git/unix-stream-server.c

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#include "git-compat-util.h"
unix-stream-server: create unix domain socket under lock Create a wrapper class for `unix_stream_listen()` that uses a ".lock" lockfile to create the unix domain socket in a race-free manner. 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). 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, we create a new socket and begin listening. Then we use "rollback" to delete the lockfile in all cases. This wrapper code conceptually exists at a higher-level than the core unix_stream_connect() and unix_stream_listen() routines that it consumes. It is isolated in a wrapper class for clarity. Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-15 21:08:27 +00:00
#include "lockfile.h"
#include "unix-socket.h"
#include "unix-stream-server.h"
#define DEFAULT_LOCK_TIMEOUT (100)
/*
* Try to connect to a unix domain socket at `path` (if it exists) and
* see if there is a server listening.
*
* We don't know if the socket exists, whether a server died and
* failed to cleanup, or whether we have a live server listening, so
* we "poke" it.
*
* We immediately hangup without sending/receiving any data because we
* don't know anything about the protocol spoken and don't want to
* block while writing/reading data. It is sufficient to just know
* that someone is listening.
*/
static int is_another_server_alive(const char *path,
const struct unix_stream_listen_opts *opts)
{
int fd = unix_stream_connect(path, opts->disallow_chdir);
if (fd >= 0) {
close(fd);
return 1;
}
return 0;
}
int unix_ss_create(const char *path,
const struct unix_stream_listen_opts *opts,
long timeout_ms,
struct unix_ss_socket **new_server_socket)
{
struct lock_file lock = LOCK_INIT;
int fd_socket;
struct unix_ss_socket *server_socket;
*new_server_socket = NULL;
if (timeout_ms < 0)
timeout_ms = DEFAULT_LOCK_TIMEOUT;
/*
* Create a lock at "<path>.lock" if we can.
*/
if (hold_lock_file_for_update_timeout(&lock, path, 0, timeout_ms) < 0)
return -1;
/*
* 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)) {
rollback_lock_file(&lock);
errno = EADDRINUSE;
return -2;
}
/*
* Create and bind to a Unix domain socket at "<path>".
*/
fd_socket = unix_stream_listen(path, opts);
if (fd_socket < 0) {
int saved_errno = errno;
rollback_lock_file(&lock);
errno = saved_errno;
return -1;
}
server_socket = xcalloc(1, sizeof(*server_socket));
server_socket->path_socket = strdup(path);
server_socket->fd_socket = fd_socket;
lstat(path, &server_socket->st_socket);
*new_server_socket = server_socket;
/*
* 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 0;
}
void unix_ss_free(struct unix_ss_socket *server_socket)
{
if (!server_socket)
return;
if (server_socket->fd_socket >= 0) {
if (!unix_ss_was_stolen(server_socket))
unlink(server_socket->path_socket);
close(server_socket->fd_socket);
}
free(server_socket->path_socket);
free(server_socket);
}
int unix_ss_was_stolen(struct unix_ss_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;
if (st_now.st_dev != server_socket->st_socket.st_dev)
return 1;
if (!S_ISSOCK(st_now.st_mode))
return 1;
return 0;
}