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

[Jim Porter]

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.

The attached patch fixes this issue. It might not be the *best* way to 
do it, but it's the least-invasive I could come up with. The 
`make-symbol' dance is a bit awkward, but Eshell already uses it for 
`eshell-command-to-value' / `eshell-do-command-to-value', so hopefully 
it's ok. I'm open to other solutions though. It's possible the bug is 
really in `eshell-do-eval', since I'd expect the original code to work, 
but `eshell-do-eval' is pretty tricky, and I don't want to poke at it 
too hard for fear that other things will break.

Long-term, the best way to fix this might be to rip out `eshell-do-eval' 
entirely, which iteratively evaluates parts of Eshell commands (so as 
not to hang Emacs) and use the generator.el machinery instead. I looked 
into that briefly, and it seems like it would be quite a bit of work. 
And I don't fully understand generator.el's implementation yet anyway...

0001-Keep-subcommands-in-pipelines-from-clobbering-the-he.patch
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