multi: reduce Win32 API calls to improve performance

1. Consolidate pre-checks into a single Curl_poll call:

This is an attempt to restructure the code in Curl_multi_wait
in such a way that less syscalls are made by removing individual
calls to Curl_socket_check via SOCKET_READABLE/SOCKET_WRITABLE.

2. Avoid resetting the WinSock event multiple times:

We finally call WSAResetEvent anyway, so specifying it as
an optional parameter to WSAEnumNetworkEvents is redundant.

3. Wakeup directly in case no sockets are being monitoring:

Fix the WinSock based implementation to skip extra waiting by
not sleeping in case no sockets are to be waited on and just
the WinSock event is being monitored for wakeup functionality.

Assisted-by: Tommy Odom
Assisted-by: Jay Satiro

Bug: curl#6146
Part of curl#6245

Author: mback2k

lib/multi.c

@@ -1086,12 +1086,10 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
   unsigned int curlfds;
   long timeout_internal;
   int retcode = 0;
-#ifndef USE_WINSOCK
   struct pollfd a_few_on_stack[NUM_POLLS_ON_STACK];
   struct pollfd *ufds = &a_few_on_stack[0];
   bool ufds_malloc = FALSE;
-#else
-  struct pollfd pre_poll;
+#ifdef USE_WINSOCK
   WSANETWORKEVENTS wsa_events;
   DEBUGASSERT(multi->wsa_event != WSA_INVALID_EVENT);
 #endif
@@ -1149,7 +1147,6 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
   }
 #endif
 
-#ifndef USE_WINSOCK
   if(nfds > NUM_POLLS_ON_STACK) {
     /* 'nfds' is a 32 bit value and 'struct pollfd' is typically 8 bytes
        big, so at 2^29 sockets this value might wrap. When a process gets
@@ -1160,9 +1157,7 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
       return CURLM_OUT_OF_MEMORY;
     ufds_malloc = TRUE;
   }
-
   nfds = 0;
-#endif
 
   /* only do the second loop if we found descriptors in the first stage run
      above */
@@ -1180,34 +1175,31 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
 #endif
         if(bitmap & GETSOCK_READSOCK(i)) {
 #ifdef USE_WINSOCK
-          if(timeout_ms && SOCKET_READABLE(sockbunch[i], 0) > 0)
-            timeout_ms = 0;
           mask |= FD_READ|FD_ACCEPT|FD_CLOSE;
-#else
+#endif
           ufds[nfds].fd = sockbunch[i];
           ufds[nfds].events = POLLIN;
           ++nfds;
-#endif
           s = sockbunch[i];
         }
         if(bitmap & GETSOCK_WRITESOCK(i)) {
 #ifdef USE_WINSOCK
-          if(timeout_ms && SOCKET_WRITABLE(sockbunch[i], 0) > 0)
-            timeout_ms = 0;
           mask |= FD_WRITE|FD_CONNECT|FD_CLOSE;
-#else
+#endif
           ufds[nfds].fd = sockbunch[i];
           ufds[nfds].events = POLLOUT;
           ++nfds;
-#endif
           s = sockbunch[i];
         }
         if(s == CURL_SOCKET_BAD) {
           break;
         }
 #ifdef USE_WINSOCK
-        if(WSAEventSelect(s, multi->wsa_event, mask) != 0)
+        if(WSAEventSelect(s, multi->wsa_event, mask) != 0) {
+          if(ufds_malloc)
+            free(ufds);
           return CURLM_INTERNAL_ERROR;
+        }
 #endif
       }
 
@@ -1219,35 +1211,18 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
   for(i = 0; i < extra_nfds; i++) {
 #ifdef USE_WINSOCK
     long mask = 0;
-    extra_fds[i].revents = 0;
-    pre_poll.fd = extra_fds[i].fd;
-    pre_poll.events = 0;
-    pre_poll.revents = 0;
-    if(extra_fds[i].events & CURL_WAIT_POLLIN) {
+    if(extra_fds[i].events & CURL_WAIT_POLLIN)
       mask |= FD_READ|FD_ACCEPT|FD_CLOSE;
-      pre_poll.events |= POLLIN;
-    }
-    if(extra_fds[i].events & CURL_WAIT_POLLPRI) {
+    if(extra_fds[i].events & CURL_WAIT_POLLPRI)
       mask |= FD_OOB;
-      pre_poll.events |= POLLPRI;
-    }
-    if(extra_fds[i].events & CURL_WAIT_POLLOUT) {
+    if(extra_fds[i].events & CURL_WAIT_POLLOUT)
       mask |= FD_WRITE|FD_CONNECT|FD_CLOSE;
-      pre_poll.events |= POLLOUT;
-    }
-    if(Curl_poll(&pre_poll, 1, 0) > 0) {
-      if(pre_poll.revents & POLLIN)
-        extra_fds[i].revents |= CURL_WAIT_POLLIN;
-      if(pre_poll.revents & POLLPRI)
-        extra_fds[i].revents |= CURL_WAIT_POLLPRI;
-      if(pre_poll.revents & POLLOUT)
-        extra_fds[i].revents |= CURL_WAIT_POLLOUT;
-      if(extra_fds[i].revents)
-        timeout_ms = 0;
-    }
-    if(WSAEventSelect(extra_fds[i].fd, multi->wsa_event, mask) != 0)
+    if(WSAEventSelect(extra_fds[i].fd, multi->wsa_event, mask) != 0) {
+      if(ufds_malloc)
+        free(ufds);
       return CURLM_INTERNAL_ERROR;
-#else
+    }
+#endif
     ufds[nfds].fd = extra_fds[i].fd;
     ufds[nfds].events = 0;
     if(extra_fds[i].events & CURL_WAIT_POLLIN)
@@ -1257,7 +1232,6 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
     if(extra_fds[i].events & CURL_WAIT_POLLOUT)
       ufds[nfds].events |= POLLOUT;
     ++nfds;
-#endif
   }
 
