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

include/curl/curl.h

@@ -1918,6 +1918,9 @@ typedef enum {
   /* alt-svc cache file name to possibly read from/write to */
   CINIT(ALTSVC, STRINGPOINT, 287),
 
+  /* maximum age of a connection to consider it for reuse (in seconds) */
+  CINIT(MAXAGE_CONN, LONG, 288),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;
 

lib/conncache.c

@@ -434,6 +434,7 @@ bool Curl_conncache_return_conn(struct connectdata *conn)
   struct connectdata *conn_candidate = NULL;
 
   conn->data = NULL; /* no owner anymore */
+  conn->lastused = Curl_now(); /* it was used up until now */
   if(maxconnects > 0 &&
      Curl_conncache_size(data) > maxconnects) {
     infof(data, "Connection cache is full, closing the oldest one.\n");
@@ -479,7 +480,7 @@ Curl_conncache_extract_bundle(struct Curl_easy *data,
 
     if(!CONN_INUSE(conn) && !conn->data) {
       /* Set higher score for the age passed since the connection was used */
-      score = Curl_timediff(now, conn->now);
+      score = Curl_timediff(now, conn->lastused);
 
       if(score > highscore) {
         highscore = score;
@@ -537,7 +538,7 @@ Curl_conncache_extract_oldest(struct Curl_easy *data)
 
       if(!CONN_INUSE(conn) && !conn->data) {
         /* Set higher score for the age passed since the connection was used */
-        score = Curl_timediff(now, conn->now);
+        score = Curl_timediff(now, conn->lastused);
 
         if(score > highscore) {
           highscore = score;

lib/setopt.c

@@ -2645,6 +2645,12 @@ static CURLcode vsetopt(struct Curl_easy *data, CURLoption option,
       return CURLE_BAD_FUNCTION_ARGUMENT;
     data->set.upkeep_interval_ms = arg;
     break;
+  case CURLOPT_MAXAGE_CONN:
+    arg = va_arg(param, long);
+    if(arg < 0)
+      return CURLE_BAD_FUNCTION_ARGUMENT;
+    data->set.maxage_conn = arg;
+    break;
   case CURLOPT_TRAILERFUNCTION:
 #ifndef CURL_DISABLE_HTTP
     data->set.trailer_callback = va_arg(param, curl_trailer_callback);

lib/url.c

@@ -541,6 +541,7 @@ CURLcode Curl_init_userdefined(struct Curl_easy *data)
   set->fnmatch = ZERO_NULL;
   set->upkeep_interval_ms = CURL_UPKEEP_INTERVAL_DEFAULT;
   set->maxconnects = DEFAULT_CONNCACHE_SIZE; /* for easy handles */
+  set->maxage_conn = 118;
   set->http09_allowed = TRUE;
   set->httpversion =
 #ifdef USE_NGHTTP2
@@ -958,6 +959,25 @@ static void prune_dead_connections(struct Curl_easy *data)
   }
 }
 
+/* A connection has to have been idle for a shorter time than 'maxage_conn' to
+   be subject for reuse. The success rate is just too low after this. */
+
+static bool conn_maxage(struct Curl_easy *data,
+                        struct connectdata *conn,
+                        struct curltime now)
+{
+  if(!conn->data) {
+    timediff_t idletime = Curl_timediff(now, conn->lastused);
+    idletime /= 1000; /* integer seconds is fine */
+
+    if(idletime/1000 > data->set.maxage_conn) {
+      infof(data, "Too old connection (%ld seconds), disconnect it\n",
+            idletime);
+      return TRUE;
+    }
+  }
+  return FALSE;
+}
 /*
  * Given one filled in connection struct (named needle), this function should
  * detect if there already is one that has all the significant details
@@ -981,6 +1001,7 @@ ConnectionExists(struct Curl_easy *data,
   bool foundPendingCandidate = FALSE;
   bool canmultiplex = IsMultiplexingPossible(data, needle);
   struct connectbundle *bundle;
+  struct curltime now = Curl_now();
 
 #ifdef USE_NTLM
   bool wantNTLMhttp = ((data->state.authhost.want &
@@ -1044,7 +1065,7 @@ ConnectionExists(struct Curl_easy *data,
         /* connect-only connections will not be reused */
         continue;
 
-      if(extract_if_dead(check, data)) {
+      if(conn_maxage(data, check, now) || extract_if_dead(check, data)) {
         /* disconnect it */
         (void)Curl_disconnect(data, check, /* dead_connection */TRUE);
         continue;

lib/urldata.h

@@ -866,6 +866,7 @@ struct connectdata {
 
   struct curltime now;     /* "current" time */
   struct curltime created; /* creation time */
+  struct curltime lastused; /* when returned to the connection cache */
   curl_socket_t sock[2]; /* two sockets, the second is used for the data
                             transfer when doing FTP */
   curl_socket_t tempsock[2]; /* temporary sockets for happy eyeballs */
@@ -1553,6 +1554,8 @@ struct UserDefined {
   long accepttimeout;   /* in milliseconds, 0 means no timeout */
   long happy_eyeballs_timeout; /* in milliseconds, 0 is a valid value */
   long server_response_timeout; /* in milliseconds, 0 means no timeout */
+  long maxage_conn;     /* in seconds, max idle time to allow a connection that
+                           is to be reused */
   long tftp_blksize;    /* in bytes, 0 means use default */
   curl_off_t filesize;  /* size of file to upload, -1 means unknown */
   long low_speed_limit; /* bytes/second */