[fuse-devel] autofs hangs with fuse

[John S. <John.Salmon@DEShawResearch.com>]

My colleagues and I just reported a bug to the autofs maintainers that 
may be of interest to fuse developers:

https://bugzilla.redhat.com/show_bug.cgi?id=1509088

Basically, the Linux automount daemon has a small window during which it 
incorrectly leaves file descriptors open when it fork/execs mount, and 
then locks up the entire automount infrastructure until those file 
descriptors are closed.  When automount mounts a fuse filesystem, those 
file descriptors stay open when mount execs the fuse daemon, and when 
the daemon calls fuse_daemonize.  Nothing can be automounted until the 
fuse daemon terminates.

This is clearly a bug in the automounter.  It's in no way fuse's fault.  
But it's possible for fuse to work around it.  In our fuse daemon, we've 
implemented:

         if(BuggyAutomountWorkaround){
             struct rlimit rl;
             getrlimit(RLIMIT_NOFILE, &rl);
             int rlopen = rl.rlim_cur;
             if(rlopen > 16384){
                 syslog(LOG_NOTICE, "Enough is enough... We're only 
closing 16k files for BuggyAutomountWorkaround even though rlim_cur is 
%d\n",
                        rlopen);
                 rlopen = 16384;
             }
             for(int fd=3; fd<rlopen; ++fd){
                 if( close(fd) == 0 ) // N.B.  These are expected to 
fail.  They *should* fail!
                     syslog(LOG_NOTICE, "Closed fd=%d.  Automount 
probably dodged a bullet.", fd);
             }
         }

Other fuse application developers might wish to do something similar if 
they expect that their filesystems may be automounted on Linux. The 
libfuse developers might also consider putting something similar in 
fuse_daemonize.

John Salmon

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