Allow creating a pipe process without a buffer

Previously, even passing :buffer nil to make-pipe-process would
create a buffer.  Now, if you explicitly call (make-pipe-process
:buffer nil), it will create a pipe process without a buffer,
just like all the other process creation functions.

* src/process.c (Fmake_pipe_process): Check for explicit :buffer
nil and don't make a buffer. (bug#79596)
* doc/lispref/processes.texi (Asynchronous Processes): Update.
* test/src/process-tests.el
(process-test-make-pipe-process-no-buffer): Add test.

doc/lispref/processes.texi

@@ -842,7 +842,8 @@ Use the string @var{name} as the process name.  As with
 @code{make-process}, it is modified if necessary to make it unique.
 
 @item :buffer @var{buffer}
-Use @var{buffer} as the process buffer.
+Use @var{buffer} as the process buffer.  If the value is @code{nil},
+the subprocess is not associated with any buffer.
 
 @item :coding @var{coding}
 If @var{coding} is a symbol, it specifies the coding system to be

src/process.c

@@ -2414,7 +2414,8 @@ arguments are defined:
 :buffer BUFFER -- BUFFER is the buffer (or buffer-name) to associate
 with the process.  Process output goes at the end of that buffer,
 unless you specify a filter function to handle the output.  If BUFFER
-is not given, the value of NAME is used.
+is not given, the value of NAME is used.  BUFFER may be also nil, meaning
+that this process is not associated with any buffer.
 
 :coding CODING -- If CODING is a symbol, it specifies the coding
 system used for both reading and writing for this process.  If CODING
@@ -2479,10 +2480,15 @@ usage:  (make-pipe-process &rest ARGS)  */)
   if (inchannel > max_desc)
     max_desc = inchannel;
 
-  buffer = plist_get (contact, QCbuffer);
-  if (NILP (buffer))
-    buffer = name;
-  buffer = Fget_buffer_create (buffer, Qnil);
+  {
+    Lisp_Object buffer_member = plist_member (contact, QCbuffer);
+    if (NILP (buffer_member))
+      buffer = name;
+    else
+      buffer = XCAR (XCDR (buffer_member));
+  }
+  if (!NILP (buffer))
+    buffer = Fget_buffer_create (buffer, Qnil);
   pset_buffer (p, buffer);
 
   pset_childp (p, contact);

test/src/process-tests.el

@@ -1053,5 +1053,11 @@ Return nil if FILENAME doesn't exist."
   (should (integerp (num-processors)))
   (should (< 0 (num-processors))))
 
+(ert-deftest process-test-make-pipe-process-no-buffer ()
+  "Test that a pipe process can be created without a buffer."
+  (should     (process-buffer (make-pipe-process :name "test")))
+  (should     (process-buffer (make-pipe-process :name "test" :buffer "test")))
+  (should-not (process-buffer (make-pipe-process :name "test" :buffer nil))))
+
 (provide 'process-tests)
 ;;; process-tests.el ends here