CURLOPT_MAXAGE_CONN: set the maximum allowed age for conn reuse

... and disconnect too old ones instead of trying to reuse.

Default max age is set to 118 seconds.

Ref: #3722
Closes #3782

docs/TODO

@@ -35,7 +35,6 @@
  1.16 Try to URL encode given URL
  1.17 Add support for IRIs
  1.18 try next proxy if one doesn't work
- 1.19 Timeout idle connections from the pool
  1.20 SRV and URI DNS records
  1.21 Have the URL API offer IDN decoding
  1.22 CURLINFO_PAUSE_STATE
@@ -373,16 +372,6 @@
 
  https://github.com/curl/curl/issues/896
 
-1.19 Timeout idle connections from the pool
-
- libcurl currently keeps connections in its connection pool for an indefinite
- period of time, until it either gets reused, gets noticed that it has been
- closed by the server or gets pruned to make room for a new connection.
-
- To reduce overhead (especially for when we add monitoring of the connections
- in the pool), we should introduce a timeout so that connections that have
- been idle for N seconds get closed.
-
 1.20 SRV and URI DNS records
 
  Offer support for resolving SRV and URI DNS records for libcurl to know which

docs/libcurl/curl_easy_setopt.3

@@ -468,6 +468,8 @@ Maximum number of connections in the connection pool. See \fICURLOPT_MAXCONNECTS
 Use a new connection. \fICURLOPT_FRESH_CONNECT(3)\fP
 .IP CURLOPT_FORBID_REUSE
 Prevent subsequent connections from re-using this. See \fICURLOPT_FORBID_REUSE(3)\fP
+.IP CURLOPT_MAXAGE_CONN
+Limit the age of connections for reuse. See \fICURLOPT_MAXAGE_CONN(3)\fP
 .IP CURLOPT_CONNECTTIMEOUT
 Timeout for the connection phase. See \fICURLOPT_CONNECTTIMEOUT(3)\fP
 .IP CURLOPT_CONNECTTIMEOUT_MS

docs/libcurl/opts/CURLOPT_MAXAGE_CONN.3

@@ -0,0 +1,65 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 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 CURLOPT_MAXAGE_CONN 3 "18 Apr 2019" "libcurl 7.65.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_MAXAGE_CONN \- max idle time allowed for reusing a connection
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_MAXAGE_CONN, long maxage);
+.SH DESCRIPTION
+Pass a long as parameter containing \fImaxage\fP - the maximum time in seconds
+that you allow an existing connection to have to be considered for reuse for
+this request.
+
+The "connection cache" that holds previously used connections. When a new
+request is to be done, it will consider any connection that matches for
+reuse. The \fICURLOPT_MAXAGE_CONN(3)\fP limit prevents libcurl from trying
+very old connections for reuse, since old connections have a high risk of not
+working and thus trying them is a performance loss and sometimes service loss
+due to the difficulties to figure out the situation. If a connection is found
+in the cache that is older than this set \fImaxage\fP, it will instead be
+closed.
+.SH DEFAULT
+Default maxage is 118 seconds.
+.SH PROTOCOLS
+All
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
+
+  /* only allow 30 seconds idle time */
+  curl_easy_setopt(curl, CURLOPT_MAXAGE_CONN, 30L);
+
+  curl_easy_perform(curl);
+}
+.fi
+.SH AVAILABILITY
+Added in libcurl 7.65.0
+.SH RETURN VALUE
+Returns CURLE_OK.
+.SH "SEE ALSO"
+.BR CURLOPT_TIMEOUT "(3), " CURLOPT_FORBID_REUSE "(3), "
+.BR CURLOPT_FRESH_CONNECT "(3), "

docs/libcurl/opts/Makefile.inc

@@ -189,6 +189,7 @@ man_MANS =                                      \
   CURLOPT_MAIL_AUTH.3                           \
   CURLOPT_MAIL_FROM.3                           \
   CURLOPT_MAIL_RCPT.3                           \
+  CURLOPT_MAXAGE_CONN.3                         \
   CURLOPT_MAXCONNECTS.3                         \
   CURLOPT_MAXFILESIZE.3                         \
   CURLOPT_MAXFILESIZE_LARGE.3                   \

docs/libcurl/symbols-in-versions

@@ -463,6 +463,7 @@ CURLOPT_LOW_SPEED_TIME          7.1
 CURLOPT_MAIL_AUTH               7.25.0
 CURLOPT_MAIL_FROM               7.20.0
 CURLOPT_MAIL_RCPT               7.20.0
+CURLOPT_MAXAGE_CONN             7.65.0
 CURLOPT_MAXCONNECTS             7.7
 CURLOPT_MAXFILESIZE             7.10.8
 CURLOPT_MAXFILESIZE_LARGE       7.11.0