multi: add CURLMOPT_NETWORK_CHANGED to signal network changed

New multi option CURLMOPT_NETWORK_CHANGED with a long bitmask value:

- CURLM_NWCOPT_CLEAR_CONNS: do not reuse existing connections, close all
  idle connections.

- CURLM_NWCOPT_CLEAR_DNS: clear the multi's DNS cache.

All other bits reserved for future extensions.

Fixes #17225
Reported-by: ウさん
Closes #17613

docs/libcurl/curl_multi_setopt.md

@@ -68,6 +68,10 @@ CURLMOPT_MAX_HOST_CONNECTIONS(3)
 
 Max simultaneously open connections. See CURLMOPT_MAX_TOTAL_CONNECTIONS(3)
 
+## CURLMOPT_NETWORK_CHANGED
+
+Signal that the network has changed. See CURLMOPT_NETWORK_CHANGED(3)
+
 ## CURLMOPT_PIPELINING
 
 Enable HTTP multiplexing. See CURLMOPT_PIPELINING(3)

docs/libcurl/opts/CURLMOPT_NETWORK_CHANGED.md

@@ -0,0 +1,80 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLMOPT_NETWORK_CHANGED
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_FRESH_CONNECT (3)
+  - CURLOPT_FORBID_REUSE (3)
+Protocol:
+  - All
+Added-in: 8.16.0
+---
+
+# NAME
+
+CURLMOPT_NETWORK_CHANGED - signal network changed
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_NETWORK_CHANGED,
+                            long value);
+~~~
+
+# DESCRIPTION
+
+Pass a long with a bitmask to tell libcurl how the multi
+handle should react. The following values in the mask are
+defined. All bits not mentioned are reserved for future
+extensions.
+
+This option can be set at any time and repeatedly. Each call only
+affects the *currently* cached connections and DNS information.
+Any connection created or DNS information added afterwards is
+cached the usual way again. Phrasing it another way: the option is
+not persisted but setting it serves as a "trigger"
+to clear the caches.
+
+The call affects only the connection and DNS cache of the multi handle
+itself and not the ones owned by SHARE handles.
+
+## CURLM_NWCOPT_CLEAR_CONNS
+
+No longer reuse any existing connection in the multi handle's
+connection cache. This closes all connections that are not in use.
+Ongoing transfers continue on the connections they operate on.
+
+## CURLM_NWCOPT_CLEAR_DNS
+
+Clear the multi handle's DNS cache.
+
+# DEFAULT
+
+0, which has no effect.
+
+# %PROTOCOLS%
+
+# EXAMPLE
+
+~~~c
+int main(void)
+{
+  CURLM *m = curl_multi_init();
+  /* do transfers on the multi handle */
+  /* do not reuse existing connections */
+  curl_multi_setopt(m, CURLMOPT_NETWORK_CHANGED, CURLM_NWCOPT_CLEAR_CONNS);
+}
+~~~
+
+# %AVAILABILITY%
+
+# RETURN VALUE
+
+curl_multi_setopt(3) returns a CURLMcode indicating success or error.
+
+CURLM_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLOPT_FORBID_REUSE.md

@@ -8,6 +8,7 @@ See-also:
   - CURLOPT_FRESH_CONNECT (3)
   - CURLOPT_MAXCONNECTS (3)
   - CURLOPT_MAXLIFETIME_CONN (3)
+  - CURLMOPT_NETWORK_CHANGED (3)
 Protocol:
   - All
 Added-in: 7.7

docs/libcurl/opts/CURLOPT_FRESH_CONNECT.md

@@ -10,6 +10,7 @@ See-also:
   - CURLOPT_FORBID_REUSE (3)
   - CURLOPT_MAXAGE_CONN (3)
   - CURLOPT_MAXLIFETIME_CONN (3)
+  - CURLMOPT_NETWORK_CHANGED (3)
 Added-in: 7.7
 ---
 

docs/libcurl/opts/Makefile.inc

@@ -108,6 +108,7 @@ man_MANS =                                      \
   CURLMOPT_MAX_PIPELINE_LENGTH.3                \
   CURLMOPT_MAX_TOTAL_CONNECTIONS.3              \
   CURLMOPT_MAXCONNECTS.3                        \
+  CURLMOPT_NETWORK_CHANGED.3                    \
   CURLMOPT_PIPELINING.3                         \
   CURLMOPT_PIPELINING_SERVER_BL.3               \
   CURLMOPT_PIPELINING_SITE_BL.3                 \

docs/libcurl/symbols-in-versions

@@ -545,6 +545,8 @@ CURLM_BAD_SOCKET                7.15.4
 CURLM_CALL_MULTI_PERFORM        7.9.6
 CURLM_CALL_MULTI_SOCKET         7.15.5
 CURLM_INTERNAL_ERROR            7.9.6
+CURLM_NWCOPT_CLEAR_CONNS        8.16.0
+CURLM_NWCOPT_CLEAR_DNS          8.16.0
 CURLM_OK                        7.9.6
 CURLM_OUT_OF_MEMORY             7.9.6
 CURLM_RECURSIVE_API_CALL        7.59.0
@@ -559,6 +561,7 @@ CURLMOPT_MAX_HOST_CONNECTIONS   7.30.0
 CURLMOPT_MAX_PIPELINE_LENGTH    7.30.0
 CURLMOPT_MAX_TOTAL_CONNECTIONS  7.30.0
 CURLMOPT_MAXCONNECTS            7.16.3
+CURLMOPT_NETWORK_CHANGED        8.16.0
 CURLMOPT_PIPELINING             7.16.0
 CURLMOPT_PIPELINING_SERVER_BL   7.30.0
 CURLMOPT_PIPELINING_SITE_BL     7.30.0