gh-156210: Fix shutil.copytree() detection of dangling relative symlinks

[lpyu001]

shutil.copytree() used the raw result of os.readlink() to determine whether a symbolic link was dangling when ignore_dangling_symlinks=True. Relative targets were therefore checked against the current working directory instead of the link's parent directory.

This could cause valid relative symbolic links to be skipped, or dangling links to be copied when the current working directory contained a matching path.

The check now uses the source link path, allowing the target to be resolved from the correct directory. Regression tests cover both cases.

• Issue: shutil.copytree() incorrectly classifies relative symlinks as dangling #156210

[picnixz]

I'm not sure this is correct. Please explain why you used srcname instead of linkto. AFAICT, the problem is linkto is the incorrect file (because readlink() resolves according to PWD and not to src).

Otherwise, I wonder whether this is deliberate and instead should be documented.

[picnixz]

I'm not sure this is correct though? srcname is the the symlink file. However, we want to resolve the relative symlink with respect to src, that is, we read the symlink content and then check if the file exists. The problem is therefore not here but rather the computation of "linkto". Instead, we should readlink(srcname) by interpreting srcname's content as relative to the src rather than PWD.

[bedevere-app]

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