Support Bash Process Substitution

[yash-fn]

the --filelist=FILE option is great to bypass command line length limits if a lot of files like for the dictionary training thing. But instead of requiring physical file which is annoying to have lying around, bash process substituitons are great! Something like:

--filelist=<(find . -type f -print)

but this won't work because zstd says:

zstd: error reading /dev/fd/63

. Seems zstd does some sort of check that required FILE to actually be a text file.

[Cyan4973]

The current implementation that reads the file list and extracts the filenames
requires knowledge of the file size for buffer allocation.
Such information is unavailable in the case of named pipe, which process substitution employs.

Supporting this use case will first require a rewrite of the function that extract the filenames.

[tansy]

Of all standard compressors the most reasonable response presented lzip.

$ gzip -k <(cat a.txt)
gzip: /dev/fd/63: Too many levels of symbolic links
$ bzip2 -k <(cat a.txt)
bzip2: Input file /dev/fd/63 is not a normal file.
$ zstd -k <(cat a.txt)
zstd: /dev/fd/63.zst: No such file or directory
$ lzip -k <(cat a.txt)
lzip: /dev/fd/63: Input file is not a regular file,
  and neither '-c' nor '-o' were specified.
$ lzip -c <(cat a.txt) > a.txt.lz

Maybe similar, crear, descriptive, though brief message would suffice.

[Cyan4973]

The issue seems that it's not possible to just write a *.zst output file into /dev/fd/.

If an output was specified, it would work :

./zstd <(cat tmp) -o tmp.zst -f
/dev/fd/11           : 32.10%   (  64.0 KiB =>   20.5 KiB, tmp.zst)

Point taken that, when no output is provided and the operation fails,
the error message is quite bland and does not describe accurately the situation.
It's currently provided by the local C runtime, via strerror(),
and quite frankly, I would have expected something like Permission denied instead.

Another solution would be statically decide a message, like zstd: /dev/fd/63.zst: Cannot open file,
that would be more accurate in this specific case,
but would also lose the ability of strerror() to provide a wider range of error messages for other scenarios.\

edit: new error message, more explicit for process substitution, at #4349:

./zstd <(cat tmp)
zstd: /dev/fd/11.zst: No such file or directory
When using process substitution (<(...)), specify an output destination with -o or -c.

[tansy]

The process substitution is essentially a pipe so these are not regular files.
Instead of making special case for it, it would better state the fact that it's not a regular file and no output was specified.

zstd: /dev/fd/63: Not a regular file
and no output specified (try -c or -o)

Or something similar.