Add facility to collect stderr of async subprocess

* src/w32.h (register_aux_fd): New function declaration.
* src/w32.c (register_aux_fd): New function.
* src/process.h (struct Lisp_Process): New member stderrproc.
* src/process.c (PIPECONN_P): New macro.
(PIPECONN1_P): New macro.
(Fdelete_process, Fprocess_status, Fset_process_buffer)
(Fset_process_filter, Fset_process_sentinel, Fstop_process)
(Fcontinue_process): Handle pipe process specially.
(create_process): Respect p->stderrproc.
(Fmake_pipe_process): New function.
(Fmake_process): Add new keyword argument :stderr.
(wait_reading_process_output): Specially handle a pipe process when
it gets an EOF.
(syms_of_process): Register Qpipe and Smake_pipe_process.

* doc/lispref/processes.texi (Asynchronous Processes): Document
`make-pipe-process' and `:stderr' keyword of `make-process'.

* lisp/subr.el (start-process): Suggest to use `make-process' handle
standard error separately.

* test/automated/process-tests.el (process-test-stderr-buffer)
(process-test-stderr-filter): New tests.

* etc/NEWS: Mention new process type `pipe' and its usage with the
`:stderr' keyword of `make-process'.

1 files changed, 70 insertions, 0 deletions

diff --git a/test/automated/process-tests.el b/test/automated/process-tests.el
index dabfbc56b78..1dab615bed0 100644
--- a/test/automated/process-tests.el
+++ b/test/automated/process-tests.el
@@ -72,4 +72,74 @@
               (should (string= (buffer-string) "arg1 = \"x &y\", arg2 = \n"))))
         (when batfile (delete-file batfile))))))
 
+(ert-deftest process-test-stderr-buffer ()
+  (skip-unless (executable-find "bash"))
+  (let* ((stdout-buffer (generate-new-buffer "*stdout*"))
+         (stderr-buffer (generate-new-buffer "*stderr*"))
+         (proc (make-process :name "test"
+                             :command (list "bash" "-c"
+                                            (concat "echo hello stdout!; "
+                                                    "echo hello stderr! >&2; "
+                                                    "exit 20"))
+                             :buffer stdout-buffer
+                             :stderr stderr-buffer))
+         (sentinel-called nil)
+         (start-time (float-time)))
+    (set-process-sentinel proc (lambda (proc msg)
+                                 (setq sentinel-called t)))
+    (while (not (or sentinel-called
+                    (> (- (float-time) start-time)
+                       process-test-sentinel-wait-timeout)))
+      (accept-process-output))
+    (cl-assert (eq (process-status proc) 'exit))
+    (cl-assert (= (process-exit-status proc) 20))
+    (should (with-current-buffer stdout-buffer
+              (goto-char (point-min))
+              (looking-at "hello stdout!")))
+    (should (with-current-buffer stderr-buffer
+              (goto-char (point-min))
+              (looking-at "hello stderr!")))))
+
+(ert-deftest process-test-stderr-filter ()
+  (skip-unless (executable-find "bash"))
+  (let* ((sentinel-called nil)
+         (stderr-sentinel-called nil)
+         (stdout-output nil)
+         (stderr-output nil)
+         (stdout-buffer (generate-new-buffer "*stdout*"))
+         (stderr-buffer (generate-new-buffer "*stderr*"))
+         (stderr-proc (make-pipe-process :name "stderr"
+                                         :buffer stderr-buffer))
+         (proc (make-process :name "test" :buffer stdout-buffer
+                             :command (list "bash" "-c"
+                                            (concat "echo hello stdout!; "
+                                                    "echo hello stderr! >&2; "
+                                                    "exit 20"))
+                             :stderr stderr-proc))
+         (start-time (float-time)))
+    (set-process-filter proc (lambda (proc input)
+                               (push input stdout-output)))
+    (set-process-sentinel proc (lambda (proc msg)
+                                 (setq sentinel-called t)))
+    (set-process-filter stderr-proc (lambda (proc input)
+                                      (push input stderr-output)))
+    (set-process-sentinel stderr-proc (lambda (proc input)
+                                        (setq stderr-sentinel-called t)))
+    (while (not (or sentinel-called
+                    (> (- (float-time) start-time)
+                       process-test-sentinel-wait-timeout)))
+      (accept-process-output))
+    (cl-assert (eq (process-status proc) 'exit))
+    (cl-assert (= (process-exit-status proc) 20))
+    (should sentinel-called)
+    (should (equal 1 (with-current-buffer stdout-buffer
+                       (point-max))))
+    (should (equal "hello stdout!\n"
+                   (mapconcat #'identity (nreverse stdout-output) "")))
+    (should stderr-sentinel-called)
+    (should (equal 1 (with-current-buffer stderr-buffer
+                       (point-max))))
+    (should (equal "hello stderr!\n"
+                   (mapconcat #'identity (nreverse stderr-output) "")))))
+
 (provide 'process-tests)