Re: [PATCH v4 10/12] unix-socket: create `unix_stream_server__listen_with_lock()`

[Jeff King <peff@peff.net>]

On Wed, Feb 17, 2021 at 09:48:46PM +0000, Jeff Hostetler via GitGitGadget wrote:

> From: Jeff Hostetler <jeffhost@microsoft.com>
> 
> Create a version of `unix_stream_listen()` that uses a ".lock" lockfile
> to create the unix domain socket in a race-free manner.

The "unix_stream_server__listen_with_lock" name is quite a mouthful.  My
first question was: don't we have an "options" struct that we can use to
tell it we're interested in using the locking strategy?

But I do find it a little weird for the feature to be at this layer at
all. I'd have thought it would make more sense in the simple-ipc layer
that implements the unix-socket backend, where app-level logic like
"it's OK to just connect to this socket and hang up in order to ping it"
might be more appropriate. We might even want to have a more robust
check (e.g., an actual "ping" that expects the server to say "yes, I'm
here").

(But also see below where I am less certain about this...)

> Unix domain sockets have a fundamental problem on Unix systems because
> they persist in the filesystem until they are deleted.  This is
> independent of whether a server is actually listening for connections.
> Well-behaved servers are expected to delete the socket when they
> shutdown.  A new server cannot easily tell if a found socket is
> attached to an active server or is leftover cruft from a dead server.
> The traditional solution used by `unix_stream_listen()` is to force
> delete the socket pathname and then create a new socket.  This solves
> the latter (cruft) problem, but in the case of the former, it orphans
> the existing server (by stealing the pathname associated with the
> socket it is listening on).

Nicely explained.

> We cannot directly use a .lock lockfile to create the socket because
> the socket is created by `bind(2)` rather than the `open(2)` mechanism
> used by `tempfile.c`.
> 
> As an alternative, we hold a plain lockfile ("<path>.lock") as a
> mutual exclusion device.  Under the lock, we test if an existing
> socket ("<path>") is has an active server.  If not, create a new
> socket and begin listening.  Then we rollback the lockfile in all
> cases.

Make sense.

> +static int is_another_server_alive(const char *path,
> +				   const struct unix_stream_listen_opts *opts)
> +{
> +	struct stat st;
> +	int fd;
> +
> +	if (!lstat(path, &st) && S_ISSOCK(st.st_mode)) {
> +		/*
> +		 * A socket-inode exists on disk at `path`, but we
> +		 * don't know whether it belongs to an active server
> +		 * or whether the last server died without cleaning
> +		 * up.
> +		 *
> +		 * Poke it with a trivial connection to try to find
> +		 * out.
> +		 */
> +		fd = unix_stream_connect(path, opts->disallow_chdir);
> +		if (fd >= 0) {
> +			close(fd);
> +			return 1;
> +		}
> +	}

The lstat() seems redundant here. unix_stream_connect() will tell us
whether there is something to connect to or not. (It's also racy with
respect to the actual connect, but since you're doing this under lock, I
don't think that matters).

> +struct unix_stream_server_socket *unix_stream_server__listen_with_lock(
> +	const char *path,
> +	const struct unix_stream_listen_opts *opts)
> +{
> +	struct lock_file lock = LOCK_INIT;
> +	int fd_socket;
> +	struct unix_stream_server_socket *server_socket;
> +
> +	/*
> +	 * Create a lock at "<path>.lock" if we can.
> +	 */
> +	if (hold_lock_file_for_update_timeout(&lock, path, 0,
> +					      opts->timeout_ms) < 0) {
> +		error_errno(_("could not lock listener socket '%s'"), path);
> +		return NULL;
> +	}

Would you want to ping to see if it's alive before creating the lock?
That would be the fast-path if we assume that a server will usually be
there once started. Or is that supposed to happen in the caller (in
which case I'd again wonder if this really should be happening in the
simple-ipc code).

> +	/*
> +	 * If another server is listening on "<path>" give up.  We do not
> +	 * want to create a socket and steal future connections from them.
> +	 */
> +	if (is_another_server_alive(path, opts)) {
> +		errno = EADDRINUSE;
> +		error_errno(_("listener socket already in use '%s'"), path);
> +		rollback_lock_file(&lock);
> +		return NULL;
> +	}

Wouldn't this be a "success" case for a caller? They did not open the
server themselves, but they are presumably happy that there is one there
now to talk to. So do we actually want to print an error to stderr?
Likewise, how do they tell the difference between this NULL and the NULL
we returned above because we couldn't take the lock? Or the NULL we
return below because there is some error creating a listening socket?

I'd think in those three cases you'd want:

  - if lock contention, pause a moment and wait for the winner to spin
    up and serve requests

  - if another server is live while we hold the lock, then we raced them
    and they won. Release the lock and start using them.

  - if we really tried to call unix_stream_listen() and that failed,
    give up now. There is some system error that is not likely to be
    fixed by trying anything more (e.g., ENAMETOOLONG).

> +	server_socket = xcalloc(1, sizeof(*server_socket));
> +	server_socket->path_socket = strdup(path);
> +	server_socket->fd_socket = fd_socket;

What do we need this server_socket for? The caller already knows the
path; they fed it to us. We do need to return the descriptor, but we
could do that directly.

> +	lstat(path, &server_socket->st_socket);

This lstat I guess is part of your "periodically check to see if we're
still the one holding the socket" strategy. We _shouldn't_ need that
anymore, with the dotlocking, but I'm OK with it as a
belt-and-suspenders check. But why are we filling in the lstat here?
This seems like something that the unix-socket code doesn't really need
to know about (though you do at least provide the complementary
"was_stolen" function here, so that part makes sense).

Again, I guess I'd find it less weird if it were happening at a layer
above. Maybe I'm really just complaining that this is in unix-socket.c.
I guess it is a separate unix_stream_server data type. Arguably that
should go in a separate file, but I guess the whole conditional
compilation of unix-socket.c makes that awkward. So maybe this is the
least-bad thing.

> +	/*
> +	 * Always rollback (just delete) "<path>.lock" because we already created
> +	 * "<path>" as a socket and do not want to commit_lock to do the atomic
> +	 * rename trick.
> +	 */
> +	rollback_lock_file(&lock);
> +
> +	return server_socket;
> +}

OK, this part makes sense to me.

> +void unix_stream_server__free(
> +	struct unix_stream_server_socket *server_socket)
> +{
> +	if (!server_socket)
> +		return;
> +
> +	if (server_socket->fd_socket >= 0) {
> +		if (!unix_stream_server__was_stolen(server_socket))
> +			unlink(server_socket->path_socket);
> +		close(server_socket->fd_socket);
> +	}
> +
> +	free(server_socket->path_socket);
> +	free(server_socket);
> +}

OK, this makes sense. We only remove it if we're still the ones holding
it. That's not done under lock, though, so it's possibly racy (somebody
steals from us while _they_ hold the lock; we check and see "not stolen"
right before they steal it, and then we unlink their stolen copy).

> +int unix_stream_server__was_stolen(
> +	struct unix_stream_server_socket *server_socket)
> +{
> +	struct stat st_now;
> +
> +	if (!server_socket)
> +		return 0;
> +
> +	if (lstat(server_socket->path_socket, &st_now) == -1)
> +		return 1;
> +
> +	if (st_now.st_ino != server_socket->st_socket.st_ino)
> +		return 1;
> +
> +	/* We might also consider the ctime on some platforms. */
> +
> +	return 0;
> +}

You probably should confirm that st.dev matches, too, since that is the
namespace for st.ino. Maybe also double check that it's still a socket
with S_ISSOCK(st_mode)?

-Peff