fprime-util purge does not always work

[celskeggs]

F´ Version	v3.5.0-22-g00c5c8e11
Affected Component	build system

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Problem Description

I interrupted the generate command with Ctrl-C, and now I have a build directory that the purge command won't remove.

# fprime-util purge
[WARNING] Build cache '[...]/build-fprime-automatic-[...]' invalid or not found. Skipping.
[WARNING] Build cache '[...]/build-fprime-automatic-[...]-ut' invalid or not found. Skipping.
# fprime-util generate --ninja
[ERROR] [...]/build-fprime-automatic-[...] already exists.

Context / Environment

Operating System: Linux
CPU Architecture: x86_64
Platform: Linux-5.15.167.4-microsoft-standard-WSL2-x86_64-with-glibc2.34
Python version: 3.9.18
CMake version: 3.26.5
Pip version: 21.2.3
Pip packages:
    fprime-tools==3.5.0
    fprime-gds==3.5.0
    fprime-fpp-*==2.2.1a1

How to Reproduce

1. Run fprime-util generate --ninja and hit Ctrl-C partway through.
2. Run fprime-util purge
3. Run fprime-util generate --ninja

Expected Behavior

fprime-util purge should remove the build directory even if it is broken, so that fprime-util generate --ninja can generate a clean build directory.

[thomas-bc]

FYI if anyone would like to take a crack at this:
The issue seems to be that we try to load the build before purging it. But since the build is invalid, it gets skipped. We may want to either skip the build loading if possible, or (more likely easier) handle the failure nicely.

For what it's worth, fprime-util purge -f does work fine.

[LeStarch]

This was originally by design. It pointed out that we shouldn't be removing directories that our tools "don't understand".

Thus purge was intended for well-formed directories where it is clear that our tools created them....and and purge -f was created for clobbering anything following the naming scheme.

I am happy to review this behavior, or change it should the community agree that one is desired behavior over the other.

[celskeggs]

I'm okay with the idea that fprime-util purge won't delete a directory that wasn't created with the F Prime tools and it can't understand. But if the directory was made with the F Prime tools, fprime-util purge should always be able to understand it.

One possibility: the first step in fprime-util generate after creating the directory could be to add a special file (.fprime-build-dir or something better named) to mark it as an F Prime directory. That way, if the build generation step fails, there will still be a marker telling fprime-util purge that it's OK to delete the directory. (If someone Ctrl-C'd fprime-util generate before it managed to create that file, then the directory will be empty, and I have no concerns with accidentally deleting an empty directory.)

[LeStarch]

I like this solution!