Targets' `add_target(s)` methods do not correctly check passed paths

Description of issue or feature request:

Judging from the docstrings and comments, add_target and add_targets are expected to update a Targets object with passed target paths, if they are relative to a targets directory base path (i.e. self._targets_directory).

However, they don’t seem to take into account that Python’s os.path.join ignores all path component arguments prior to a component that looks like an absolute path.

Current behavior:

• Works as expected (?) if passed target path is relative to a targets directory base path or absolute but falls into the targets directory:

# repo._targets_directory == "/path/to/repo/targets"
repo.targets.add_target("foo.txt")
repo.targets.add_target("/path/to/repo/targets/bar.txt)
# Successfully adds "foo.txt" and "bar.txt" 

• Fails if the passed target path is not absolute and does not exist relative to targets directory base path “/path/to/repo/targets”.

# repo._targets_directory == "/path/to/repo/targets"
repo.targets.add_target("repo/targets/foo.txt")
# Fails with "'/path/to/repo/targets/repo/targets/foo.txt' is not a valid file in the repository's targets directory"

• Fails if the passed target path is absolute and does not exist

#repo._targets_directory == "/path/to/repo/targets"
repo.targets.add_target("/path/does/not/exist/foo.txt")
# Fails with "'/path/does/not/exist/foo.txt' is not a valid file in the repository's targets directory"

• Has unexpected behavior if the passed path is absolute and exists!

Expected behavior: Revise intended behavior, update implementation accordingly.

[joshuagl]

add_target could just add any paths passed to the internal representation of targets metadata, checking that they are relative (and maybe don’t contain “…”?). And only later, when the corresponding files are actually needed, i.e. to generate hashes, should it fail gracefully if any of the paths can’t be resolved into a file relative to the targets directory base path.

This feels like a sensible approach, that handles @lukpueh’s least-surprise concerns above and we can address my debugability concern with appropriate logging.

[lukpueh]

I agree with your debugging concern, @joshuagl. I am concerned with least surprise. 😃

To me, and this should answer your other question, none of above cases seems fully surprise-free. Here are some concerns:

• Why can I pass an absolute path, if the spec says that targetpaths should be relative?
• Why should I pass an absolute path, if it must be relative to an “unknown” base path (i.e. self._targets_directory), whose portion in the path gets stripped anyway?
• Why must a relative path be relative to an “unknown” base path, and not relative to my cwd?