Mention any extant downstreams in close warning.

Suggested by Brian Rak.

putty.h

@@ -652,6 +652,12 @@ struct BackendVtable {
     /* Only implemented in the SSH protocol: check whether a
      * connection-sharing upstream exists for a given configuration. */
     bool (*test_for_upstream)(const char *host, int port, Conf *conf);
+    /* Special-purpose function to return additional information to put
+     * in a "are you sure you want to close this session" dialog;
+     * return NULL if no such info, otherwise caller must free.
+     * Only implemented in the SSH protocol, to warn about downstream
+     * connections that would be lost if this one were terminated. */
+    char *(*close_warn_text)(Backend *be);
 
     /* 'id' is a machine-readable name for the backend, used in
      * saved-session storage. 'displayname' is a human-readable name

raw.c

@@ -324,6 +324,7 @@ const BackendVtable raw_backend = {
     .unthrottle = raw_unthrottle,
     .cfg_info = raw_cfg_info,
     .test_for_upstream = NULL,
+    .close_warn_text = NULL,
     .id = "raw",
     .displayname = "Raw",
     .protocol = PROT_RAW,

rlogin.c

@@ -422,6 +422,7 @@ const BackendVtable rlogin_backend = {
     .unthrottle = rlogin_unthrottle,
     .cfg_info = rlogin_cfg_info,
     .test_for_upstream = NULL,
+    .close_warn_text = NULL,
     .id = "rlogin",
     .displayname = "Rlogin",
     .protocol = PROT_RLOGIN,

ssh.c

@@ -691,6 +691,19 @@ static bool ssh_test_for_upstream(const char *host, int port, Conf *conf)
     return ret;
 }
 
+static char *ssh_close_warn_text(Backend *be)
+{
+    Ssh *ssh = container_of(be, Ssh, backend);
+    if (!ssh->connshare)
+        return NULL;
+    int ndowns = share_ndownstreams(ssh->connshare);
+    if (ndowns == 0)
+        return NULL;
+    char *msg = dupprintf("This will also close %d downstream connection%s.",
+                          ndowns, ndowns==1 ? "" : "s");
+    return msg;
+}
+
 static const PlugVtable Ssh_plugvt = {
     .log = ssh_socket_log,
     .closing = ssh_closing,
@@ -1204,6 +1217,7 @@ const BackendVtable ssh_backend = {
     .unthrottle = ssh_unthrottle,
     .cfg_info = ssh_cfg_info,
     .test_for_upstream = ssh_test_for_upstream,
+    .close_warn_text = ssh_close_warn_text,
     .id = "ssh",
     .displayname = "SSH",
     .protocol = PROT_SSH,
@@ -1227,6 +1241,7 @@ const BackendVtable sshconn_backend = {
     .unthrottle = ssh_unthrottle,
     .cfg_info = ssh_cfg_info,
     .test_for_upstream = ssh_test_for_upstream,
+    .close_warn_text = ssh_close_warn_text,
     .id = "ssh-connection",
     .displayname = "Bare ssh-connection",
     .protocol = PROT_SSHCONN,

unix/gtkwin.c

@@ -618,13 +618,21 @@ gint delete_window(GtkWidget *widget, GdkEvent *event, GtkFrontend *inst)
          */
         if (!find_and_raise_dialog(inst, DIALOG_SLOT_WARN_ON_CLOSE)) {
             char *title = dupcat(appname, " Exit Confirmation");
+            char *msg, *additional = NULL;
+            if (inst && inst->backend && inst->backend->vt->close_warn_text) {
+                additional = inst->backend->vt->close_warn_text(inst->backend);
+            }
+            msg = dupprintf("Are you sure you want to close this session?%s%s",
+                            additional ? "\n" : "",
+                            additional ? additional : "");
             GtkWidget *dialog = create_message_box(
-                inst->window, title,
-                "Are you sure you want to close this session?",
+                inst->window, title, msg,
                 string_width("Most of the width of the above text"),
                 false, &buttons_yn, warn_on_close_callback, inst);
             register_dialog(&inst->seat, DIALOG_SLOT_WARN_ON_CLOSE, dialog);
             sfree(title);
+            sfree(msg);
+            sfree(additional);
         }
         return true;
     }

windows/window.c

@@ -2122,16 +2122,23 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
       case WM_CREATE:
         break;
       case WM_CLOSE: {
-        char *str;
+        char *title, *msg, *additional = NULL;
         show_mouseptr(true);
-        str = dupprintf("%s Exit Confirmation", appname);
+        title = dupprintf("%s Exit Confirmation", appname);
+        if (backend && backend->vt->close_warn_text) {
+            additional = backend->vt->close_warn_text(backend);
+        }
+        msg = dupprintf("Are you sure you want to close this session?%s%s",
+                        additional ? "\n" : "",
+                        additional ? additional : "");
         if (session_closed || !conf_get_bool(conf, CONF_warn_on_close) ||
-            MessageBox(hwnd,
-                       "Are you sure you want to close this session?",
-                       str, MB_ICONWARNING | MB_OKCANCEL | MB_DEFBUTTON1)
+            MessageBox(hwnd, msg, title,
+                       MB_ICONWARNING | MB_OKCANCEL | MB_DEFBUTTON1)
             == IDOK)
             DestroyWindow(hwnd);
-        sfree(str);
+        sfree(title);
+        sfree(msg);
+        sfree(additional);
         return 0;
       }
       case WM_DESTROY: