Allow creating processes where only one of stdin or stdout is a PTY

* src/lisp.h (emacs_spawn):
* src/callproc.c (emacs_spawn): Add PTY_IN and PTY_OUT arguments to
specify which streams should be set up as a PTY.
(call_process): Adjust call to 'emacs_spawn'.

* src/process.h (Lisp_Process): Replace 'pty_flag' with 'pty_in' and
'pty_out'.

* src/process.c (is_pty_from_symbol): New function.
(make-process): Allow :connection-type to be a cons cell, and allow
using a stderr process with a PTY for stdin/stdout.
(create_process): Handle creating a process where only one of stdin or
stdout is a PTY.

* lisp/eshell/esh-proc.el (eshell-needs-pipe, eshell-needs-pipe-p):
Remove.
(eshell-gather-process-output): Use 'make-process' and set
':connection-type' as needed by the value of 'eshell-in-pipeline-p'.

* lisp/net/tramp.el (tramp-handle-make-process):
* lisp/net/tramp-adb.el (tramp-adb-handle-make-process):
* lisp/net/tramp-sh.el (tramp-sh-handle-make-process): Don't signal an
error when ':connection-type' is a cons cell.

* test/src/process-tests.el
(process-test-sentinel-wait-function-working-p): Allow passing PROC
in, and rework into...
(process-test-wait-for-sentinel): ... this.
(process-test-sentinel-accept-process-output)
(process-test-sentinel-sit-for, process-test-quoted-batfile)
(process-test-stderr-filter): Use 'process-test-wait-for-sentinel'.
(make/process/test-connection-type): New function.
(make-process/connection-type/pty, make-process/connection-type/pty-2)
(make-process/connection-type/pipe)
(make-process/connection-type/pipe-2)
(make-process/connection-type/in-pty)
(make-process/connection-type/out-pty)
(make-process/connection-type/pty-with-stderr-buffer)
(make-process/connection-type/out-pty-with-stderr-buffer): New tests.

* test/lisp/eshell/esh-proc-tests.el (esh-proc-test--detect-pty-cmd):
New variable.
(esh-proc-test/pipeline-connection-type/no-pipeline)
(esh-proc-test/pipeline-connection-type/first)
(esh-proc-test/pipeline-connection-type/middle)
(esh-proc-test/pipeline-connection-type/last): New tests.

* doc/lispref/processes.texi (Asynchronous Processes): Document new
':connection-type' behavior.
(Output from Processes): Remove caveat about ':stderr' forcing
'make-process' to use pipes.

* etc/NEWS: Announce this change (bug#56025).

1 files changed, 10 insertions, 18 deletions

diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index 1ef8fc3d03a..e253ab9de03 100644
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -705,12 +705,13 @@ coding system will apply.  @xref{Default Coding Systems}.
 Initialize the type of device used to communicate with the subprocess.
 Possible values are @code{pty} to use a pty, @code{pipe} to use a
 pipe, or @code{nil} to use the default derived from the value of the
-@code{process-connection-type} variable.  This parameter and the value
-of @code{process-connection-type} are ignored if a non-@code{nil}
-value is specified for the @code{:stderr} parameter; in that case, the
-type will always be @code{pipe}.  On systems where ptys are not
-available (MS-Windows), this parameter is likewise ignored, and pipes
-are used unconditionally.
+@code{process-connection-type} variable.  If @var{type} is a cons cell
+@w{@code{(@var{input} . @var{output})}}, then @var{input} will be used
+for standard input and @var{output} for standard output (and standard
+error if @code{:stderr} is @code{nil}).
+
+On systems where ptys are not available (MS-Windows), this parameter
+is ignored, and pipes are used unconditionally.
 
 @item :noquery @var{query-flag}
 Initialize the process query flag to @var{query-flag}.
@@ -1530,20 +1531,11 @@ a buffer, which is called the associated buffer of the process
 default filter discards the output.
 
   If the subprocess writes to its standard error stream, by default
-the error output is also passed to the process filter function.  If
-Emacs uses a pseudo-TTY (pty) for communication with the subprocess,
-then it is impossible to separate the standard output and standard
-error streams of the subprocess, because a pseudo-TTY has only one
-output channel.  In that case, if you want to keep the output to those
-streams separate, you should redirect one of them to a file---for
-example, by using an appropriate shell command via
-@code{start-process-shell-command} or a similar function.
-
-  Alternatively, you could use the @code{:stderr} parameter with a
+the error output is also passed to the process filter function.
+Alternatively, you could use the @code{:stderr} parameter with a
 non-@code{nil} value in a call to @code{make-process}
 (@pxref{Asynchronous Processes, make-process}) to make the destination
-of the error output separate from the standard output; in that case,
-Emacs will use pipes for communicating with the subprocess.
+of the error output separate from the standard output.
 
   When a subprocess terminates, Emacs reads any pending output,
 then stops reading output from that subprocess.  Therefore, if the