curl_multi_poll: a sister to curl_multi_wait() that waits more

Repeatedly we see problems where using curl_multi_wait() is difficult or
just awkward because if it has no file descriptor to wait for
internally, it returns immediately and leaves it to the caller to wait
for a small amount of time in order to avoid occasional busy-looping.

This is often missed or misunderstood, leading to underperforming
applications.

This change introduces curl_multi_poll() as a replacement drop-in
function that accepts the exact same set of arguments. This function
works identically to curl_multi_wait() - EXCEPT - for the case when
there's nothing to wait for internally, as then this function will by
itself wait for a "suitable" short time before it returns. This
effectiely avoids all risks of busy-looping and should also make it less
likely that apps "over-wait".

This also changes the curl tool to use this funtion internally when
doing parallel transfers and changes curl_easy_perform() to use it
internally.

Closes #4163

1 files changed, 2 insertions, 40 deletions

diff --git a/src/tool_operate.c b/src/tool_operate.c
index 7cb2f01f9..946dc7cca 100644
--- a/src/tool_operate.c
+++ b/src/tool_operate.c
@@ -1904,23 +1904,6 @@ static CURLcode create_transfers(struct GlobalConfig *global,
   return result;
 }
 
-/* portable millisecond sleep */
-static void wait_ms(int ms)
-{
-#if defined(MSDOS)
-  delay(ms);
-#elif defined(WIN32)
-  Sleep(ms);
-#elif defined(HAVE_USLEEP)
-  usleep(1000 * ms);
-#else
-  struct timeval pending_tv;
-  pending_tv.tv_sec = ms / 1000;
-  pending_tv.tv_usec = (ms % 1000) * 1000;
-  (void)select(0, NULL, NULL, NULL, &pending_tv);
-#endif
-}
-
 static long all_added; /* number of easy handles currently added */
 
 static int add_parallel_transfers(struct GlobalConfig *global,
@@ -1971,30 +1954,9 @@ static CURLcode parallel_transfers(struct GlobalConfig *global,
     return result;
 
   while(!done && !mcode && still_running) {
-    int numfds;
-    struct timeval before = tvnow();
-    long delta;
-
-    mcode = curl_multi_wait(multi, NULL, 0, 1000, &numfds);
-    delta = tvdiff(tvnow(), before);
-
-    if(!mcode) {
-      if(!numfds && (delta < 30)) {
-        long sleep_ms;
-
-        /* If it returns without any file descriptor instantly, we need to
-           avoid busy-looping during periods where it has nothing particular
-           to wait for */
-        curl_multi_timeout(multi, &sleep_ms);
-        if(sleep_ms) {
-          if(sleep_ms > 1000)
-            sleep_ms = 1000;
-          wait_ms((int)sleep_ms);
-        }
-      }
-
+    mcode = curl_multi_poll(multi, NULL, 0, 1000, NULL);
+    if(!mcode)
       mcode = curl_multi_perform(multi, &still_running);
-    }
 
     progress_meter(global, &start, FALSE);