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, 3 insertions, 19 deletions

diff --git a/lib/easy.c b/lib/easy.c
index 616ad28b8..b8d94c740 100644
--- a/lib/easy.c
+++ b/lib/easy.c
@@ -602,27 +602,11 @@ static CURLcode easy_transfer(struct Curl_multi *multi)
 
   while(!done && !mcode) {
     int still_running = 0;
-    bool gotsocket = FALSE;
-
-    mcode = Curl_multi_wait(multi, NULL, 0, 1000, NULL, &gotsocket);
-
-    if(!mcode) {
-      if(!gotsocket) {
-        long sleep_ms;
-
-        /* If it returns without any filedescriptor 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;
-          Curl_wait_ms((int)sleep_ms);
-        }
-      }
 
+    mcode = curl_multi_poll(multi, NULL, 0, 1000, NULL);
+
+    if(!mcode)
       mcode = curl_multi_perform(multi, &still_running);
-    }
 
     /* only read 'still_running' if curl_multi_perform() return OK */
     if(!mcode && !still_running) {