upkeep: add a connection upkeep API: curl_easy_conn_upkeep()

Add functionality so that protocols can do custom keepalive on their
connections, when an external API function is called.

Add docs for the new options in 7.62.0

Closes #1641

1 files changed, 77 insertions, 0 deletions

diff --git a/docs/libcurl/curl_easy_conn_upkeep.3 b/docs/libcurl/curl_easy_conn_upkeep.3
new file mode 100644
index 000000000..f95f0ab22
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+++ b/docs/libcurl/curl_easy_conn_upkeep.3
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+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2018, 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_easy_conn_upkeep 3 "31 Oct 2018" "libcurl 7.62.0" "libcurl Manual"
+.SH NAME
+curl_easy_conn_upkeep - Perform any connection upkeep checks.
+.SH SYNOPSIS
+.B #include <curl/curl.h>
+
+.BI "CURLcode curl_easy_conn_upkeep(CURL *" handle ");"
+.SH DESCRIPTION
+
+Some protocols have "connection upkeep" mechanisms. These mechanisms usually
+send some traffic on existing connections in order to keep them alive; this
+can prevent connections from being closed due to overzealous firewalls, for
+example.
+
+Currently the only protocol with a connection upkeep mechanism is HTTP/2: when
+the connection upkeep interval is exceeded and \fIcurl_easy_conn_upkeep(3)\fP
+is called, an HTTP/2 PING frame is sent on the connection.
+
+This function must be explicitly called in order to perform the upkeep work.
+The connection upkeep interval is set with
+\fICURLOPT_CONN_UPKEEP_INTERVAL_MS(3)\fP.
+
+.SH AVAILABILITY
+Added in 7.62.0.
+.SH RETURN VALUE
+On success, returns \fBCURLE_OK\fP.
+
+On failure, returns the appropriate error code.
+
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  /* Make a connection to an HTTP/2 server. */
+  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
+
+  /* Set the interval to 30000ms / 30s */
+  curl_easy_setopt(curl, CURLOPT_CONN_UPKEEP_INTERVAL_MS, 30000L);
+
+  curl_easy_perform(curl);
+
+  /* Perform more work here. */
+
+  /* While the connection is being held open, curl_easy_conn_upkeep() can be
+     called. If curl_easy_conn_upkeep() is called and the time since the last
+     upkeep exceeds the interval, then an HTTP/2 PING is sent. */
+  curl_easy_conn_upkeep(curl);
+
+  /* Perform more work here. */
+
+  /* always cleanup */
+  curl_easy_cleanup(curl);
+}
+
+.fi