Allow fpm to change the working directory

[awvwgk]

This allow to use fpm -C <PATH> to change the working directory and enables the automatic discovery of a package manifest in the parent directories.

Explicitly changing of directories

-C, --directory PATH: Change working directory to PATH before running any command.

After checking out this pull request locally, you should be able to build the example package by:

fpm run -- build -C example_packages/hello_fpm

Automatic discovery of manifest files

With a directory changing mechanism we can also easily implement a mechanism to ”find” the package manifest in a parent directory. For this purpose fpm will retrieve the current working directory and check if a package manifest exists in any of the parent directories. By changing to this directory, all other commands can be used directly without modification.

You should be able to check this behavior with

fpm run -- build -C example_packages/hello_fpm/app

If you copy / install the fpm executable you can also check the automatic discovery without the -C / --directory option. It should work just the same.

Please try to break this feature.

---

Caveats

This introduces preprocessor usage into fpm, not sure if we are actually able to avoid it without using compiler extensions at this point.

Related #481, #172

[certik]

I don't think this PR closes #481 (or its duplicate #172). To close it, it should work by running fpm without any -C option.

I think this PR closes #411 and #410. My only question would be whether it the flag should be called -C or rather --directory as suggested in those issues.

[certik]

It's fine for this PR, but down the road we should move all these platform specific ifdefs into a platform.f90 or compatibility.f90 file, and then just use such a file here.

[awvwgk]

I was tempted to do:

#ifndef _WIN32
character(len=*), parameter :: chdir_symbol = "chdir"
#else
character(len=*), parameter :: chdir_symbol = "_chdir"
#endif
! ...
function chdir(...) bind(C, name=chdir_symbol)

But GFortran didn't like those, unfortunately.

[certik]

I was going to try exactly that to see if it works! Then we could have the chdir_symbol in platform.f90. As it is now, we'll have to move the whole chdir definition into platform.f90, which is fine too.

[awvwgk]

I used string concatenation in the past with bind(C, name=namespace//"...") so in principle this seems to work. But here I'm hitting:

./src/fpm_os.F90:18:29:

   18 |                 bind(C, name=chdir_symbol)
      |                             1
Error: Parameter ‘chdir_symbol’ at (1) has not been declared or is a variable, which does not reduce to a constant expression

[awvwgk]

Maybe this is not possible because the interface is a separate scoping unit not automatically inheriting the module scope. Using import will also not work here because it would make the parameter available after it has been used. That's a tricky one, maybe worth a thread on discourse?

[awvwgk]

Posted here: https://fortran-lang.discourse.group/t/constant-expressions-for-bind-c-name/1319

Let's see if somebody more knowledgeable in the Fortran standard can help us out here.

[LKedward]

Yes, this would only close #411. The idea for #410 is subtly different and would actually require a different command line option so that it can be used in combination with #411.

Edit: Milan gives a good example and explanation in #411 for how they could be used in combination

[awvwgk]

Okay, than let's try this:

1. fpm can change with -C or --directory the working directory.
2. fpm will search within the file system boundaries for an fpm.toml file, if it finds a package manifest, it will change to this directory

Give it a try:

❯ fpm run -- build -C example_packages/hello_fpm/app
fpm: Entering directory '/home/awvwgk/projects/src/git/fortran-package-manager/example_packages/hello_fpm/app'
fpm: Entering directory '/home/awvwgk/projects/src/git/fortran-package-manager/example_packages/hello_fpm'
fpm: Leaving directory '/home/awvwgk/projects/src/git/fortran-package-manager/example_packages/hello_fpm'
fpm: Leaving directory '/home/awvwgk/projects/src/git/fortran-package-manager/example_packages/hello_fpm/app'

[LKedward]

Autodiscovery seems to works nicely but would it not be better to change back to the original directory after the model has been built? This would allow me to do fpm run or fpm test in a different directory to the directory containing the manifest (like #410). By example, if I run git status in a subdirectory, then it finds the git repo in the parent directory but still executes the subcommand as if it's in the current directory.

It seems that we still need two separate options:

• --manifest-path (as in Allow running fpm from out-of-package directory #411) which only affects the project root and defaults to the autodiscovery process here
• --directory as implemented here, change directory before doing anything

What do you think?

[awvwgk]

I think we will need both, knowledge about the working directory and the project root at every point in fpm.

With this patch right now a command like

fpm -C example_packages/hello_fpm run --runner cp -- .

will copy the executable into example_packages/hello_fpm rather than the actual current working directory. Changing this in fpm requires some deeper modification how we handle file paths and how we spawn subprocesses, but this is definitely a desirable change.

[LKedward]

Thanks Sebastian @awvwgk. Changes look good to me for addressing #411 and #172, and I haven't been able to break it. I'm not too worried about the specific syntax in the fpm_os.F90 since this design decision is more suited to discussion in the stdlib_os repo.

[certik]

It looks good to me now. Thanks!

[LKedward]

This looks good to go so with two approvals I'll now merge. Thanks @awvwgk