Re: [PATCH] Do not clear the value of st_dev in File I/O's stat()

[Pedro Alves <palves at redhat dot com>]

[Hi Corinna, adding you in case you still happen to remember
any of this and have any input.  If not, that's fine.]

On 05/17/2018 09:28 AM, Julio Guerra wrote:
> Do not clear the value of st_dev in the fileio stat structure sent to the
> target. It prevents from being able to check the file type on the target.
> Note that the fileio function fstat `remote_fileio_func_fstat()` doesn't clear
> this field.

Curious.  This looked like was written in a way like you'd write if you
had a reason to so I did some archaeology to try to find what it was, see
if the reason is still valid.

I found the original File I/O protocol proposal here:

  https://www.sourceware.org/ml/gdb/2002-11/msg00107.html

and in there, in the intro, it said:

~~~~~
  The protocol is only used for files on the host file system and
  for I/O on the console.  Character or block special devices, pipes,
  named pipes or sockets or any other communication method on the host
  system are not supported by this protocol.
~~~~~

and further below, in "B.3 struct stat", it says:

~~~~~
  The values of several fields have a restricted meaning and/or
  range of values.

    st_dev:	0	file
		1	console
~~~~~

And the current manual also says:

 @item st_dev
 A value of 0 represents a file, 1 the console.

And it looks like early on, in:

 https://sourceware.org/ml/gdb-patches/2003-03/msg00221.html

we had:

 +static void
 +remote_fileio_to_fio_stat (struct stat *st, struct fio_stat *fst)
 +{
 +  /* `st_dev' is set in the calling function */
 +  remote_fileio_to_fio_uint ((long) st->st_ino, fst->fst_ino);

Note the comment.  

I can't find the comment in the current sources, but it was there
up until:

 commit 791c00567a7ccbae3d71e3b63ac43c0b555079dc
 Author:     Gary Benson <gbenson@redhat.com>
 AuthorDate: Wed Mar 11 17:53:57 2015 +0000

    Move remote_fileio_to_fio_stat to gdb/common


and note that remote_fileio_func_fstat still does:

  if (fd == FIO_FD_CONSOLE_IN || fd == FIO_FD_CONSOLE_OUT)
    {
      host_to_fileio_uint (1, fst.fst_dev);
                          ^^^

"fstat" was only added later on, along the work that Gary did
around that commit above, for this series:

 https://sourceware.org/ml/gdb-patches/2015-03/msg00286.html

... which introduced "fstat" in the protocol, and in that case,
the 0/1 st_dev issue was either missed or ignored, so
fstat is not following the documented values.

Now I'm not sure what to do.  If we let the real st_dev
through, what shall we fill it in, in the "console" files
case?

> 
> 2018-05-16  Julio Guerra  <julio@farjump.io>
> 
> 	* remote-fileio.c: do not clear the value of st_dev in File I/O's stat().

Nit, you'd write:

gdb/ChangeLog:
2018-05-16  Julio Guerra  <julio@farjump.io>

	* remote-fileio.c (remote_fileio_func_stat): Do not clear the
	value of st_dev.

Thanks,
Pedro Alves