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From Jeff Trawick <traw...@attglobal.net>
Subject Re: challenges creating processes from threaded environment?
Date Sat, 24 May 2003 11:34:12 GMT
William A. Rowe, Jr. wrote:
> At 08:02 PM 5/23/2003, Jeff Trawick wrote:

> OWWW!  That's a case I hadn't considered.  However, the right answer
> is to close the children's side of the pipes, always.

apr_proc_create() does that unconditionally in the parent path after fork()

it should be changed to register a cleanup-for-exec on the parent's side 
of the pipes...  I suspect that this particular hole can be filled 
immediately before mutexing and related API compatibility concerns are 
worked out, unless somebody can come up with what sort of app this would 
break

>  If they are left open,
> it's much harder to detect that the child has died/you've reached EOF.
> 
> 
>>For mod_ext_filter I can hold a thread mutex around the critical section  above, 
 >and that protects it from other instances of mod_ext_filter doing fork()
 >and inheriting the wrong handle, but it does nothing for other threads
 >calling apr_proc_create().
> 
> 
> The right answer, in part, is to close those child handles.

here I'm confused because it looks like apr_proc_create() already does that

>  The second side
> is to protect that window where we declare the handles as keep-open on
> exec to the point where we've already fork()ed.

agreed

>>It would seem that all of the pipe creation and child cleanup 
 >registration needs to be done inside apr_proc_create() so that
 >it can hold a mutex and all callers of apr_proc_create() will
 >be happy.  I didn't look at it to know if that would require
 >API changes.  I wouldn't be shocked.
> 
> 
> For the small window between declaring the handles keep-open,
> fork()ing the child and then closing them in the parent, we absolutely
> need a mutex to keep things clean.

yes; basically, get a mutex at start of apr_proc_create, then create 
pipes as noted in apr_procattr_io_set(), then fork() then close child 
side of handles and register cleanup-on-exec for other handles,
then release mutex

so apr_procattr_io_set() is changed to note what to do later in 
apr_proc_create() instead of actually creating the pipes

all the while, keep track of which if any function is permanently lost 
this way and also which changes require potential adjustment in the app :)


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