Fix crash in utmp-stamping on Wayland.

Thanks to Colin Watson for the report: if pterm is both
able (appropriately setgid) and willing (given the right options) to
stamp utmp, it will use $DISPLAY as the location to write into utmp,
and segfault if it's not set. But in a Wayland-only system it might
very well not be set.

To fix this I've generalised seat_get_x_display() into
seat_get_display(), so that it can also return a display id string
like "wayland-0" if that's what's appropriate. So now in that
situation pterm will stamp utmp with a Wayland display id in place of
an X11 one.

However, seat_get_x_display() was also used to retrieve an X11 display
name specifically so as to populate $DISPLAY in the terminal's shell.
So the new seat_get_display() has a parameter to constrain the
returned display to be of a particular type, or NULL if that type
isn't available.

As a final fallback, in case seat_get_display(seat, SDISP_ANY) might
_still_ manage to return NULL for any reason, we catch that and turn
it into the empty string before stamping utmp, so that we still won't
segfault.

proxy/sshproxy.c

@@ -583,7 +583,7 @@ static const SeatVtable SshProxy_seat_vt = {
     .prompt_descriptions = sshproxy_prompt_descriptions,
     .is_utf8 = nullseat_is_never_utf8,
     .echoedit_update = nullseat_echoedit_update,
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
     .get_window_pixel_size = nullseat_get_window_pixel_size,
     .stripctrl_new = sshproxy_stripctrl_new,

pscp.c

@@ -81,7 +81,7 @@ static const SeatVtable pscp_seat_vt = {
     .prompt_descriptions = console_prompt_descriptions,
     .is_utf8 = nullseat_is_never_utf8,
     .echoedit_update = nullseat_echoedit_update,
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
     .get_window_pixel_size = nullseat_get_window_pixel_size,
     .stripctrl_new = console_stripctrl_new,

psftp.c

@@ -62,7 +62,7 @@ static const SeatVtable psftp_seat_vt = {
     .prompt_descriptions = console_prompt_descriptions,
     .is_utf8 = nullseat_is_never_utf8,
     .echoedit_update = nullseat_echoedit_update,
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
     .get_window_pixel_size = nullseat_get_window_pixel_size,
     .stripctrl_new = console_stripctrl_new,

putty.h

@@ -1070,6 +1070,11 @@ void seat_dialog_text_free(SeatDialogText *sdt);
 PRINTF_LIKE(3, 4) void seat_dialog_text_append(
     SeatDialogText *sdt, SeatDialogTextType type, const char *fmt, ...);
 
+/* Parameter to seat_get_display */
+typedef enum SeatDisplayType {
+    SDISP_X11, SDISP_ANY
+} SeatDisplayType;
+
 /*
  * Data type 'Seat', which is an API intended to contain essentially
  * everything that a back end might need to talk to its client for:
@@ -1299,10 +1304,13 @@ struct SeatVtable {
     void (*echoedit_update)(Seat *seat, bool echoing, bool editing);
 
     /*
-     * Return the local X display string relevant to a seat, or NULL
-     * if there isn't one or if the concept is meaningless.
+     * Return a string describing the GUI display (e.g. X11 or
+     * Wayland) relevant to a seat, or NULL if there isn't one or if
+     * the concept is meaningless. If dtype is not SDISP_ANY then only
+     * a display string of the requested type will be returned, or
+     * NULL if the available display is of a different type.
      */
-    const char *(*get_x_display)(Seat *seat);
+    const char *(*get_display)(Seat *seat, SeatDisplayType dtype);
 
