bug#25549: 25.1; eshell grep gives inconsistent output

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tags 25549 moreinfo unreproducible
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me@alandmoore.com writes:

> When using grep on a large file in eshell, the output is sometimes
>   inconsistent. For example, when grepping for a string in my
> server's apache2 access log (~5 MB) I got anywhere from around 200 to
> around 1300 results, just running the same grep command over and over
> (the file itself was not changing significantly).
>
> The bug cannot be consistently reproduced, but has happened in multiple
> situations and others have reportedly reproduced it per discussion on
> reddit
> (https://www.reddit.com/r/emacs/comments/5qbd70/trampeshellgrep_returning_wildly_different/).

Could you give some hints on what is needed to reproduce this?  Is it
the size of the grepped file?  Number of matches?  Are there some
non-ascii characters in the file?  Is Tramp required (the first post in
the linked thread mentions it, but some other comments seem to suggest
it's not required)?  You mention the file was not changing
"significantly", does this happen also when file stays *exactly*
identical?

I tried doubling up xdisp.c until it was over 5 MB, but grepping for "int"
in eshell returned a consistent number of matches.