Re: [PATCH] Add facility to collect stderr of async subprocess

[Tino Calancha <tino.calancha@gmail.com>]

Daiki Ueno <ueno@gnu.org> writes:

> Daiki Ueno <ueno@gnu.org> writes:
> I've pushed it to master.

How about to allow direct standard-error output to a different buffer
in start-process as well?

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From 087970e4952476d94c43927ed96b1b960fc316fc Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Wed, 5 Oct 2016 11:59:25 +0900
Subject: [PATCH] start-process: Allow separate standard-error from
 standard-output

* lisp/subr.el (start-process): Allow buffer being a cons
(STDOUT-BUFFER . STDERR-BUFFER); the car is the buffer-or-name
for the standard outout, the cdr is the buffer-or-name for the
standard error.
Update doc string.
* lisp/simple.el (async-shell-command):
Use start-process-shell-command instead of start-process.
If error-buffer is non-nil, then write standard-error output there.
* etc/TODO (Other features we would like): Drop this task from TODO list.
* doc/lispref/processes.texi (Asynchronous Processes, Output from Processes):
Update manual.
; * etc/NEWS (Lisp Changes in Emacs 26.1): Add news entry.
---
 doc/lispref/processes.texi | 17 ++++++++++++++---
 etc/NEWS                   |  3 +++
 etc/TODO                   |  3 ---
 lisp/simple.el             | 10 +++++++---
 lisp/subr.el               | 20 ++++++++++----------
 5 files changed, 34 insertions(+), 19 deletions(-)

diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index 87c0b5c..4fbdf47 100644
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -745,7 +745,11 @@ Asynchronous Processes
 for the new subprocess in Lisp.  The argument @var{name} specifies the
 name for the process object; as with @code{make-process}, it is
 modified if necessary to make it unique.  The buffer
-@var{buffer-or-name} is the buffer to associate with the process.
+@var{buffer-or-name} is the buffer to associate with the process.  It
+might be a cons @w{@code{(@var{stdout-buffer} . @var{stderr-buffer})}}
+as well; in this case @var{stdout-buffer} is the buffer to associate with
+the standard output of the process, and @var{stderr-buffer} is the buffer
+to associate with its error output.
 
 If @var{program} is @code{nil}, Emacs opens a new pseudoterminal (pty)
 and associates its input and output with @var{buffer-or-name}, without
@@ -1359,8 +1363,15 @@ Output from Processes
 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
+output channel.  In case you want to keep the output to those
+streams separate, you should provide also a buffer for the standard
+error.  Then Emacs doesn't use a pseudo-TTY for communication with
+the subprocess.  The way to do that is to start the subprocess with
+the argument @var{buffer-or-name} being a cons
+@w{@code{(@var{stdout-buffer} . @var{stderr-buffer})}}; in that case,
+the subprocess writes its standard output in @var{stdout-buffer} and
+writes its error output in @var{stderr-buffer}.
+Another way could be 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.
 
diff --git a/etc/NEWS b/etc/NEWS
index bd94c94..a60d859 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -499,6 +499,9 @@ function 'check-declare-errmsg' has been removed.
 \f
 * Lisp Changes in Emacs 26.1
 
+** Now 'start-process' can direct standard-error output to a different
+buffer.
+
 ** New function undo-amalgamate-change-group to get rid of undo-boundaries
 between two states.
 
diff --git a/etc/TODO b/etc/TODO
index fe0e2ac..6749afd 100644
--- a/etc/TODO
+++ b/etc/TODO
@@ -489,9 +489,6 @@ from the emacsclient process.
   immediately, then replace it later.  So that C-s a with
   input method latin-1-postfix would immediately search for an a.
 
-** Give start-process the ability to direct standard-error
-  output to a different filter.
-
 ** Give desktop.el a feature to switch between different named desktops.
 
 ** Add a cpio mode, more or less like tar mode.
diff --git a/lisp/simple.el b/lisp/simple.el
index 70bd759..4c734f2 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -3480,9 +3480,13 @@ shell-command
 		  (display-buffer buffer '(nil (allow-no-window . t)))
                   (shell-command--save-pos-or-erase)
 		  (setq default-directory directory)
-		  (setq proc (start-process "Shell" buffer shell-file-name
-					    shell-command-switch command))
-		  (setq mode-line-process '(":%s"))
+		  (setq proc (start-process-shell-command
+                              "Shell"
+                              (if error-buffer
+                                  `(,buffer . ,error-buffer)
+                                buffer)
+                              command)
+                        mode-line-process '(":%s"))
 		  (require 'shell) (shell-mode)
 		  (set-process-sentinel proc 'shell-command-sentinel)
 		  ;; Use the comint filter for proper handling of carriage motion
diff --git a/lisp/subr.el b/lisp/subr.el
index b143812..03b902e 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -1989,24 +1989,24 @@ start-process
 NAME is name for process.  It is modified if necessary to make it unique.
 BUFFER is the buffer (or buffer name) to associate with the process.
 
-Process output (both standard output and standard error streams) goes
+If buffer is a cons (STDOUT-BUFFER . STDERR-BUFFER), then the standard
+output and the standard error appear in different buffers.  Otherwise,
+process output (both standard output and standard error streams) goes
 at end of BUFFER, unless you specify an output stream or filter
 function to handle the output.  BUFFER may also be nil, meaning that
 this process is not associated with any buffer.
 
 PROGRAM is the program file name.  It is searched for in `exec-path'
 \(which see).  If nil, just associate a pty with the buffer.  Remaining
-arguments are strings to give program as arguments.
-
-If you want to separate standard output from standard error, use
-`make-process' or invoke the command through a shell and redirect
-one of them using the shell syntax."
+arguments are strings to give program as arguments."
   (unless (fboundp 'make-process)
     (error "Emacs was compiled without subprocess support"))
-  (apply #'make-process
-	 (append (list :name name :buffer buffer)
-		 (if program
-		     (list :command (cons program program-args))))))
+  (apply #'make-process :name name
+         (append (if (consp buffer)
+                     (list :buffer (car buffer) :stderr (cdr buffer))
+                   (list :buffer buffer))
+                 (if program
+                     (list :command (cons program program-args))))))
 
 (defun process-lines (program &rest args)
   "Execute PROGRAM with ARGS, returning its output as a list of lines.
-- 
2.9.3

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In GNU Emacs 25.1.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.0)
 of 2016-10-04 built on calancha-pc
Repository revision: 13ba5af7427bdbd022a9d449dc2987d6a96591eb