[bug] Can't create package that has symlinks when the conan cache is a symlink

When the conan cache dir (~/.conan) is a symlink (for /tmp/.conan for example) and I’m trying to create a package that includes internal symlinks, the conan create command fails. The failure point is when the export_source folder contents (in the cache) gets copied over to the source dir. Looking at: https://github.com/conan-io/conan/blob/ff7b8e6703e8407773968517d68424b9ec59aa30/conans/util/files.py#L412 You can see that the usage of os.path.realpath would resolve the src’s symlink, which would completely throw off the os.path.relpath call. The solution is to simply remove the os.path.realpath:

out_of_source = os.path.relpath(linkto, src).startswith(".")

Environment Details (include every applicable attribute)

• Operating System+version: Ubuntu 18.04
• Compiler+version: Any
• Conan version: 1.40.0
• Python version: 3.8.5

Steps to reproduce (Include if Applicable)

mkdir /tmp/.conan
ln -s /tmp/.conan -t ~/
conan new symlink_bug/0.1.0
touch original_file
ln -s original_file link_file
# add the following code to beginning of SymlinkBugConan::build()
#        with open('link_file', 'r') as f:
#            print("link_file found!")
conan create .

Result:

ERROR: symlink_bug/0.1.0: Error in build() method, line 33
        with open('link_file', 'r') as f:
        FileNotFoundError: [Errno 2] No such file or directory: 'link_file'

Expected Result: Package gets created

[memsharded]

Because the intent of Python devs was to deprecate distutils implementation in favor of shutil one (https://bugs.python.org/issue4843), and Conan necessary support (2.7, 3.5, 3.6…) didn’t implement the necessary functionality in shutil, raising if the directory already exists (only copying to an empty directory). Apparently in 3.8 it is possible, but Conan still needs to support 3.6, so not a possibility.

[tapia]

Just for the record, the PR related to this issue is #9584