win32 gpoll: overcome the 64 handles limit

This is a new polling method allowing to poll more than 64 handles
based on the glib one.
When we reach the limit of 64 we create a thread and we poll
on that thread for a batch of handles this way we overcome the limit.

https://gitlab.gnome.org/GNOME/glib/issues/1071

glib/gpoll.c

@@ -72,6 +72,7 @@
 #ifdef G_OS_WIN32
 #define STRICT
 #include <windows.h>
+#include <process.h>
 #endif /* G_OS_WIN32 */
 
 #include "gpoll.h"
@@ -130,6 +131,7 @@ g_poll (GPollFD *fds,
 
 static int
 poll_rest (GPollFD *msg_fd,
+           GPollFD *stop_fd,
            HANDLE  *handles,
            GPollFD *handle_to_fd[],
            gint     nhandles,
@@ -211,113 +213,279 @@ poll_rest (GPollFD *msg_fd,
       /* If no timeout and handles to poll, recurse to poll them,
        * too.
        */
-      recursed_result = poll_rest (NULL, handles, handle_to_fd, nhandles, 0);
+      recursed_result = poll_rest (NULL, stop_fd, handles, handle_to_fd, nhandles, 0);
       return (recursed_result == -1) ? -1 : 1 + recursed_result;
     }
   else if (ready >= WAIT_OBJECT_0 && ready < WAIT_OBJECT_0 + nhandles)
     {
+      int retval;
+
       f = handle_to_fd[ready - WAIT_OBJECT_0];
       f->revents = f->events;
       if (_g_main_poll_debug)
         g_print ("  got event %p\n", (HANDLE) f->fd);
 
+      /* Do not count the stop_fd */
+      retval = (f != stop_fd) ? 1 : 0;
+
       /* If no timeout and polling several handles, recurse to poll
        * the rest of them.
        */
       if (timeout_ms == 0 && nhandles > 1)
-	{
+        {
-	  /* Poll the handles with index > ready */
+          /* Poll the handles with index > ready */
-          HANDLE  *shorter_handles;
+          HANDLE *shorter_handles;
           GPollFD **shorter_handle_to_fd;
-          gint     shorter_nhandles;
+          gint shorter_nhandles;
 
           shorter_handles = &handles[ready - WAIT_OBJECT_0 + 1];
           shorter_handle_to_fd = &handle_to_fd[ready - WAIT_OBJECT_0 + 1];
           shorter_nhandles = nhandles - (ready - WAIT_OBJECT_0 + 1);
 
-	  recursed_result = poll_rest (NULL, shorter_handles, shorter_handle_to_fd, shorter_nhandles, 0);
+          recursed_result = poll_rest (NULL, stop_fd, shorter_handles, shorter_handle_to_fd, shorter_nhandles, 0);
-	  return (recursed_result == -1) ? -1 : 1 + recursed_result;
+          return (recursed_result == -1) ? -1 : retval + recursed_result;
-	}
+        }
-      return 1;
+      return retval;
     }
 
   return 0;
 }
 
-gint
+typedef struct
-g_poll (GPollFD *fds,
-	guint    nfds,
-	gint     timeout)
 {
   HANDLE handles[MAXIMUM_WAIT_OBJECTS];
   GPollFD *handle_to_fd[MAXIMUM_WAIT_OBJECTS];
-  GPollFD *msg_fd = NULL;
+  GPollFD *msg_fd;
-  GPollFD *f;
+  GPollFD *stop_fd;
-  gint nhandles = 0;
+  gint nhandles;
+  gint timeout_ms;
+} GWin32PollThreadData;
+
+static gint
+poll_single_thread (GWin32PollThreadData *data)
+{
   int retval;
 
