On May 13, 2025, at 8:23 PM, Vincent Veyron <vv.li...@wanadoo.fr> wrote:
> Is there a way I could do :
>
> $r->print('<h3>Building tar file</h3>') ;
>
> every second or other until the tar file is built? I can't seem to think of
> one.
>
> My handler calls an sql script that dumps the database with system() :
>
> @args = ('psql', '-f', '/path/to/script/to/export_raw_data.sql', '-v',
> 'id_client=' . $r->pnotes('session')->{id_client}, '-v', 'database=' .
> $database, 'postgres') ;
>
> system(@args) == 0 or warn "system @args failed: $!" ;
>
> then waits for the resulting file before proceeding to compress and tar it,
> also with a system() call.
>
> How can I wrap this in a loop that re-sends the 'Building tar file' message
> until the file exists?
Well, the standard way of doing that is you fork a child process to run the
system command while the parent process loops and prints a period (or whatever
you prefer) every 1 second (make sure you call flush() after printing!) while
waiting for the child process to finish. Lots of good examples of this can be
found here:
https://stackoverflow.com/questions/3193091/showing-progress-whilst-running-a-system-command-in-perl/
The problem with this is that forking from a mod_perl process is more
complicated than it is in a normal Perl process. The usual advice is to use a
module called Apache2::SubProcess instead:
https://metacpan.org/pod/Apache2::SubProcess
But I don't know that that will work for what you want to do. I've never
actually used that module, and I can't find an example in my Internet searches
that shows the parent process doing something while the subprocess executes.
Maybe try the example using detach_script.pl and then modify that
detach_script.pl to build the tar file or execute psql or do whatever you want.
If that doesn't work, you can still fork from mod_perl, but you just need to
know how to do it properly. Make sure you read this:
https://stackoverflow.com/questions/471681/how-do-i-fork-properly-with-mod-perl2/
Since I'm not convinced Apache2::SubProcess would actually work here, I would
probably go with the "fork properly with mod_perl2" method, personally.
I also recommend that you still put an upper time limit on your subprocess and
not just loop infinitely. 30 minutes to an hour seems like a reasonable choice.
Depends on how big your tar files can be, of course.
When/if you get something along these lines working the way you want, please
share your solution here on the mailing list for others to learn from.
Hope this helps,
Ed
