Use IPv6 localhost when family is 'ipv6

This fixes Bug#34193

* src/process.c (Fmake_network_process): Explicitly use ::1 when
using IPv6 with 'local.  Update docstring.

* test/lisp/net/network-stream-tests.el
(make-ipv6-tcp-server-with-unspecified-port):
(make-ipv6-tcp-server-with-specified-port): Test creating ipv6
local server.
(make-server): Add optional family argument, default ipv4
(echo-server-with-local-ipv4): Test connecting to 'local ipv4
(echo-server-with-local-ipv6): Test connecting to 'local ipv6

* doc/lispref/processes.texi (Network Processes): Describe
behavior when using 'local.

* etc/NEWS: Document new 'make-network-process' behavior when
connecting to 'local with ipv6.

4 files changed, 79 insertions, 5 deletions

diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index fd6686e8828..7b02759b307 100644
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -2640,7 +2640,9 @@ Specify the host to connect to.  @var{host} should be a host name or
 Internet address, as a string, or the symbol @code{local} to specify
 the local host.  If you specify @var{host} for a server, it must
 specify a valid address for the local host, and only clients
-connecting to that address will be accepted.
+connecting to that address will be accepted.  When using @code{local},
+by default IPv4 will be used, specify a @var{family} of @code{ipv6} to
+override this.
 
 @item :service @var{service}
 @var{service} specifies a port number to connect to; or, for a server,
diff --git a/etc/NEWS b/etc/NEWS
index cac379fe7eb..2e3d92f2515 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -324,6 +324,11 @@ write alists of variables to ".dir-locals.el".  This is the same
 syntax that you can see in the example of a ".dir-locals.el" file in
 the node "(emacs) Directory Variables" of the user manual.
 
++++
+** Network connections using 'local can now use IPv6.
+'make-network-process' now uses the correct loopback address when
+asked to use :host 'local and :family 'ipv6.
+
 
 * Changes in Specialized Modes and Packages in Emacs 27.1
 
diff --git a/src/process.c b/src/process.c
index 728c14a7624..9502ef461e8 100644
--- a/src/process.c
+++ b/src/process.c
@@ -3733,6 +3733,8 @@ also nil, meaning that this process is not associated with any buffer.
 address.  The symbol `local' specifies the local host.  If specified
 for a server process, it must be a valid name or address for the local
 host, and only clients connecting to that address will be accepted.
+`local' will use IPv4 by default, use a FAMILY of 'ipv6 to override
+this.
 
 :service SERVICE -- SERVICE is name of the service desired, or an
 integer specifying a port number to connect to.  If SERVICE is t,
@@ -3983,14 +3985,24 @@ usage: (make-network-process &rest ARGS)  */)
 #ifdef HAVE_LOCAL_SOCKETS
       if (family != AF_LOCAL)
 #endif
-        host = build_string ("127.0.0.1");
+        {
+        if (family == AF_INET6)
+          host = build_string ("::1");
+        else
+          host = build_string ("127.0.0.1");
+        }
     }
   else
     {
       if (EQ (host, Qlocal))
+        {
         /* Depending on setup, "localhost" may map to different IPv4 and/or
            IPv6 addresses, so it's better to be explicit (Bug#6781).  */
-        host = build_string ("127.0.0.1");
+        if (family == AF_INET6)
+          host = build_string ("::1");
+        else
+          host = build_string ("127.0.0.1");
+        }
       CHECK_STRING (host);
     }
 
diff --git a/test/lisp/net/network-stream-tests.el b/test/lisp/net/network-stream-tests.el
index 6ad0c25903f..6151c3064c4 100644
--- a/test/lisp/net/network-stream-tests.el
+++ b/test/lisp/net/network-stream-tests.el
@@ -71,12 +71,39 @@
                  (= (aref (process-contact server :local) 4) 57869)))
     (delete-process server)))
 
-(defun make-server (host)
+(ert-deftest make-ipv6-tcp-server-with-unspecified-port ()
+  (let ((server
+         (make-network-process
+          :name "server"
+          :server t
+          :noquery t
+          :family 'ipv6
+          :service t
+          :host 'local)))
+    (should (and (arrayp (process-contact server :local))
+                 (numberp (aref (process-contact server :local) 8))
+                 (> (aref (process-contact server :local) 8) 0)))
+    (delete-process server)))
+
+(ert-deftest make-ipv6-tcp-server-with-specified-port ()
+  (let ((server
+         (make-network-process
+          :name "server"
+          :server t
+          :noquery t
+          :family 'ipv6
+          :service 57870
+          :host 'local)))
+    (should (and (arrayp (process-contact server :local))
+                 (= (aref (process-contact server :local) 8) 57870)))
+    (delete-process server)))
+
+(defun make-server (host &optional family)
   (make-network-process
    :name "server"
    :server t
    :noquery t
-   :family 'ipv4
+   :family (or family 'ipv4)
    :coding 'raw-text-unix
    :buffer (get-buffer-create "*server*")
    :service t
@@ -129,6 +156,34 @@
       (should (equal (buffer-string) "foo\n")))
     (delete-process server)))
 
+(ert-deftest echo-server-with-local-ipv4 ()
+  (let* ((server (make-server 'local 'ipv4))
+         (port (aref (process-contact server :local) 4))
+         (proc (make-network-process :name "foo"
+                                     :buffer (generate-new-buffer "*foo*")
+                                     :host 'local
+                                     :family 'ipv4
+                                     :service port)))
+    (with-current-buffer (process-buffer proc)
+      (process-send-string proc "echo foo")
+      (sleep-for 0.1)
+      (should (equal (buffer-string) "foo\n")))
+    (delete-process server)))
+
+(ert-deftest echo-server-with-local-ipv6 ()
+  (let* ((server (make-server 'local 'ipv6))
+         (port (aref (process-contact server :local) 8))
+         (proc (make-network-process :name "foo"
+                                     :buffer (generate-new-buffer "*foo*")
+                                     :host 'local
+                                     :family 'ipv6
+                                     :service port)))
+    (with-current-buffer (process-buffer proc)
+      (process-send-string proc "echo foo")
+      (sleep-for 0.1)
+      (should (equal (buffer-string) "foo\n")))
+    (delete-process server)))
+
 (ert-deftest echo-server-with-ip ()
   (let* ((server (make-server 'local))
          (port (aref (process-contact server :local) 4))