OK to fork if you
do not use it afterwards.  A patch with a better wording might
be in order:-)

Also I'm guessing that restriction is only relevant on Unix:
Hopefully Windows locking doesn't have flock's insane semantics.
I don't know Windows though.


(...)
* Seems like there is currently no call to SetHandleInformation with
HANDLE_FLAG_INHERIT=0 for Winows, should that be added?


MDB was originally written with Unix in mind.  I'm guessing the "no
multiple handles" restriction is only relevant on Unix: Hopefully
Windows locking doesn't have flock's insane semantics.
But I don't know Windows.

OTOH it won't hurt to add close-on-fork/exec flags for everything, not
just the Unix lockfile descriptor.  Would need to factor the opens out
to a separate function to avoid excessive code and #ifdef duplication,
I never got around to it.  (Or rather, I drafted something but it got
complicated and I didn't want to spend time testing it on Windows.)

Regarding your next message:

The unix version only uses O_DIRECT if psize >= OS psize
because O_DIRECT typically requires alignment on OS page
boundaries, or something like that.  Should be commented.
Didn't find anything similar in the Windows doc, but again,
I don't know Windows.  Maybe Howard knows more.