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, 52 insertions, 0 deletions

diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index 3e9cc50de52..f228921137c 100644
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -741,6 +741,58 @@ Initialize the process filter to @var{filter}.
 
 @item :sentinel @var{sentinel}
 Initialize the process sentinel to @var{sentinel}.
+
+@item :stderr @var{stderr}
+Associate @var{stderr} with the standard error of the process.
+@var{stderr} is either a buffer or a pipe process created with
+@code{make-pipe-process}.
+@end table
+
+The original argument list, modified with the actual connection
+information, is available via the @code{process-contact} function.
+@end defun
+
+@defun make-pipe-process &rest args
+This function creates a bidirectional pipe which can be attached to a
+child process (currently only useful with the @code{:stderr} keyword
+of @code{make-process}).
+
+The arguments @var{args} are a list of keyword/argument pairs.
+Omitting a keyword is always equivalent to specifying it with value
+@code{nil}, except for @code{:coding}.
+Here are the meaningful keywords:
+
+@table @asis
+@item :name @var{name}
+Use the string @var{name} as the process name.  It is modified if
+necessary to make it unique.
+
+@item :buffer @var{buffer}
+Use @var{buffer} as the process buffer.
+
+@item :coding @var{coding}
+If @var{coding} is a symbol, it specifies the coding system to be
+used for both reading and writing of data from and to the
+connection.  If @var{coding} is a cons cell
+@w{@code{(@var{decoding} . @var{encoding})}}, then @var{decoding}
+will be used for reading and @var{encoding} for writing.
+
+If @var{coding} is @code{nil}, the default rules for finding the
+coding system will apply.  @xref{Default Coding Systems}.
+
+@item :noquery @var{query-flag}
+Initialize the process query flag to @var{query-flag}.
+@xref{Query Before Exit}.
+
+@item :stop @var{stopped}
+If @var{stopped} is non-@code{nil}, start the process in the
+``stopped'' state.
+
+@item :filter @var{filter}
+Initialize the process filter to @var{filter}.
+
+@item :sentinel @var{sentinel}
+Initialize the process sentinel to @var{sentinel}.
 @end table
 
 The original argument list, modified with the actual connection