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curl_multi_poll: a sister to curl_multi_wait() that waits more

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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
This commit is contained in:
Daniel Stenberg 2019-07-29 13:41:00 +02:00
parent cee7639b79
commit 02346abc32
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9 changed files with 160 additions and 79 deletions
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16
include/curl/multi.h
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@ -7,7 +7,7 @@
* | (__| |_| | _ <| |___
* \___|\___/|_| \_\_____|
*
* Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
* Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
*
* This software is licensed as described in the file COPYING, which
* you should have received as part of this distribution. The terms
@ -173,6 +173,20 @@ CURL_EXTERN CURLMcode curl_multi_wait(CURLM *multi_handle,
int timeout_ms,
int *ret);
/*
* Name: curl_multi_poll()
*
* Desc: Poll on all fds within a CURLM set as well as any
* additional fds passed to the function.
*
* Returns: CURLMcode type, general multi error code.
*/
CURL_EXTERN CURLMcode curl_multi_poll(CURLM *multi_handle,
struct curl_waitfd extra_fds[],
unsigned int extra_nfds,
int timeout_ms,
int *ret);
/*
* Name: curl_multi_perform()
*