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From Jeff Trawick <traw...@attglobal.net>
Subject Re: challenges creating processes from threaded environment?
Date Tue, 27 May 2003 17:40:33 GMT
another general problem:

between the time a descriptor is allocated by the kernel (by open(), 
pipe(), socket(), accept(), etc.) and the time that a child cleanup can 
be registered for it, apr_proc_create() could have fork()-ed and allowed 
a descriptor to be inappropriately inherited

in order to fix this problem in general:

Establish a critical section covering the path between descriptor 
allocation and registration of the cleanup; any number of threads 
allocating descriptors can enter that critical section at any given 
time, but a thread performing a fork() must have exclusive access 
(read-only access vs. read-write access).

APIs that allocate descriptors may allow the caller to specify whether 
or not the file should be inherited, and APIs without such a parameter 
default to not-inherited and the user can call apr_foo_inherit_set(), as 
with apr_file_open() currently.

issues:

open() and accept() are potentially blocking...  we can't hold the mutex 
for this critical section across an open() or accept() since that could 
deadlock the APR application.  Thus, the descriptor would be allocated 
by the kernel and inherited by new processes created by other threads in 
the window prior to registering a child cleanup.

hack for open(): disable blocking on the open of a named pipe by passing 
O_NONBLOCK (not without side-effects)

hack for accept() on a blocking socket: make it non-blocking 
temporarily, wait in select() or poll() until appropriate time, get 
mutex, call accept(), release mutex

--/--

back to the original complaint about pipes created by 
apr_procattr_io_set() and the related apr_proc_create()...

the client side of those pipes actually must be inherited by the one new 
process intended to use them, so whatever was done to avoid arbitrary 
processes inheriting them must be undone for the intended process in 
apr_proc_create() while holding the critical section


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