servers: add option to build without window handler (Windows)

curl already built like this in UWP mode; add a macro to allow building
regular Win32 targets too, meaning without the hidden GUI window, window
handler, and thus without support for shutting down `servers.exe`
gracefully. To test the server in CI as a plain console app and remove
one component/variable from the system, in an effort chasing flaky runs.

The graceful option was used by `taskkill` (without `-f`) in runtests,
and it has been disabled (in favor of always `-f` (forced) kill), in an
attempt to avoid taskkill hanging. Those resolved, but general CI
flakiness remained. We see `taskkill` invoked seldom per job, and in
most of these cases it doesn't find the PID requested. It means the
graceful option is not actually used since 2025-04, and rather rarely
before that. Also just today, the callback and main loop received a few
updates to avoid potentially unsafe calls and interaction with the ctrl
handler.

Not expecting anything breakthrough here. Also graceful shutdown
is a useful feature to cleanup properly and to be in sync with other
platforms.

Also: GHA/windows: enable in Windows CI jobs.

Ref: https://learn.microsoft.com/windows/console/registering-a-control-handler-function#listen-with-hidden-window-example

Follow-up to 7db9947fcf #22045
Follow-up to 9ea48811fe #22487
Follow-up to b11e0026f7 #22495
Follow-up to 208b87744e #21039
Follow-up to f450f3801b #19897
Follow-up to 2701ac6a4d #19421
Follow-up to 4e203f65a1 #17054

Closes #22496

.github/workflows/windows.yml

@@ -368,6 +368,7 @@ jobs:
           MATRIX_TYPE: '${{ matrix.type }}'
           TFLAGS: '${{ matrix.tflags }}'
         run: |
+          CPPFLAGS+=' -DCURL_DEBUG_NO_WIN32_WND'
           if [ "${MATRIX_TEST}" = 'uwp' ]; then
             CPPFLAGS+=' -DWINSTORECOMPAT -DWINAPI_FAMILY=WINAPI_FAMILY_APP -D_WIN32_WINNT=0x0a00'
             if [[ "${MATRIX_ENV}" != 'clang'* ]]; then
@@ -581,7 +582,7 @@ jobs:
             ver: '16.1.0'
             url: 'https://github.com/skeeto/w64devkit/releases/download/v2.8.0/w64devkit-x64-2.8.0.7z.exe'
             SHA256: 6252bf34fe2231a55ac7f03d482b36d2c7c58697990551bba508102cfb3f342e
-            config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF -DENABLE_UNIX_SOCKETS=OFF -DCURL_GCC_ANALYZER=ON'
+            config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF -DENABLE_UNIX_SOCKETS=OFF -DCURL_GCC_ANALYZER=ON -DCMAKE_C_FLAGS=-DCURL_DEBUG_NO_WIN32_WND'
             type: 'Debug'
           - name: 'schannel'  # mingw-w64 10.0
             sys: 'mingw64'
@@ -590,7 +591,7 @@ jobs:
             ver: '9.5.0'
             url: 'https://github.com/brechtsanders/winlibs_mingw/releases/download/9.5.0-10.0.0-msvcrt-r1/winlibs-x86_64-posix-seh-gcc-9.5.0-mingw-w64msvcrt-10.0.0-r1.7z'
             SHA256: 41637132ea7dc36a7f86a1961eaa334c380b5a3423d36aecb481cabcd006e3fe
-            config: '-DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF -DCURL_DISABLE_VERBOSE_STRINGS=ON'
+            config: '-DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF -DCURL_DISABLE_VERBOSE_STRINGS=ON -DCMAKE_C_FLAGS=-DCURL_DEBUG_NO_WIN32_WND'
             type: 'Release'
             tflags: 'skiprun'
           - name: 'schannel mbedtls U'  # mingw-w64 6.0
@@ -1013,6 +1014,7 @@ jobs:
             cflags=''
             rcflags=''
             ldflags=''
+            cflags+=' -DCURL_DEBUG_NO_WIN32_WND'
             if [ "${MATRIX_PLAT}" = 'uwp' ]; then
               options+=' -DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=10.0'
               cflags+=' -DWINAPI_FAMILY=WINAPI_FAMILY_PC_APP'

tests/server/util.c

@@ -33,6 +33,10 @@
 #include <share.h>
 #endif
 
+#if defined(CURL_WINDOWS_UWP) && !defined(CURL_DEBUG_NO_WIN32_WND)
+#define CURL_DEBUG_NO_WIN32_WND
+#endif
+
 void loghex(const unsigned char *buffer, ssize_t len)
 {
   char data[12000];
@@ -450,7 +454,7 @@ static BOOL WINAPI ctrl_event_handler(DWORD dwCtrlType)  /* stay signal-safe */
   return TRUE;
 }
 
-#ifndef CURL_WINDOWS_UWP
+#ifndef CURL_DEBUG_NO_WIN32_WND
 static DWORD thread_main_id = 0;
 static HANDLE thread_main_window = NULL;
 static HWND hidden_main_window = NULL;
@@ -538,7 +542,7 @@ static DWORD WINAPI main_window_loop(void *lpParameter)
   hidden_main_window = NULL;
   return (DWORD)msg.wParam;
 }
-#endif /* CURL_WINDOWS_UWP */
+#endif /* !CURL_DEBUG_NO_WIN32_WND */
 #endif /* !_WIN32 */
 
 void install_signal_handlers(bool keep_sigalrm)
@@ -592,7 +596,7 @@ void install_signal_handlers(bool keep_sigalrm)
   if(!SetConsoleCtrlHandler(ctrl_event_handler, TRUE))
     logmsg("cannot install CTRL event handler");
 
-#ifndef CURL_WINDOWS_UWP
+#ifndef CURL_DEBUG_NO_WIN32_WND
   thread_main_window = CreateThread(NULL, 0, &main_window_loop,
                                     GetModuleHandle(NULL), 0, &thread_main_id);
   if(!thread_main_window || !thread_main_id)
@@ -629,7 +633,7 @@ void restore_signal_handlers(bool keep_sigalrm)
 #endif
 #else /* _WIN32 */
   (void)SetConsoleCtrlHandler(ctrl_event_handler, FALSE);
-#ifndef CURL_WINDOWS_UWP
+#ifndef CURL_DEBUG_NO_WIN32_WND
   if(thread_main_window && thread_main_id) {
     if(PostThreadMessage(thread_main_id, WM_APP, 0, 0)) {
       if(WaitForSingleObjectEx(thread_main_window, INFINITE, TRUE)) {