On Oct 3, 2008, at 1:03 PM, William A. Rowe, Jr. wrote:
If we compare this to apr_shm_create / apr_shm_attach, it's pretty
clear we
are just missing the mechanics to solve this. apr_proc|
global_mutex_attach
is missing, and this should just be mopped up for apr 1.4.0, and
absolutely
before 2.0.0.
The other solutions that come to mind are all very, very crufty
hacks against
specific solutions, e.g. an apr_os_proc_mutex_put sort of solution.
It really
is the API that needs repair.
Assuming Bill's right and this is actually a missing API, let me add
some urgency with a use case.
What I'm doing is implementing PUT, for the AtomPub protocol, against
a file-backed store. Since it's file-backed, it fits very nicely in
with httpd's default code paths.
For PUT, I want to be able to support conditional update with ETags
and If-Match headers; done correctly, this can guarantee against lost
updates. But this totally obviously needs a critical section. After
you've done map_to_storage to find the file your r->uri maps to, you
*really* need to grab a global_mutex against that file while you
compute the ETag and compare it to the If-Match value, and either
reject or accept-and-implement the PUT.
Summary: you can't implement PUT correctly inside httpd unless you can
create and attach to arbitrary mutexes at runtime.
I've actually figured out a way to kludge around this, but it's kinda
gross.
-Tim
