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

["Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>]

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.

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, create a new
socket and begin listening.  Then we rollback the lockfile in all
cases.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 unix-socket.c | 115 ++++++++++++++++++++++++++++++++++++++++++++++++++
 unix-socket.h |  29 +++++++++++++
 2 files changed, 144 insertions(+)

diff --git a/unix-socket.c b/unix-socket.c
index 1eaa8cf759c0..647bbde37f97 100644
--- a/unix-socket.c
+++ b/unix-socket.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "lockfile.h"
 #include "unix-socket.h"
 
 static int chdir_len(const char *orig, int len)
@@ -132,3 +133,117 @@ int unix_stream_listen(const char *path,
 	errno = saved_errno;
 	return -1;
 }
+
+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;
+		}
+	}
+
+	return 0;
+}
+
+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;
+	}
+
+	/*
+	 * 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;
+	}
+
+	/*
+	 * Create and bind to a Unix domain socket at "<path>".
+	 */
+	fd_socket = unix_stream_listen(path, opts);
+	if (fd_socket < 0) {
+		error_errno(_("could not create listener socket '%s'"), path);
+		rollback_lock_file(&lock);
+		return NULL;
+	}
+
+	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);
+
+	/*
+	 * 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;
+}
+
+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);
+}
+
+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;
+}
diff --git a/unix-socket.h b/unix-socket.h
index 2c0b2e79d7b3..8faf5b692f90 100644
--- a/unix-socket.h
+++ b/unix-socket.h
@@ -2,14 +2,17 @@
 #define UNIX_SOCKET_H
 
 struct unix_stream_listen_opts {
+	long timeout_ms;
 	int listen_backlog_size;
 	unsigned int disallow_chdir:1;
 };
 
+#define DEFAULT_UNIX_STREAM_LISTEN_TIMEOUT (100)
 #define DEFAULT_UNIX_STREAM_LISTEN_BACKLOG (5)
 
 #define UNIX_STREAM_LISTEN_OPTS_INIT \
 { \
+	.timeout_ms = DEFAULT_UNIX_STREAM_LISTEN_TIMEOUT, \
 	.listen_backlog_size = DEFAULT_UNIX_STREAM_LISTEN_BACKLOG, \
 	.disallow_chdir = 0, \
 }
@@ -18,4 +21,30 @@ int unix_stream_connect(const char *path, int disallow_chdir);
 int unix_stream_listen(const char *path,
 		       const struct unix_stream_listen_opts *opts);
 
+struct unix_stream_server_socket {
+	char *path_socket;
+	struct stat st_socket;
+	int fd_socket;
+};
+
+/*
+ * Create a Unix Domain Socket at the given path under the protection
+ * of a '.lock' lockfile.
+ */
+struct unix_stream_server_socket *unix_stream_server__listen_with_lock(
+	const char *path,
+	const struct unix_stream_listen_opts *opts);
+
+/*
+ * Close and delete the socket.
+ */
+void unix_stream_server__free(
+	struct unix_stream_server_socket *server_socket);
+
+/*
+ * Return 1 if the inode of the pathname to our socket changes.
+ */
+int unix_stream_server__was_stolen(
+	struct unix_stream_server_socket *server_socket);
+
 #endif /* UNIX_SOCKET_H */
-- 
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