     /*
      * Return the X11 id of the X terminal window relevant to a seat,
@@ -1426,8 +1434,8 @@ static inline bool seat_is_utf8(Seat *seat)
 { return seat->vt->is_utf8(seat); }
 static inline void seat_echoedit_update(Seat *seat, bool ec, bool ed)
 { seat->vt->echoedit_update(seat, ec, ed); }
-static inline const char *seat_get_x_display(Seat *seat)
-{ return seat->vt->get_x_display(seat); }
+static inline const char *seat_get_display(Seat *seat, SeatDisplayType dtype)
+{ return seat->vt->get_display(seat, dtype); }
 static inline bool seat_get_windowid(Seat *seat, long *id_out)
 { return seat->vt->get_windowid(seat, id_out); }
 static inline bool seat_get_window_pixel_size(Seat *seat, int *w, int *h)
@@ -1515,7 +1523,7 @@ const SeatDialogPromptDescriptions *nullseat_prompt_descriptions(Seat *seat);
 bool nullseat_is_never_utf8(Seat *seat);
 bool nullseat_is_always_utf8(Seat *seat);
 void nullseat_echoedit_update(Seat *seat, bool echoing, bool editing);
-const char *nullseat_get_x_display(Seat *seat);
+const char *nullseat_get_display(Seat *seat, SeatDisplayType dtype);
 bool nullseat_get_windowid(Seat *seat, long *id_out);
 bool nullseat_get_window_pixel_size(Seat *seat, int *width, int *height);
 StripCtrlChars *nullseat_stripctrl_new(

ssh/server.c

@@ -126,7 +126,7 @@ static const SeatVtable server_seat_vt = {
     .prompt_descriptions = nullseat_prompt_descriptions,
     .is_utf8 = nullseat_is_never_utf8,
     .echoedit_update = nullseat_echoedit_update,
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
     .get_window_pixel_size = nullseat_get_window_pixel_size,
     .stripctrl_new = nullseat_stripctrl_new,

ssh/sesschan.c

@@ -203,7 +203,7 @@ static const SeatVtable sesschan_seat_vt = {
     .prompt_descriptions = nullseat_prompt_descriptions,
     .is_utf8 = nullseat_is_never_utf8,
     .echoedit_update = nullseat_echoedit_update,
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
     .get_window_pixel_size = sesschan_get_window_pixel_size,
     .stripctrl_new = nullseat_stripctrl_new,
@@ -310,7 +310,7 @@ static void sesschan_start_backend(sesschan *sess, const char *cmd)
      * confusingly set in the absence of that.
      *
      * (DISPLAY must also be cleared, but pty.c will do that anyway
-     * when our get_x_display method returns NULL.)
+     * when our get_display method returns NULL.)
      */
     static const char *const env_to_unset[] = {
         "XAUTHORITY", "SSH_AUTH_SOCK", "SSH_AGENT_PID",

stubs/null-seat.c

@@ -37,7 +37,8 @@ SeatPromptResult nullseat_confirm_weak_cached_hostkey(
 bool nullseat_is_never_utf8(Seat *seat) { return false; }
 bool nullseat_is_always_utf8(Seat *seat) { return true; }
 void nullseat_echoedit_update(Seat *seat, bool echoing, bool editing) {}
-const char *nullseat_get_x_display(Seat *seat) { return NULL; }
+const char *nullseat_get_display(Seat *seat, SeatDisplayType dtype)
+{ return NULL; }
 bool nullseat_get_windowid(Seat *seat, long *id_out) { return false; }
 bool nullseat_get_window_pixel_size(
     Seat *seat, int *width, int *height) { return false; }

unix/plink.c

@@ -419,7 +419,7 @@ static const SeatVtable plink_seat_vt = {
     .prompt_descriptions = console_prompt_descriptions,
     .is_utf8 = nullseat_is_never_utf8,
     .echoedit_update = plink_echoedit_update,
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
     .get_window_pixel_size = nullseat_get_window_pixel_size,
     .stripctrl_new = console_stripctrl_new,

unix/pty.c

@@ -958,7 +958,10 @@ Backend *pty_backend_create(
                 close(pty_utmp_helper_pipe);
                 pty_utmp_helper_pipe = -1;
             } else {
-                const char *location = seat_get_x_display(pty->seat);
+                const char *location = seat_get_display(pty->seat, SDISP_ANY);
+                if (!location)
+                    location = "";
+
                 int len = strlen(location)+1, pos = 0; /* +1 to include NUL */
                 while (pos < len) {
                     int ret = write(pty_utmp_helper_pipe,
@@ -1133,7 +1136,7 @@ Backend *pty_backend_create(
              * terminal to match the display the terminal itself is
              * on.
              */
-            const char *x_display = seat_get_x_display(pty->seat);
+            const char *x_display = seat_get_display(pty->seat, SDISP_X11);
             if (x_display) {
                 char *x_display_env_var = dupprintf("DISPLAY=%s", x_display);
                 putenv(x_display_env_var);

