Add Multipath TCP (MPTCP) support (Core)

Multipath TCP (MPTCP), standardized in RFC8684 [1],
is a TCP extension that enables a TCP connection to
use different paths.

Multipath TCP has been used for several use cases.
On smartphones, MPTCP enables seamless handovers between
cellular and Wi-Fi networks while preserving established
connections. This use-case is what pushed Apple to use
MPTCP since 2013 in multiple applications [2]. On dual-stack
hosts, Multipath TCP enables the TCP connection to
automatically use the best performing path, either IPv4
or IPv6. If one path fails, MPTCP automatically uses
the other path.

To benefit from MPTCP, both the client and the server
have to support it. Multipath TCP is a backward-compatible
TCP extension that is enabled by default on recent
Linux distributions (Debian, Ubuntu, Redhat, ...). Multipath
TCP is included in the Linux kernel since version 5.6 [3].
To use it on Linux, an application must explicitly enable
it when creating the socket. No need to change anything
else in the application.

Adding the possibility to create MPTCP sockets would thus
be a really fine addition to httpd, by allowing clients
to make use of their different interfaces.

This patch introduces the possibility to listen with MPTCP
sockets. Note however that these changes are only available
on Linux, as IPPROTO_MPTCP is Linux specific for the time being.

To do so, we extended the Listen directive to include
a "multipathtcp" option, allowing to create MPTCP sockets
instead of regular TCP ones:

Listen 80 options=multipathtcp

We then store this information in flags for the listen directive
and create sockets appropriately according to this value.

Link: https://www.rfc-editor.org/rfc/rfc8684.html [1]
Link: https://www.tessares.net/apples-mptcp-story-so-far/ [2]
Link: https://www.mptcp.dev [3]

docs/manual/mod/mpm_common.xml

@@ -278,6 +278,12 @@ Listen 192.170.2.5:8000
       to the same port.  (If the server is built with IPv4-mapped
       addresses <em>disabled</em>, this is the default behaviour and
       this option has no effect.)</li>
+
+      <li><code>multipathtcp</code>: Enable the use of
+      <a href="https://mptcp.dev">Multipath TCP (MPTCP)</a> for the
+      sockets. Beware that this option is currently limited to Linux
+      only.
+      </li>
     </ul>
 
     <note><title>Error condition</title>

include/ap_listen.h

@@ -44,6 +44,7 @@ typedef apr_status_t (*accept_function)(void **csd, ap_listen_rec *lr, apr_pool_
 #define AP_LISTEN_FREEBIND        (0x0002)
 #define AP_LISTEN_REUSEPORT       (0x0004)
 #define AP_LISTEN_V6ONLY          (0x0008)
+#define AP_LISTEN_MPTCP           (0x0010)
 
 /**
  * @brief Apache's listeners record.

server/listen.c

@@ -496,6 +496,12 @@ static const char *alloc_listener(process_rec *process, const char *addr,
 
     while (sa) {
         ap_listen_rec *new;
+        int sock_proto = 0;
+
+#ifdef IPPROTO_MPTCP
+        if (flags & AP_LISTEN_MPTCP)
+            sock_proto = IPPROTO_MPTCP;
+#endif
 
         /* this has to survive restarts */
         new = apr_palloc(process->pool, sizeof(ap_listen_rec));
@@ -509,7 +515,7 @@ static const char *alloc_listener(process_rec *process, const char *addr,
         sa = sa->next;
 
         status = apr_socket_create(&new->sd, new->bind_addr->family,
-                                    SOCK_STREAM, 0, process->pool);
+                                    SOCK_STREAM, sock_proto, process->pool);
 
 #if APR_HAVE_IPV6
         /* What could happen is that we got an IPv6 address, but this system
@@ -864,6 +870,7 @@ AP_DECLARE(apr_status_t) ap_duplicate_listeners(apr_pool_t *p, server_rec *s,
             char *hostname;
             apr_port_t port;
             apr_sockaddr_t *sa;
+            int sock_proto = 0;
 #ifdef HAVE_SYSTEMD
             if (use_systemd) {
                 int thesock;
@@ -891,8 +898,12 @@ AP_DECLARE(apr_status_t) ap_duplicate_listeners(apr_pool_t *p, server_rec *s,
                 duplr->bind_addr = sa;
                 duplr->next = NULL;
                 duplr->flags = lr->flags;
+#ifdef IPPROTO_MPTCP
+                if (duplr->flags & AP_LISTEN_MPTCP)
+                    sock_proto = IPPROTO_MPTCP;
+#endif
                 stat = apr_socket_create(&duplr->sd, duplr->bind_addr->family,
-                                         SOCK_STREAM, 0, p);
+                                         SOCK_STREAM, sock_proto, p);
                 if (stat != APR_SUCCESS) {
                     ap_log_perror(APLOG_MARK, APLOG_CRIT, 0, p, APLOGNO(02640)
                                 "ap_duplicate_listeners: for address %pI, "
@@ -1038,6 +1049,13 @@ static const char *parse_listen_flags(apr_pool_t *temp_pool, const char *arg,
             flags |= AP_LISTEN_REUSEPORT;
         else if (ap_cstr_casecmp(token, "v6only") == 0)
             flags |= AP_LISTEN_V6ONLY;
+        else if (ap_cstr_casecmp(token, "multipathtcp") == 0)
+#ifdef IPPROTO_MPTCP
+            flags |= AP_LISTEN_MPTCP;
+#else
+            return apr_psprintf(temp_pool, "Listen option '%s' in '%s' is not supported on this system",
+                                token, arg);
+#endif
         else
             return apr_psprintf(temp_pool, "Unknown Listen option '%s' in '%s'",
                                 token, arg);