When using mdbook build some_dir, preprocessor using relative path doesn't work

[Lorak-mmk]

Problem

If I use a preprocessor written in Python, like in documentation example: https://rust-lang.github.io/mdBook/format/configuration/preprocessors.html#provide-your-own-command , it works correctly when I execute mdbook build - so the book.toml file is in the same directory.
In the project I'm working on, book is in the docs subdirectory, so it's usually built with mdbook build docs. But when using this custom preprocessor, it doesn't work when invoked like this, because the PWD is still the main directory, instead of docs directory.
This is counterintuitive and limits the ability to use preprocessors.

I created a workaround in the project ( scylladb/scylla-rust-driver#910 ):

[preprocessor.sphinx]
command = '''bash -c '
if test -f "./docs/sphinx_preprocessor.py"; then
    python ./docs/sphinx_preprocessor.py "$@" 
else 
    python ./sphinx_preprocessor.py "$@"
fi' run_preprocessor
'''

but it's ugly and only allows executing from root or docs directories.

Steps

1. Have some book in a directory foo.
2. Create a preprocessor called bar.py in foo directory.
3. Add entry to book.toml:

[preprocessor.bar]
command = "python bar.py"

4. Go to parent of foo directory
5. Execute mdbook build foo
6. Book is built, but without using preprocessor, as bar.py couldn't be found.

Possible Solution(s)

Before executing preprocessor command change the PWD to the root of the book

Notes

No response

Version

mdbook v0.4.36

[ehuss]

Thanks for the report! I believe this is a duplicate of #1424, so closing in favor of that.