This solves a problem where sometimes FEX would spin up a new process
while FEXMountDaemon was in the process of shutting down. Breaking
things on both sides.
Now the race conditions are squashed that I could see.
Also fixes one race where FEX is starting up and FEXMountDaemon is
spinning up. This case is where FEX managed to pull the lock file just
before it got deleted. Then sent the FEXMountDaemon a request to be
observed. With DGRAM sockets we would fire and forget. Use a STREAM with
a ack result so we know we can return.
The FEXMountDaemon no longer uses the inotify interface for refcounting
instances of FEX.
The inotify interface fails to send close events when an application
crashes. Which is either an API oversight or intentional choice.
Now to use FEXMountDaemon the FEX process must send the daemon a pipe
fd.
The FEXMountDaemon then uses the write end of the pipe to determine if
the read end of the pipe is still open. It does this using the epoll API
and ref counting how many pipes are still active.
This is possible since epoll will tell us if pipe status has changed to
error. Signalling to the write end that the read end has closed for
whatever reason.
Now we only use the "lock" file to remove races and tell the new
instances of FEX where the rootfs is mounted
Now that we deparent FEXMountDaemon we can no longer use
PR_SET_PDEATHSIG.
Instead we rely on the ref counting and checking the pipe status to see
if the original parent has left us.
Further improvements that could be done in the future is that every user
of the mount point talks to the daemon to give it a pipe to check is
still live, since the ref counting sometimes is incorrect.
This allows FEXMountDaemon to remove FEXInterpreter as its parent.
Instead becoming the parent of whatever the current reaper process is.
Do it as early as possible this way FEXInterpreter won't get an
erroneous SIGCHLD.
This isn't quite a 100% clean sweep of IWYU.
There are some false positives where clang fails.
Additionally there are still a few missed in the frontend side of things
that I didn't get to
Instead of watching to ensure our parent process is still alive. Mount the
squashfs once and use a combination of file leases and inotify to
ref count how many processes are using the rootfs.
This makes it so in the common case, the FEXMountDaemon only ever executes once
and runs until all FEX processes stop running.
In the rare edge case there is a race condition where multiple FEXMountDaemon
applications will start, but only one will end up mounting the squashfs.
In this case, one application wins and the one that failed to grab the lease
will wait until the other one completes.
With this change, squashfs should be reasonable to use now.
This is a tool that communicates with FEXLoader/FEXInterpreter to automatically mount
squashfs based rootfs files on execve.
This tool will launch automatically if you have a squashfs based rootfs selected.
Once the rootfs is mounted, it will watch for the parent FEX processe to completely exit.
Once the parent FEX exits it will unmount and cleanup after itself.
This tool has a dependency on your host having FUSE, fusermount, and squashfuse applications.
If anything goes wrong in the bringup process then it propagates the erro up the chain and will
let FEX know that it couldn't mount.
Ryan Houdek