vsock-mux ssh proxy

allow the ssh-proxy to connect to cloud-hypervisor/Firecracker guests,
via their unix-domain socket to AF_VSOCK multiplexer:

https://github.com/cloud-hypervisor/cloud-hypervisor/blob/main/docs/vsock.md
https://github.com/firecracker-microvm/firecracker/blob/main/docs/vsock.md

src/ssh-generator/ssh-proxy.c

@@ -5,6 +5,7 @@
 #include <unistd.h>
 
 #include "fd-util.h"
+#include "io-util.h"
 #include "iovec-util.h"
 #include "log.h"
 #include "main-func.h"
@@ -79,6 +80,50 @@ static int process_unix(const char *path) {
         return 0;
 }
 
+static int process_vsock_mux(const char *path, const char *port) {
+        int r;
+
+        assert(path);
+        assert(port);
+
+        /* We assume the path is absolute unless it starts with a dot (or is already explicitly absolute) */
+        _cleanup_free_ char *prefixed = NULL;
+        if (!STARTSWITH_SET(path, "/", "./")) {
+                prefixed = strjoin("/", path);
+                if (!prefixed)
+                        return log_oom();
+
+                path = prefixed;
+        }
+
+        _cleanup_close_ int fd = socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0);
+        if (fd < 0)
+                return log_error_errno(errno, "Failed to allocate AF_UNIX socket: %m");
+
+        r = connect_unix_path(fd, AT_FDCWD, path);
+        if (r < 0)
+                return log_error_errno(r, "Failed to connect to AF_UNIX socket %s: %m", path);
+
+        /* Based on the protocol as defined here:
+         * https://github.com/cloud-hypervisor/cloud-hypervisor/blob/main/docs/vsock.md
+         * https://github.com/firecracker-microvm/firecracker/blob/main/docs/vsock.md */
+        _cleanup_free_ char *connect_cmd = NULL;
+        connect_cmd = strjoin("CONNECT ", port, "\n");
+        if (!connect_cmd)
+                return log_oom();
+
+        r = loop_write(fd, connect_cmd, SIZE_MAX);
+        if (r < 0)
+                return log_error_errno(r, "Failed to send CONNECT to %s:%s: %m", path, port);
+
+        r = send_one_fd_iov(STDOUT_FILENO, fd, &IOVEC_NUL_BYTE, /* n_iovec= */ 1, /* flags= */ 0);
+        if (r < 0)
+                return log_error_errno(r, "Failed to send socket via STDOUT: %m");
+
+        log_debug("Successfully sent AF_UNIX socket via STDOUT.");
+        return 0;
+}
+
 static int run(int argc, char* argv[]) {
 
         log_setup();
@@ -96,6 +141,10 @@ static int run(int argc, char* argv[]) {
         if (p)
                 return process_unix(p);
 
+        p = startswith(host, "vsock-mux/");
+        if (p)
+                return process_vsock_mux(p, port);
+
         return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Don't know how to parse host name specification: %s", host);
 }