[SVN-2078] Hook scripts hang if writing a lot to stderr

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.0.1
• Fix Version/s: 1.3.0
• Component/s: libsvn_repos

Environment

Linux

• Assignee: Unassigned
• Reporter: Subversion Importer
• Created: 05/Oct/04 23:04
• Updated: 27/Oct/18 16:39
• Resolved: 10/Mar/09 15:22

Description

I'm writing some hook scripts, and while debugging, at least, they write a lot
of text to stderr.  I noticed my scripts sometimes hang.  I believe I have
tracked it down to libsvn_repos/hooks.c.  The run_hook_cmd() code creates a pipe
and passes the write end of the pipe to the hook script as standard error.  It
then runs the hook, and if it fails, reads error information from the read end
of the pipe.  But if the pipe fills, the hook blocks while writing to the write
end of the pipe.  Since nobody is yet reading from the read end of the pipe, it
blocks indefinitely.  I don't know anything about the apr functions that are
used in this file, but is there a way to run the command asynchronously?  You
could then continue execution in hook.c and read everything from the read end of
the pipe while the hook runs.  When EOF is seen on the pipe, call some
(hopefully available) apr function like waitpid() to wait for the hook to
complete and return the exit status.  Then proceed as today by checking the exit
status.  This algorithm would ensure that the hook won't hang while writing to
the pipe.

If you want to limit the amount of text the hook can write, you can always start
throwing away data after a certain number of lines, to prevent a bad hook from
causing hook.c to consume too much memory.

Even documentating that hooks shouldn't write too much to stderr seems
inadequate to me.  In unusual situations (e.g., some required network connection
is down or some required filesystem isn't mounted), the hook could
unintentionally produce a lot of error messages.  You don't want the hook to
hang in a situation like this.

Original issue reported by jph

Issue Links

• is duplicated by: SVN-2661 Should read hook output before reaping.