uxserver: option to generate numeric "exit-signal".

This mimics a bug in some old SSH servers for which PuTTY contains
compensation code (parsing an incoming "exit-signal" two ways and
seeing which one worked). I completely rewrote that code in commit
7535f645a, as part of the BinarySource rework. Now I can finally test
it sensibly.

sesschan.c

@@ -608,13 +608,15 @@ static bool sesschan_seat_eof(Seat *seat)
 static void sesschan_notify_remote_exit(Seat *seat)
 {
     sesschan *sess = container_of(seat, sesschan, seat);
-    ptrlen signame;
-    char *sigmsg;
 
     if (!sess->backend)
         return;
 
-    signame = pty_backend_exit_signame(sess->backend, &sigmsg);
+    bool got_signal = false;
+    if (!sess->ssc->exit_signal_numeric) {
+        char *sigmsg;
+        ptrlen signame = pty_backend_exit_signame(sess->backend, &sigmsg);
+
         if (signame.len) {
             if (!sigmsg)
                 sigmsg = dupstr("");
@@ -623,9 +625,21 @@ static void sesschan_notify_remote_exit(Seat *seat)
                 sess->c, signame, false, ptrlen_from_asciz(sigmsg));
 
             sfree(sigmsg);
-    } else {
-        sshfwd_send_exit_status(sess->c, backend_exitcode(sess->backend));
+
+            got_signal = true;
         }
+    } else {
+        int signum = pty_backend_exit_signum(sess->backend);
+
+        if (signum >= 0) {
+            sshfwd_send_exit_signal_numeric(sess->c, signum, false,
+                                            PTRLEN_LITERAL(""));
+            got_signal = true;
+        }
+    }
+
+    if (!got_signal)
+        sshfwd_send_exit_status(sess->c, backend_exitcode(sess->backend));
 
     sess->seen_exit = true;
     queue_toplevel_callback(sesschan_check_close_callback, sess);

sshserver.h

@@ -2,6 +2,8 @@ typedef struct AuthPolicy AuthPolicy;
 
 struct SshServerConfig {
     ptrlen banner;        /* banner.ptr == NULL indicates no banner */
+
+    bool exit_signal_numeric;          /* mimic an old server bug */
 };
 
 Plug *ssh_server_plug(
@@ -94,6 +96,7 @@ Channel *sesschan_new(SshChannel *c, LogContext *logctx,
 Backend *pty_backend_create(
     Seat *seat, LogContext *logctx, Conf *conf, char **argv, const char *cmd,
     struct ssh_ttymodes ttymodes, bool pipes_instead_of_pty);
+int pty_backend_exit_signum(Backend *be);
 ptrlen pty_backend_exit_signame(Backend *be, char **aux_msg);
 
 /*

unix/uxpty.c

@@ -1526,17 +1526,23 @@ static int pty_exitcode(Backend *be)
         return WEXITSTATUS(pty->exit_code);
 }
 
-ptrlen pty_backend_exit_signame(Backend *be, char **aux_msg)
+int pty_backend_exit_signum(Backend *be)
 {
     Pty *pty = container_of(be, Pty, backend);
-    int sig;
-
-    *aux_msg = NULL;
 
     if (!pty->finished || !WIFSIGNALED(pty->exit_code))
-	return PTRLEN_LITERAL("");
+	return -1;
 
-    sig = WTERMSIG(pty->exit_code);
+    return WTERMSIG(pty->exit_code);
+}
+
+ptrlen pty_backend_exit_signame(Backend *be, char **aux_msg)
+{
+    *aux_msg = NULL;
+
+    int sig = pty_backend_exit_signum(be);
+    if (sig < 0)
+	return PTRLEN_LITERAL("");
 
 #define TRANSLATE_SIGNAL(s) do                          \
     {                                                   \

unix/uxserver.c

@@ -542,6 +542,8 @@ int main(int argc, char **argv)
             ssc.banner = ptrlen_from_strbuf(sb);
         } else if (longoptarg(arg, "--bannertext", &val, &argc, &argv)) {
             ssc.banner = ptrlen_from_asciz(val);
+        } else if (longoptnoarg(arg, "--exitsignum")) {
+            ssc.exit_signal_numeric = true;
         } else if (longoptarg(arg, "--sshlog", &val, &argc, &argv) ||
                    longoptarg(arg, "-sshlog", &val, &argc, &argv)) {
             Filename *logfile = filename_from_str(val);