unix/window.c

@@ -402,7 +402,7 @@ static void gtk_seat_notify_remote_exit(Seat *seat);
 static void gtk_seat_update_specials_menu(Seat *seat);
 static void gtk_seat_set_busy_status(Seat *seat, BusyStatus status);
 #ifndef NOT_X_WINDOWS
-static const char *gtk_seat_get_x_display(Seat *seat);
+static const char *gtk_seat_get_display(Seat *seat, SeatDisplayType dtype);
 static bool gtk_seat_get_windowid(Seat *seat, long *id);
 #endif
 static void gtk_seat_set_trust_status(Seat *seat, bool trusted);
@@ -430,10 +430,10 @@ static const SeatVtable gtk_seat_vt = {
     .is_utf8 = gtk_seat_is_utf8,
     .echoedit_update = nullseat_echoedit_update,
 #ifdef NOT_X_WINDOWS
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
 #else
-    .get_x_display = gtk_seat_get_x_display,
+    .get_display = gtk_seat_get_display,
     .get_windowid = gtk_seat_get_windowid,
 #endif
     .get_window_pixel_size = gtk_seat_get_window_pixel_size,
@@ -4331,11 +4331,11 @@ void modalfatalbox(const char *p, ...)
 }
 
 #ifndef NOT_X_WINDOWS
-static const char *gtk_seat_get_x_display(Seat *seat)
+static const char *gtk_seat_get_display(Seat *seat, SeatDisplayType dtype)
 {
-    if (GDK_IS_X11_DISPLAY(gdk_display_get_default()))
-        return gdk_get_display();
+    if (dtype == SDISP_X11 && !GDK_IS_X11_DISPLAY(gdk_display_get_default()))
         return NULL;
+    return gdk_get_display();
 }
 
 static bool gtk_seat_get_windowid(Seat *seat, long *id)

utils/tempseat.c

@@ -149,10 +149,10 @@ static bool tempseat_is_utf8(Seat *seat)
     return seat_is_utf8(ts->realseat);
 }
 
-static const char *tempseat_get_x_display(Seat *seat)
+static const char *tempseat_get_display(Seat *seat, SeatDisplayType dtype)
 {
     TempSeat *ts = container_of(seat, TempSeat, seat);
-    return seat_get_x_display(ts->realseat);
+    return seat_get_display(ts->realseat, dtype);
 }
 
 static bool tempseat_get_windowid(Seat *seat, long *id_out)
@@ -348,7 +348,7 @@ static const struct SeatVtable tempseat_vt = {
     .prompt_descriptions = tempseat_prompt_descriptions,
     .is_utf8 = tempseat_is_utf8,
     .echoedit_update = tempseat_echoedit_update,
-    .get_x_display = tempseat_get_x_display,
+    .get_display = tempseat_get_display,
     .get_windowid = tempseat_get_windowid,
     .get_window_pixel_size = tempseat_get_window_pixel_size,
     .stripctrl_new = tempseat_stripctrl_new,

windows/plink.c

@@ -107,7 +107,7 @@ static const SeatVtable plink_seat_vt = {
     .prompt_descriptions = console_prompt_descriptions,
     .is_utf8 = nullseat_is_never_utf8,
     .echoedit_update = plink_echoedit_update,
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
     .get_window_pixel_size = nullseat_get_window_pixel_size,
     .stripctrl_new = console_stripctrl_new,

windows/window.c

@@ -267,7 +267,7 @@ static const SeatVtable win_seat_vt = {
     .prompt_descriptions = win_seat_prompt_descriptions,
     .is_utf8 = win_seat_is_utf8,
     .echoedit_update = nullseat_echoedit_update,
-    .get_x_display = nullseat_get_x_display,
+    .get_display = nullseat_get_display,
     .get_windowid = nullseat_get_windowid,
     .get_window_pixel_size = win_seat_get_window_pixel_size,
     .stripctrl_new = win_seat_stripctrl_new,