[PATCH 07/10] unix-socket: create gentle version of unix_stream_listen()

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

From: Jeff Hostetler <jeffhost@microsoft.com>

Create a gentle version of `unix_stream_listen()`.  This version does
not call `die()` if a socket-fd cannot be created and does not assume
that it is safe to `unlink()` an existing socket-inode.

`unix_stream_listen()` uses `unix_stream_socket()` helper function to
create the socket-fd.  Avoid that helper because it calls `die()` on
errors.

`unix_stream_listen()` always tries to `unlink()` the socket-path before
calling `bind()`.  If there is an existing server/daemon already bound
and listening on that socket-path, our `unlink()` would have the effect
of disassociating the existing server's bound-socket-fd from the socket-path
without notifying the existing server.  The existing server could continue
to service existing connections (accepted-socket-fd's), but would not
receive any futher new connections (since clients rendezvous via the
socket-path).  The existing server would effectively be offline but yet
appear to be active.

Furthermore, `unix_stream_listen()` creates an opportunity for a brief
race condition for connecting clients if they try to connect in the
interval between the forced `unlink()` and the subsequent `bind()` (which
recreates the socket-path that is bound to a new socket-fd in the current
process).

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 unix-socket.c | 39 +++++++++++++++++++++++++++++++++++++++
 unix-socket.h |  8 ++++++++
 2 files changed, 47 insertions(+)

diff --git a/unix-socket.c b/unix-socket.c
index 19ed48be990..3a9ffc32268 100644
--- a/unix-socket.c
+++ b/unix-socket.c
@@ -121,3 +121,42 @@ int unix_stream_listen(const char *path)
 	errno = saved_errno;
 	return -1;
 }
+
+int unix_stream_listen_gently(const char *path,
+			      const struct unix_stream_listen_opts *opts)
+{
+	int fd = -1;
+	int bind_successful = 0;
+	int saved_errno;
+	struct sockaddr_un sa;
+	struct unix_sockaddr_context ctx;
+
+	if (unix_sockaddr_init(&sa, path, &ctx) < 0)
+		goto fail;
+
+	fd = socket(AF_UNIX, SOCK_STREAM, 0);
+	if (fd < 0)
+		goto fail;
+
+	if (opts->force_unlink_before_bind)
+		unlink(path);
+
+	if (bind(fd, (struct sockaddr *)&sa, sizeof(sa)) < 0)
+		goto fail;
+	bind_successful = 1;
+
+	if (listen(fd, opts->listen_backlog_size) < 0)
+		goto fail;
+
+	unix_sockaddr_cleanup(&ctx);
+	return fd;
+
+fail:
+	saved_errno = errno;
+	unix_sockaddr_cleanup(&ctx);
+	close(fd);
+	if (bind_successful)
+		unlink(path);
+	errno = saved_errno;
+	return -1;
+}
diff --git a/unix-socket.h b/unix-socket.h
index e271aeec5a0..253f579f087 100644
--- a/unix-socket.h
+++ b/unix-socket.h
@@ -4,4 +4,12 @@
 int unix_stream_connect(const char *path);
 int unix_stream_listen(const char *path);
 
+struct unix_stream_listen_opts {
+	int listen_backlog_size;
+	unsigned int force_unlink_before_bind:1;
+};
+
+int unix_stream_listen_gently(const char *path,
+			      const struct unix_stream_listen_opts *opts);
+
 #endif /* UNIX_SOCKET_H */
-- 
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