bug#49925: cat -E interprets sentinel newline at the end of buffer as an actual newline after a \r

[Pádraig Brady]

On 07/08/2021 14:07, Michael Debertol wrote:
> Hi,
>
> after https://lists.gnu.org/archive/html/coreutils/2021-02/msg00003.html
> (unreleased), the behavior of cat -E was changed so that it prints "^M$"
> for "\r\n" line endings.
>
> Whenever it sees a \r "cat -E" checks if the byte after is a \n, however
> that \n might be the sentinel value that is inserted at the end of a buffer.
>
> This is a problem in two cases:
>
> - When a \r is at the end of the input. `printf "\r" | cat -E` will
> print "^M", even though there is no "\n" after the "\r". FWIW,
> tests/misc/cat-E.sh expects a "^M" for a trailing "\r", but I think
> that's wrong.

This was intentional (as per the test) as I was thinking
we can provide more info here in the edge case that \r is the last char of a 
file.
However it's incorrect as you suggest, as cat can't treat files independently.

> - When the file is too big to fit into one buffer. If you try to "cat
> -E" a big file (mutliple megabytes) that consists of only "\r", cat will
> print a few "^M" whenever it hits the end of a buffer in the middle of
> the file and at the end.

That indeed is a bug.

So we need to track handling of \r across buffer and file boundaries.
The attached does that, and I'll apply later.

marking this as done,

thanks!
Pádraig

cat-E-trailing-CR.patch
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