CURLOPT_MAXLIFETIME_CONN: maximum allowed lifetime for conn reuse

... and close connections that are too old instead of reusing them.

By default, this behavior is disabled.

Bug: https://curl.se/mail/lib-2021-09/0058.html
Closes #7751

docs/libcurl/curl_easy_setopt.3

@@ -492,7 +492,9 @@ 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
+Limit the age (idle time) of connections for reuse. See \fICURLOPT_MAXAGE_CONN(3)\fP
+.IP CURLOPT_MAXLIFETIME_CONN
+Limit the age (since creation) of connections for reuse. See \fICURLOPT_MAXLIFETIME_CONN(3)\fP
 .IP CURLOPT_CONNECTTIMEOUT
 Timeout for the connection phase. See \fICURLOPT_CONNECTTIMEOUT(3)\fP
 .IP CURLOPT_CONNECTTIMEOUT_MS

docs/libcurl/opts/CURLOPT_FORBID_REUSE.3

@@ -57,3 +57,4 @@ Always
 Returns CURLE_OK
 .SH "SEE ALSO"
 .BR CURLOPT_FRESH_CONNECT "(3), " CURLOPT_MAXCONNECTS "(3), "
+.BR CURLOPT_MAXLIFETIME_CONN "(3), "

docs/libcurl/opts/CURLOPT_MAXAGE_CONN.3

@@ -29,8 +29,8 @@ CURLOPT_MAXAGE_CONN \- max idle time allowed for reusing a connection
 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.
+that you allow an existing connection to have been idle 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
@@ -62,4 +62,4 @@ Added in libcurl 7.65.0
 Returns CURLE_OK.
 .SH "SEE ALSO"
 .BR CURLOPT_TIMEOUT "(3), " CURLOPT_FORBID_REUSE "(3), "
-.BR CURLOPT_FRESH_CONNECT "(3), "
+.BR CURLOPT_FRESH_CONNECT "(3), " CURLOPT_MAXLIFETIME_CONN "(3), "

docs/libcurl/opts/CURLOPT_MAXLIFETIME_CONN.3

@@ -0,0 +1,66 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 2021, 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.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_MAXLIFETIME_CONN 3 "10 Nov 2021" "libcurl 7.80.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_MAXLIFETIME_CONN \- max lifetime (since creation) allowed for reusing a connection
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_MAXLIFETIME_CONN, long maxlifetime);
+.SH DESCRIPTION
+Pass a long as parameter containing \fImaxlifetime\fP - the maximum time in
+seconds, since the creation of the connection, that you allow an existing
+connection to have to be considered for reuse for this request.
+
+libcurl features a 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_MAXLIFETIME_CONN(3)\fP limit prevents libcurl from
+trying very old connections for reuse. This can be used for client-side load
+balancing. If a connection is found in the cache that is older than this set
+\fImaxlifetime\fP, it will instead be closed once any in-progress transfers
+complete.
+
+If set to 0, this behavior is disabled: all connections are eligible for reuse.
+.SH DEFAULT
+Default \fImaxlifetime\fP is 0 seconds (i.e., disabled).
+.SH PROTOCOLS
+All
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
+
+  /* only allow each connection to be reused for 30 seconds */
+  curl_easy_setopt(curl, CURLOPT_MAXLIFETIME_CONN, 30L);
+
+  curl_easy_perform(curl);
+}
+.fi
+.SH AVAILABILITY
+Added in libcurl 7.80.0
+.SH RETURN VALUE
+Returns CURLE_OK.
+.SH "SEE ALSO"
+.BR CURLOPT_TIMEOUT "(3), " CURLOPT_FORBID_REUSE "(3), "
+.BR CURLOPT_FRESH_CONNECT "(3), " CURLOPT_MAXAGE_CONN "(3), "

docs/libcurl/opts/Makefile.inc

@@ -228,6 +228,7 @@ man_MANS =                                      \
   CURLOPT_MAXCONNECTS.3                         \
   CURLOPT_MAXFILESIZE.3                         \
   CURLOPT_MAXFILESIZE_LARGE.3                   \
+  CURLOPT_MAXLIFETIME_CONN.3                    \
   CURLOPT_MAXREDIRS.3                           \
   CURLOPT_MAX_RECV_SPEED_LARGE.3                \
   CURLOPT_MAX_SEND_SPEED_LARGE.3                \

docs/libcurl/symbols-in-versions

@@ -499,6 +499,7 @@ CURLOPT_MAXAGE_CONN             7.65.0
 CURLOPT_MAXCONNECTS             7.7
 CURLOPT_MAXFILESIZE             7.10.8
 CURLOPT_MAXFILESIZE_LARGE       7.11.0
+CURLOPT_MAXLIFETIME_CONN        7.80.0
 CURLOPT_MAXREDIRS               7.5
 CURLOPT_MAX_RECV_SPEED_LARGE    7.15.5
 CURLOPT_MAX_SEND_SPEED_LARGE    7.15.5