-  if (_g_main_poll_debug)
-    g_print ("g_poll: waiting for");
-
-  for (f = fds; f < &fds[nfds]; ++f)
-    {
-      if (f->fd == G_WIN32_MSG_HANDLE && (f->events & G_IO_IN))
-        {
-          if (_g_main_poll_debug && msg_fd == NULL)
-            g_print (" MSG");
-          msg_fd = f;
-        }
-      else if (f->fd > 0)
-        {
-          if (nhandles == MAXIMUM_WAIT_OBJECTS)
-            {
-              g_warning ("Too many handles to wait for!");
-              break;
-            }
-          else
-            {
-              if (_g_main_poll_debug)
-                g_print (" %p", (HANDLE) f->fd);
-              handle_to_fd[nhandles] = f;
-              handles[nhandles++] = (HANDLE) f->fd;
-            }
-        }
-      f->revents = 0;
-    }
-
-  if (_g_main_poll_debug)
-    g_print ("\n");
-
-  if (timeout == -1)
-    timeout = INFINITE;
-
   /* Polling for several things? */
-  if (nhandles > 1 || (nhandles > 0 && msg_fd != NULL))
+  if (data->nhandles > 1 || (data->nhandles > 0 && data->msg_fd != NULL))
     {
       /* First check if one or several of them are immediately
        * available
        */
-      retval = poll_rest (msg_fd, handles, handle_to_fd, nhandles, 0);
+      retval = poll_rest (data->msg_fd, data->stop_fd, data->handles, data->handle_to_fd, data->nhandles, 0);
 
