Make make-{network,serial}-process handle :coding nil consistently

The handling of :coding nil was different between
make-{network,serial}-process and make-{pipe}process.  Now they all
handle :coding nil as if :coding had not been specified.

* process.c (Fmake_serial_process)
(set_network_socket_coding_system): Use plist-get to check if
:coding has been specified instead of plist-member, to ensure that
":coding nil" does not override coding-system-for-{read,write}.

* network-stream-tests.el (check-network-process-coding-system-bind)
(check-network-process-coding-system-no-override)
(check-network-process-coding-system-override): New tests.

* etc/NEWS: Describe change in make-network-process and
make-serial-process :coding behavior.

1 files changed, 6 insertions, 10 deletions

diff --git a/src/process.c b/src/process.c
index 07881d6c5d3..e6d18fbaad2 100644
--- a/src/process.c
+++ b/src/process.c
@@ -3188,14 +3188,12 @@ usage:  (make-serial-process &rest ARGS)  */)
                        BUF_ZV_BYTE (XBUFFER (buffer)));
     }
 
-  tem = Fplist_member (contact, QCcoding);
-  if (!NILP (tem) && (!CONSP (tem) || !CONSP (XCDR (tem))))
-    tem = Qnil;
+  tem = Fplist_get (contact, QCcoding);
 
   val = Qnil;
   if (!NILP (tem))
     {
-      val = XCAR (XCDR (tem));
+      val = tem;
       if (CONSP (val))
         val = XCAR (val);
     }
@@ -3209,7 +3207,7 @@ usage:  (make-serial-process &rest ARGS)  */)
   val = Qnil;
   if (!NILP (tem))
     {
-      val = XCAR (XCDR (tem));
+      val = tem;
       if (CONSP (val))
         val = XCDR (val);
     }
@@ -3244,16 +3242,14 @@ set_network_socket_coding_system (Lisp_Object proc, Lisp_Object host,
   Lisp_Object coding_systems = Qt;
   Lisp_Object val;
 
-  tem = Fplist_member (contact, QCcoding);
-  if (!NILP (tem) && (!CONSP (tem) || !CONSP (XCDR (tem))))
-    tem = Qnil;  /* No error message (too late!).  */
+  tem = Fplist_get (contact, QCcoding);
 
   /* Setup coding systems for communicating with the network stream.  */
   /* Qt denotes we have not yet called Ffind_operation_coding_system.  */
 
   if (!NILP (tem))
     {
-      val = XCAR (XCDR (tem));
+      val = tem;
       if (CONSP (val))
         val = XCAR (val);
     }
@@ -3287,7 +3283,7 @@ set_network_socket_coding_system (Lisp_Object proc, Lisp_Object host,
 
   if (!NILP (tem))
     {
-      val = XCAR (XCDR (tem));
+      val = tem;
       if (CONSP (val))
         val = XCDR (val);
     }