On 24 August 2016 at 06:27, Hallvard Breien Furuseth <
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:-)
Happy to do that.
MDB was originally written with Unix in mind. I'm guessing the
multiple handles" restriction is only relevant on Unix: Hopefully
Windows locking doesn't have flock's insane semantics.
But I don't know Windows.
According to the CreateFileW docs, calling the it without a security
descriptor (which is what LMDB does) makes it behave like O_CLOEXEC. So
it's only the POSIX code that is affected.
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
to a separate function to avoid excessive code and #ifdef duplication,
I've got a patch for this which I'll submit once I get approval.
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.
According to man 2 open O_DIRECT alignment is file system specific from 2.6
onwards, but "usually" 512bytes FWIW. Not sure how that would affect this