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, 10 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 91729e4aaec..fa333640548 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -317,6 +317,16 @@ optional argument specifying whether to follow symbolic links.
 ** 'parse-time-string' can now parse ISO 8601 format strings,
 such as "2020-01-15T16:12:21-08:00".
 
+---
+** 'make-network-process', 'make-serial-process' :coding behavior change.
+Previously, passing ":coding nil" to either of these functions would
+override any non-nil binding for 'coding-system-for-read' and
+'coding-system-for-write'.  For consistency with 'make-process' and
+'make-pipe-process', passing ":coding nil" is now ignored.  No code in
+Emacs depended on the previous behavior; if you really want the
+process' coding-system to be nil, use 'set-process-coding-system'
+after the process has been created, or pass in ":coding '(nil nil)".
+
 
 * Changes in Emacs 28.1 on Non-Free Operating Systems