 #ifdef ENABLE_WAKEUP
@@ -1270,53 +1244,59 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
 #endif
 #endif
 
+#if defined(ENABLE_WAKEUP) && defined(USE_WINSOCK)
+  if(nfds || use_wakeup) {
+#else
   if(nfds) {
-    /* wait... */
+#endif
+    int pollrc;
 #ifdef USE_WINSOCK
-    WSAWaitForMultipleEvents(1, &multi->wsa_event, FALSE, timeout_ms, FALSE);
+    if(nfds)
+      pollrc = Curl_poll(ufds, nfds, 0); /* just pre-check with WinSock */
+    else
+      pollrc = 0;
+    if(pollrc <= 0) /* now wait... if not ready during the pre-check above */
+      WSAWaitForMultipleEvents(1, &multi->wsa_event, FALSE, timeout_ms, FALSE);
 #else
-    int pollrc = Curl_poll(ufds, nfds, timeout_ms);
+    pollrc = Curl_poll(ufds, nfds, timeout_ms); /* wait... */
 #endif
 
-#ifdef USE_WINSOCK
-    /* With Winsock, we have to run this unconditionally to call
-       WSAEventSelect(fd, event, 0) on all the sockets */
-    {
-      retcode = 0;
-#else
     if(pollrc > 0) {
       retcode = pollrc;
+#ifdef USE_WINSOCK
+    }
+    /* With WinSock, we have to run the following section unconditionally
+       to call WSAEventSelect(fd, event, 0) on all the sockets */
+    {
 #endif
       /* copy revents results from the poll to the curl_multi_wait poll
          struct, the bit values of the actual underlying poll() implementation
          may not be the same as the ones in the public libcurl API! */
       for(i = 0; i < extra_nfds; i++) {
+        unsigned r = ufds[curlfds + i].revents;
         unsigned short mask = 0;
 #ifdef USE_WINSOCK
         wsa_events.lNetworkEvents = 0;
-        mask = extra_fds[i].revents;
-        if(WSAEnumNetworkEvents(extra_fds[i].fd, multi->wsa_event,
-                                &wsa_events) == 0) {
+        if(WSAEnumNetworkEvents(extra_fds[i].fd, NULL, &wsa_events) == 0) {
           if(wsa_events.lNetworkEvents & (FD_READ|FD_ACCEPT|FD_CLOSE))
             mask |= CURL_WAIT_POLLIN;
           if(wsa_events.lNetworkEvents & (FD_WRITE|FD_CONNECT|FD_CLOSE))
             mask |= CURL_WAIT_POLLOUT;
           if(wsa_events.lNetworkEvents & FD_OOB)
             mask |= CURL_WAIT_POLLPRI;
-          if(ret && wsa_events.lNetworkEvents != 0)
+          if(ret && pollrc <= 0 && wsa_events.lNetworkEvents != 0)
             retcode++;
         }
         WSAEventSelect(extra_fds[i].fd, multi->wsa_event, 0);
-#else
-        unsigned r = ufds[curlfds + i].revents;
-
+        if(pollrc <= 0)
+          continue;
+#endif
         if(r & POLLIN)
           mask |= CURL_WAIT_POLLIN;
         if(r & POLLOUT)
           mask |= CURL_WAIT_POLLOUT;
         if(r & POLLPRI)
           mask |= CURL_WAIT_POLLPRI;
-#endif
         extra_fds[i].revents = mask;
       }
 
@@ -1331,17 +1311,8 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
           for(i = 0; i < MAX_SOCKSPEREASYHANDLE; i++) {
             if(bitmap & (GETSOCK_READSOCK(i) | GETSOCK_WRITESOCK(i))) {
               wsa_events.lNetworkEvents = 0;
-              if(WSAEnumNetworkEvents(sockbunch[i], multi->wsa_event,
-                                      &wsa_events) == 0) {
-                if(ret && wsa_events.lNetworkEvents != 0)
-                  retcode++;
-              }
-              if(ret && !timeout_ms && wsa_events.lNetworkEvents == 0) {
-                if((bitmap & GETSOCK_READSOCK(i)) &&
-                   SOCKET_READABLE(sockbunch[i], 0) > 0)
-                  retcode++;
-                else if((bitmap & GETSOCK_WRITESOCK(i)) &&
-                   SOCKET_WRITABLE(sockbunch[i], 0) > 0)
+              if(WSAEnumNetworkEvents(sockbunch[i], NULL, &wsa_events) == 0) {
+                if(ret && pollrc <= 0 && wsa_events.lNetworkEvents != 0)
                   retcode++;
               }
               WSAEventSelect(sockbunch[i], multi->wsa_event, 0);
@@ -1382,16 +1353,15 @@ static CURLMcode multi_wait(struct Curl_multi *multi,
     }
   }
 
-#ifndef USE_WINSOCK
   if(ufds_malloc)
     free(ufds);
-#endif
   if(ret)
     *ret = retcode;
-  if(!extrawait || nfds)
-    /* if any socket was checked */
-    ;
-  else {
+#if defined(ENABLE_WAKEUP) && defined(USE_WINSOCK)
+  if(extrawait && !nfds && !use_wakeup) {
+#else
+  if(extrawait && !nfds) {
+#endif
     long sleep_ms = 0;
 
     /* Avoid busy-looping when there's nothing particular to wait for */