`shutil.copytree()` incorrectly classifies relative symlinks as dangling

Bug report

Bug description:

Summary

shutil.copytree(..., symlinks=False, ignore_dangling_symlinks=True) tests a relative symlink target against the process current working directory rather than against the directory containing the symlink. Consequently, a valid relative symlink can be silently omitted from a successful copy, and a truly dangling symlink can still cause shutil.Error when the current working directory happens to contain a file with the same name as its target.

Affected code

Lib/shutil.py, _copytree():

linkto = os.readlink(srcname)
...
if not os.path.exists(linkto) and ignore_dangling_symlinks:
    continue

os.readlink() returns the raw target. A relative target is interpreted by the operating system relative to the symlink's parent directory, but os.path.exists(linkto) interprets it relative to the process current working
directory.

Reproducer

import os
import shutil
import tempfile

with tempfile.TemporaryDirectory() as root:
    src = os.path.join(root, "src")
    dst = os.path.join(root, "dst")
    os.mkdir(src)
    with open(os.path.join(src, "target"), "w") as f:
        f.write("data")
    os.symlink("target", os.path.join(src, "link"))

    old_cwd = os.getcwd()
    try:
        os.chdir(root)  # root/target does not exist
        shutil.copytree(src, dst, ignore_dangling_symlinks=True)
        print("exception: none")
    finally:
        os.chdir(old_cwd)

    print("source link exists?", os.path.lexists(os.path.join(src, "link")))
    print("destination entries:", sorted(os.listdir(dst)))
    print("destination link exists?", os.path.lexists(os.path.join(dst, "link")))

Actual output:

exception: none
source link exists? True
destination entries: ['target']
destination link exists? False

The source symlink is valid: src/link -> target resolves to src/target. With symlinks=False, the destination should contain a regular link file with the contents of src/target, in addition to target. Instead, the operation returns successfully with an incomplete destination tree.

The inverse case also fails: if src/link -> target is actually dangling but the current working directory contains target, it is not ignored and copytree() raises shutil.Error after creating the destination directory.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-156214

Bắt đầu trong Lib/shutil.py tại _copytree() và xem xét PR được liên kết gh-156214 trước khi thực hiện thay đổi. Tái hiện các trường hợp symlink tương đối từ issue, sau đó thêm hoặc cập nhật phạm vi kiểm thử hồi quy trong các bài kiểm thử shutil hiện có để các liên kết hợp lệ được sao chép và các liên kết bị hỏng bị bỏ qua bất kể thư mục làm việc hiện tại.