       /* If not, and we have a significant timeout, poll again with
        * timeout then. Note that this will return indication for only
        * one event, or only for messages.
        */
-      if (retval == 0 && (timeout == INFINITE || timeout > 0))
+      if (retval == 0 && (data->timeout_ms == INFINITE || data->timeout_ms > 0))
-	retval = poll_rest (msg_fd, handles, handle_to_fd, nhandles, timeout);
+        retval = poll_rest (data->msg_fd, data->stop_fd, data->handles, data->handle_to_fd, data->nhandles, data->timeout_ms);
     }
   else
     {
       /* Just polling for one thing, so no need to check first if
        * available immediately
        */
-      retval = poll_rest (msg_fd, handles, handle_to_fd, nhandles, timeout);
+      retval = poll_rest (data->msg_fd, data->stop_fd, data->handles, data->handle_to_fd, data->nhandles, data->timeout_ms);
+    }
+
+  return retval;
+}
+
+static void
+fill_poll_thread_data (GPollFD              *fds,
+                       guint                 nfds,
+                       gint                  timeout_ms,
+                       GPollFD              *stop_fd,
+                       GWin32PollThreadData *data)
+{
+  GPollFD *f;
+
+  data->timeout_ms = timeout_ms;
+
+  if (stop_fd != NULL)
+    {
+      if (_g_main_poll_debug)
+        g_print (" Stop FD: %p", (HANDLE) stop_fd->fd);
+
+      g_assert (data->nhandles < MAXIMUM_WAIT_OBJECTS);
+
+      data->stop_fd = stop_fd;
+      data->handle_to_fd[data->nhandles] = stop_fd;
+      data->handles[data->nhandles++] = (HANDLE) stop_fd->fd;
+    }
+
+  for (f = fds; f < &fds[nfds]; ++f)
+    {
+      if ((data->nhandles == MAXIMUM_WAIT_OBJECTS) ||
+          (data->msg_fd != NULL && (data->nhandles == MAXIMUM_WAIT_OBJECTS - 1)))
+        {
+          g_warning ("Too many handles to wait for!");
+          break;
+        }
+
+      if (f->fd == G_WIN32_MSG_HANDLE && (f->events & G_IO_IN))
+        {
+          if (_g_main_poll_debug && data->msg_fd == NULL)
+            g_print (" MSG");
+          data->msg_fd = f;
+        }
+      else if (f->fd > 0)
+        {
+          if (_g_main_poll_debug)
+            g_print (" %p", (HANDLE) f->fd);
+          data->handle_to_fd[data->nhandles] = f;
+          data->handles[data->nhandles++] = (HANDLE) f->fd;
+        }
+
+      f->revents = 0;
+    }
+}
+
+static guint __stdcall
+poll_thread_run (gpointer user_data)
+{
+  GWin32PollThreadData *data = user_data;
+
+  /* Docs say that it is safer to call _endthreadex by our own:
+   * https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/endthread-endthreadex
+   */
+  _endthreadex (poll_single_thread (data));
+
+  g_assert_not_reached ();
+
+  return 0;
+}
+
+/* One slot for a possible msg object or the stop event */
+#define MAXIMUM_WAIT_OBJECTS_PER_THREAD (MAXIMUM_WAIT_OBJECTS - 1)
+
+gint
+g_poll (GPollFD *fds,
+	guint    nfds,
+	gint     timeout)
+{
+  guint nthreads, threads_remain;
+  HANDLE thread_handles[MAXIMUM_WAIT_OBJECTS];
+  GWin32PollThreadData *threads_data;
+  GPollFD stop_event = { 0, };
+  GPollFD *f;
+  guint i, fds_idx = 0;
+  DWORD ready;
+  DWORD thread_retval;
+  int retval;
+  GPollFD *msg_fd = NULL;
+
+  if (timeout == -1)
+    timeout = INFINITE;
+
+  /* Simple case without extra threads */
+  if (nfds <= MAXIMUM_WAIT_OBJECTS)
+    {
+      GWin32PollThreadData data = { 0, };
+
+      if (_g_main_poll_debug)
+        g_print ("g_poll: waiting for");
+
+      fill_poll_thread_data (fds, nfds, timeout, NULL, &data);
+
+      if (_g_main_poll_debug)
+        g_print ("\n");
+
+      retval = poll_single_thread (&data);
+      if (retval == -1)
+        for (f = fds; f < &fds[nfds]; ++f)
+          f->revents = 0;
+
+      return retval;
+    }
+
+  if (_g_main_poll_debug)
+    g_print ("g_poll: polling with threads\n");
+
+  nthreads = nfds / MAXIMUM_WAIT_OBJECTS_PER_THREAD;
+  threads_remain = nfds % MAXIMUM_WAIT_OBJECTS_PER_THREAD;
+  if (threads_remain > 0)
+    nthreads++;
+
+  if (nthreads > MAXIMUM_WAIT_OBJECTS_PER_THREAD)
+    {
+      g_warning ("Too many handles to wait for in threads!");
+      nthreads = MAXIMUM_WAIT_OBJECTS_PER_THREAD;
+    }
+
+#if GLIB_SIZEOF_VOID_P == 8
+  stop_event.fd = (gint64)CreateEventW (NULL, TRUE, FALSE, NULL);
+#else
+  stop_event.fd = (gint)CreateEventW (NULL, TRUE, FALSE, NULL);
+#endif
+  stop_event.events = G_IO_IN;
+
+  threads_data = g_new0 (GWin32PollThreadData, nthreads);
+  for (i = 0; i < nthreads; i++)
+    {
+      guint thread_fds;
+      guint ignore;
+
+      if (i == (nthreads - 1) && threads_remain > 0)
+        thread_fds = threads_remain;
+      else
+        thread_fds = MAXIMUM_WAIT_OBJECTS_PER_THREAD;
+
+      fill_poll_thread_data (fds + fds_idx, thread_fds, timeout, &stop_event, &threads_data[i]);
+      fds_idx += thread_fds;
+
+      /* We must poll for messages from the same thread, so poll it along with the threads */
+      if (threads_data[i].msg_fd != NULL)
+        {
+          msg_fd = threads_data[i].msg_fd;
+          threads_data[i].msg_fd = NULL;
+        }
+
+      thread_handles[i] = (HANDLE) _beginthreadex (NULL, 0, poll_thread_run, &threads_data[i], 0, &ignore);
+    }
+
+  /* Wait for at least one thread to return */
+  if (msg_fd != NULL)
+    ready = MsgWaitForMultipleObjectsEx (nthreads, thread_handles, timeout,
+                                         QS_ALLINPUT, MWMO_ALERTABLE);
+  else
+    ready = WaitForMultipleObjects (nthreads, thread_handles, timeout > 0, timeout);
+
+  /* Signal the stop in case any of the threads did not stop yet */
+  if (!SetEvent ((HANDLE)stop_event.fd))
+    {
+      gchar *emsg = g_win32_error_message (GetLastError ());
+      g_warning ("gpoll: failed to signal the stop event: %s", emsg);
+      g_free (emsg);
+    }
+
+  /* Wait for the rest of the threads to finish */
+  WaitForMultipleObjects (nthreads, thread_handles, TRUE, INFINITE);
+
+  /* The return value of all the threads give us all the fds that changed state */
+  retval = 0;
+  if (msg_fd != NULL && ready == WAIT_OBJECT_0 + nthreads)
+    {
+      msg_fd->revents |= G_IO_IN;
+      retval = 1;
+    }
+
+  for (i = 0; i < nthreads; i++)
+    {
+      if (GetExitCodeThread (thread_handles[i], &thread_retval))
+        retval = retval == -1 ? -1 : thread_retval == -1 ? -1 : retval + thread_retval;
+
+      CloseHandle (thread_handles[i]);
     }
 
   if (retval == -1)
     for (f = fds; f < &fds[nfds]; ++f)
       f->revents = 0;
 
+  g_free (threads_data);
+  CloseHandle ((HANDLE)stop_event.fd);
+
   return retval;
 }