sd-bus: cleanup ssh sessions (Closes: #8076)

we still invoke ssh unnecessarily when there in incompatible or erreneous input
The fallow-up to finish that would make the code a bit more verbose,
as it would require repeating this bit:
```
        r = bus_connect_transport(arg_transport, arg_host, false, &bus);
        if (r < 0) {
                log_error_errno(r, "Failed to create bus connection: %m");
                goto finish;
        }

        sd_bus_set_allow_interactive_authorization(bus, arg_ask_password);
```
in every verb, after parsing.

v2: add waitpid() to avoid a zombie process, switch to SIGTERM from SIGKILL
v3: refactor, wait in bus_start_address()

src/libsystemd/sd-bus/bus-internal.h

@@ -296,6 +296,7 @@ struct sd_bus {
         unsigned n_memfd_cache;
 
         pid_t original_pid;
+        pid_t busexec_pid;
 
         sd_event_source *input_io_event_source;
         sd_event_source *output_io_event_source;

src/libsystemd/sd-bus/bus-socket.c

@@ -937,18 +937,18 @@ int bus_socket_connect(sd_bus *b) {
 
 int bus_socket_exec(sd_bus *b) {
         int s[2], r;
-        pid_t pid;
 
         assert(b);
         assert(b->input_fd < 0);
         assert(b->output_fd < 0);
         assert(b->exec_path);
+        assert(b->busexec_pid == 0);
 
         r = socketpair(AF_UNIX, SOCK_STREAM|SOCK_NONBLOCK|SOCK_CLOEXEC, 0, s);
         if (r < 0)
                 return -errno;
 
-        r = safe_fork_full("(sd-busexec)", s+1, 1, FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS, &pid);
+        r = safe_fork_full("(sd-busexec)", s+1, 1, FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS, &b->busexec_pid);
         if (r < 0) {
                 safe_close_pair(s);
                 return r;

src/libsystemd/sd-bus/sd-bus.c

@@ -22,8 +22,10 @@
 #include <netdb.h>
 #include <poll.h>
 #include <pthread.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <sys/mman.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #include "sd-bus.h"
@@ -1099,6 +1101,13 @@ static int bus_parse_next_address(sd_bus *b) {
         return 1;
 }
 
+static void bus_kill_exec(sd_bus *bus) {
+        if (pid_is_valid(bus->busexec_pid) > 0) {
+                sigterm_wait(bus->busexec_pid);
+                bus->busexec_pid = 0;
+        }
+}
+
 static int bus_start_address(sd_bus *b) {
         int r;
 
@@ -1108,6 +1117,8 @@ static int bus_start_address(sd_bus *b) {
                 bus_close_io_fds(b);
                 bus_close_inotify_fd(b);
 
+                bus_kill_exec(b);
+
                 /* If you provide multiple different bus-addresses, we
                  * try all of them in order and use the first one that
                  * succeeds. */
@@ -1506,6 +1517,9 @@ _public_ void sd_bus_close(sd_bus *bus) {
         if (bus_pid_changed(bus))
                 return;
 
+        /* Don't leave ssh hanging around */
+        bus_kill_exec(bus);
+
         bus_set_state(bus, BUS_CLOSED);
 
         sd_bus_detach_event(bus);
@@ -1523,6 +1537,9 @@ _public_ sd_bus* sd_bus_flush_close_unref(sd_bus *bus) {
         if (!bus)
                 return NULL;
 
+        /* Have to do this before flush() to prevent hang */
+        bus_kill_exec(bus);
+
         sd_bus_flush(bus);
         sd_bus_close(bus);