bug#55590: [PATCH v2] 29.0.50; Eshell subcommands clobber pipelines and produce incorrect output

[Jim Porter]

On 5/22/2022 10:39 PM, Sean Whitton wrote:
> On Sun 22 May 2022 at 08:43pm -07, Jim Porter wrote:
> > On 5/22/2022 8:34 PM, Jim Porter wrote:
> > > Starting from `emacs -Q -f eshell', enter the following command:
> > >
> > >     echo ${*echo hi} | rev
> > >
> > > The output is:
> > >
> > >     ~ $ ih
> > >
> > > That is, the output of the command ended up *after* the prompt, when it
> > > should be before.
[snip]
> Couldn't you just defvar a couple of vars for this purpose?

Do you mean just defvar-ing `headproc' and `tailproc' (and renaming 
them, of course)? That works for my example above, but falls apart for 
more complex cases such as:

  echo ${*echo hi | rev} | tr a-z A-Z

The Eshell manual also says the following command is broken (output 
comes after the prompt). It works correctly with my patch, but just 
adding defvars isn't enough:

  for i in 1 2 3 { grep -q a b && *echo hi } | wc -l

The "Command evaluation macros" comment section in 
lisp/eshell/esh-cmd.el indicates that there are some odd limitations on 
these macros, so that might be part of the issue. I'm still a bit 
confused as to why the dance in my patch is necessary, but I'm ok with 
any implementation that passes the tests.

Attached is an updated patch with an additional test for the 
more-complex case and the manual entry about this bug removed.

0001-Keep-subcommands-in-pipelines-from-clobbering-the-he.patch
Description: Text document