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

2 files changed, 74 insertions, 0 deletions

diff --git a/docs/libcurl/opts/CURLOPT_CONN_UPKEEP_INTERVAL_MS.3 b/docs/libcurl/opts/CURLOPT_CONN_UPKEEP_INTERVAL_MS.3
new file mode 100644
index 000000000..e2545a5c8
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_CONN_UPKEEP_INTERVAL_MS.3
@@ -0,0 +1,73 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  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 CURLOPT_CONN_UPKEEP_INTERVAL_MS 3 "31 Oct 2018" "libcurl 7.62.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_CONN_UPKEEP_INTERVAL_MS \- connection upkeep interval
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_CONN_UPKEEP_INTERVAL_MS, long upkeep_interval_ms);
+.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.
+
+The user needs to explicitly call \fIcurl_easy_conn_upkeep(3)\fP in order to
+perform the upkeep work.
+
+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.
+
+.SH DEFAULT
+CURL_UPKEEP_INTERVAL_DEFAULT (currently defined as 60000L, which is 60 seconds)
+.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
+.SH AVAILABILITY
+Added in 7.62.0
+.SH RETURN VALUE
+Returns CURLE_OK
diff --git a/docs/libcurl/opts/Makefile.inc b/docs/libcurl/opts/Makefile.inc
index b7f664a38..869f9812e 100644
--- a/docs/libcurl/opts/Makefile.inc
+++ b/docs/libcurl/opts/Makefile.inc
@@ -94,6 +94,7 @@ man_MANS =                                      \
   CURLOPT_CHUNK_END_FUNCTION.3                  \
   CURLOPT_CLOSESOCKETDATA.3                     \
   CURLOPT_CLOSESOCKETFUNCTION.3                 \
+  CURLOPT_CONN_UPKEEP_INTERVAL_MS.3             \
   CURLOPT_CONNECTTIMEOUT.3                      \
   CURLOPT_CONNECTTIMEOUT_MS.3                   \
   CURLOPT_CONNECT_ONLY.3                        \