Windows: move GUI timer handling into a utils module.

Previously, the timing.c subsystem worked in Windows PuTTY by means of
scheduling WM_TIMER messages to be sent to the terminal window. Now it
uses a separate hidden window instead, and all the machinery for
handling that window lives on its own in windows/utils/gui-timing.c.

Most immediately, this removes a use of wgs.term_hwnd that will become
awkward when I move that structure in a following commit. But also, it
will make it easier to add the same timing subsystem to unrelated GUI
programs, such as Windows Pageant: if we ever decide to implement
automatic deletion or re-encryption of unused keys after a timeout,
this will help make that easier.

windows/CMakeLists.txt

@@ -16,6 +16,7 @@ add_sources_from_current_dir(utils
   utils/getdlgitemtext_alloc.c
   utils/get_system_dir.c
   utils/get_username.c
+  utils/gui-timing.c
   utils/interprocess_mutex.c
   utils/is_console_handle.c
   utils/load_system32_dll.c

windows/platform.h

@@ -793,4 +793,6 @@ bool aux_match_done(AuxMatchOpt *amo);
 char *save_screenshot(HWND hwnd, const char *outfile);
 void gui_terminal_ready(HWND hwnd, Seat *seat, Backend *backend);
 
+void setup_gui_timing(void);
+
 #endif /* PUTTY_WINDOWS_PLATFORM_H */

windows/utils/gui-timing.c

@@ -0,0 +1,56 @@
+#include "putty.h"
+
+#define TIMING_CLASS_NAME "PuTTYTimerWindow"
+#define TIMING_TIMER_ID 1234
+static long timing_next_time;
+static HWND timing_hwnd;
+
+static LRESULT CALLBACK TimingWndProc(HWND hwnd, UINT message,
+                                      WPARAM wParam, LPARAM lParam)
+{
+    switch (message) {
+      case WM_TIMER:
+        if ((UINT_PTR)wParam == TIMING_TIMER_ID) {
+            unsigned long next;
+
+            KillTimer(hwnd, TIMING_TIMER_ID);
+            if (run_timers(timing_next_time, &next)) {
+                timer_change_notify(next);
+            } else {
+            }
+        }
+        return 0;
+    }
+    return DefWindowProc(hwnd, message, wParam, lParam);
+}
+
+void setup_gui_timing(void)
+{
+    WNDCLASS wndclass;
+
+    memset(&wndclass, 0, sizeof(wndclass));
+    wndclass.lpfnWndProc = TimingWndProc;
+    wndclass.hInstance = hinst;
+    wndclass.lpszClassName = TIMING_CLASS_NAME;
+
+    RegisterClass(&wndclass);
+
+    timing_hwnd = CreateWindow(
+        TIMING_CLASS_NAME, "PuTTY: hidden timing window",
+        WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT,
+        100, 100, NULL, NULL, hinst, NULL);
+    ShowWindow(timing_hwnd, SW_HIDE);
+}
+
+void timer_change_notify(unsigned long next)
+{
+    unsigned long now = GETTICKCOUNT();
+    long ticks;
+    if (now - next < INT_MAX)
+        ticks = 0;
+    else
+        ticks = next - now;
+    KillTimer(timing_hwnd, TIMING_TIMER_ID);
+    SetTimer(timing_hwnd, TIMING_TIMER_ID, ticks, NULL);
+    timing_next_time = next;
+}

windows/window.c

@@ -142,9 +142,6 @@ static const SessionSpecial *specials = NULL;
 static HMENU specials_menu = NULL;
 static int n_specials = 0;
 
-#define TIMING_TIMER_ID 1234
-static long timing_next_time;
-
 static struct {
     HMENU menu;
 } popup_menus[2];
@@ -564,6 +561,8 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 
     init_winfuncs();
 
+    setup_gui_timing();
+
     conf = conf_new();
 
     /*
@@ -2106,19 +2105,6 @@ static void win_seat_notify_remote_exit(Seat *seat)
     queue_toplevel_callback(exit_callback, NULL);
 }
 
-void timer_change_notify(unsigned long next)
-{
-    unsigned long now = GETTICKCOUNT();
-    long ticks;
-    if (now - next < INT_MAX)
-        ticks = 0;
-    else
-        ticks = next - now;
-    KillTimer(wgs.term_hwnd, TIMING_TIMER_ID);
-    SetTimer(wgs.term_hwnd, TIMING_TIMER_ID, ticks, NULL);
-    timing_next_time = next;
-}
-
 static void conf_cache_data(void)
 {
     /* Cache some items from conf to speed lookups in very hot code */
@@ -2195,17 +2181,6 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
     int resize_action;
 
     switch (message) {
-      case WM_TIMER:
-        if ((UINT_PTR)wParam == TIMING_TIMER_ID) {
-            unsigned long next;
-
-            KillTimer(hwnd, TIMING_TIMER_ID);
-            if (run_timers(timing_next_time, &next)) {
-                timer_change_notify(next);
-            } else {
-            }
-        }
-        return 0;
       case WM_CREATE:
         break;
       case WM_CLOSE: {