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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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1
docs/libcurl/Makefile.inc
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@ -46,6 +46,7 @@ man_MANS = \
curl_multi_info_read.3 \
curl_multi_init.3 \
curl_multi_perform.3 \
curl_multi_poll.3 \
curl_multi_remove_handle.3 \
curl_multi_setopt.3 \
curl_multi_socket.3 \

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docs/libcurl/curl_multi_poll.3 Normal file
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@ -0,0 +1,110 @@
.\" **************************************************************************
.\" * _ _ ____ _
.\" * Project ___| | | | _ \| |
.\" * / __| | | | |_) | |
.\" * | (__| |_| | _ <| |___
.\" * \___|\___/|_| \_\_____|
.\" *
.\" * 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
.\" * are also available at https://curl.haxx.se/docs/copyright.html.
.\" *
.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
.\" * copies of the Software, and permit persons to whom the Software is
.\" * furnished to do so, under the terms of the COPYING file.
.\" *
.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
.\" * KIND, either express or implied.
.\" *
.\" **************************************************************************
.TH curl_multi_poll 3 "29 Jul 2019" "libcurl 7.66.0" "libcurl Manual"
.SH NAME
curl_multi_poll - polls on all easy handles in a multi handle
.SH SYNOPSIS
.nf
#include <curl/curl.h>
CURLMcode curl_multi_poll(CURLM *multi_handle,
struct curl_waitfd extra_fds[],
unsigned int extra_nfds,
int timeout_ms,
int *numfds);
.ad
.SH DESCRIPTION
\fIcurl_multi_poll(3)\fP polls all file descriptors used by the curl easy
handles contained in the given multi handle set. It will block until activity
is detected on at least one of the handles or \fItimeout_ms\fP has passed.
Alternatively, if the multi handle has a pending internal timeout that has a
shorter expiry time than \fItimeout_ms\fP, that shorter time will be used
instead to make sure timeout accuracy is reasonably kept.
The calling application may pass additional curl_waitfd structures which are
similar to \fIpoll(2)\fP's pollfd structure to be waited on in the same call.
On completion, if \fInumfds\fP is non-NULL, it will be populated with the
total number of file descriptors on which interesting events occurred. This
number can include both libcurl internal descriptors as well as descriptors
provided in \fIextra_fds\fP.
If no extra file descriptors are provided and libcurl has no file descriptor
to offer to wait for, this function will instead wait during \fItimeout_ms\fP
milliseconds (or shorter if an internal timer indicates so). This is the
detail that makes this function different than \fIcurl_multi_wait(3)\fP.
This function is encouraged to be used instead of select(3) when using the
multi interface to allow applications to easier circumvent the common problem
with 1024 maximum file descriptors.
.SH curl_waitfd
.nf
struct curl_waitfd {
curl_socket_t fd;
short events;
short revents;
};
.fi
.IP CURL_WAIT_POLLIN
Bit flag to curl_waitfd.events indicating the socket should poll on read
events such as new data received.
.IP CURL_WAIT_POLLPRI
Bit flag to curl_waitfd.events indicating the socket should poll on high
priority read events such as out of band data.
.IP CURL_WAIT_POLLOUT
Bit flag to curl_waitfd.events indicating the socket should poll on write
events such as the socket being clear to write without blocking.
.SH EXAMPLE
.nf
CURL *easy_handle;
CURLM *multi_handle;
/* add the individual easy handle */
curl_multi_add_handle(multi_handle, easy_handle);
do {
CURLMcode mc;
int numfds;
mc = curl_multi_perform(multi_handle, &still_running);
if(mc == CURLM_OK) {
/* wait for activity or timeout */
mc = curl_multi_poll(multi_handle, NULL, 0, 1000, &numfds);
}
if(mc != CURLM_OK) {
fprintf(stderr, "curl_multi failed, code %d.\\n", mc);
break;
}
} while(still_running);
curl_multi_remove_handle(multi_handle, easy_handle);
.fi
.SH RETURN VALUE
CURLMcode type, general libcurl multi interface error code. See
\fIlibcurl-errors(3)\fP
.SH AVAILABILITY
This function was added in libcurl 7.66.0.
.SH "SEE ALSO"
.BR curl_multi_fdset "(3), " curl_multi_perform "(3), " curl_multi_wait "(3)"