[PATCH 3 of 4] Mail: added MPTCP support

[Anthony Doeraene]

# HG changeset patch
# User Anthony Doeraene <anthony.doeraene.dev at gmail.com>
# Date 1723532381 -7200
#      Tue Aug 13 08:59:41 2024 +0200
# Node ID 19780d9b772a8b7192c20fc0fe7890dd381f0cce
# Parent  d5b3c722c6796f5b163821b9a8402457420ade4a
Mail: added MPTCP support.

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.

The 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.

Even if MPTCP is supported by different OS, only Linux supports the
`IPPROTO_MPTCP` protocol, which is why this feature is currently
limited to Linux only.

This patch adds a new parameter 'multipath' to the 'listen' directive
in the Mail module.

Co-developed-by: Maxime Dourov <mux99 at live.be>

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]

diff -r d5b3c722c679 -r 19780d9b772a src/mail/ngx_mail.c
--- a/src/mail/ngx_mail.c	Tue Aug 13 08:55:43 2024 +0200
+++ b/src/mail/ngx_mail.c	Tue Aug 13 08:59:41 2024 +0200
@@ -332,6 +332,7 @@
             ls->log.data = &ls->addr_text;
             ls->log.handler = ngx_accept_log_error;
 
+            ls->protocol = addr[i].opt.protocol;
             ls->backlog = addr[i].opt.backlog;
             ls->rcvbuf = addr[i].opt.rcvbuf;
             ls->sndbuf = addr[i].opt.sndbuf;
diff -r d5b3c722c679 -r 19780d9b772a src/mail/ngx_mail.h
--- a/src/mail/ngx_mail.h	Tue Aug 13 08:55:43 2024 +0200
+++ b/src/mail/ngx_mail.h	Tue Aug 13 08:59:41 2024 +0200
@@ -47,6 +47,7 @@
     int                     tcp_keepintvl;
     int                     tcp_keepcnt;
 #endif
+    int                     protocol;
     int                     backlog;
     int                     rcvbuf;
     int                     sndbuf;
diff -r d5b3c722c679 -r 19780d9b772a src/mail/ngx_mail_core_module.c
--- a/src/mail/ngx_mail_core_module.c	Tue Aug 13 08:55:43 2024 +0200
+++ b/src/mail/ngx_mail_core_module.c	Tue Aug 13 08:59:41 2024 +0200
@@ -477,6 +477,13 @@
 #endif
         }
 
+#ifdef IPPROTO_MPTCP
+        if (ngx_strcmp(value[i].data, "multipath") == 0) {
+            ls->protocol = IPPROTO_MPTCP;
+            continue;
+        }
+#endif
+
         if (ngx_strncmp(value[i].data, "so_keepalive=", 13) == 0) {
 
             if (ngx_strcmp(&value[i].data[13], "on") == 0) {