[RFE] Support reflink (CoW copies)

[lukts30]

Currently any copy operation done by meson is a full (duplicating) copy even if the operating system and file system support copy-on-write file copies.

Applies to both fs.copyfile & meson install

Tools / languages that already make use of CoW file copies:

• cp, mv, and install from GNU coreutils since 9.0 (2021-09-24)
  • enabled by default since 9.1 (2022-04-15)
• jdk20 (2023)
• rust & cargo (ca. 2019)
• dotnet (2022)
• python3
  • shutil: add reflink=False to file copy functions to control clone/CoW copies (use copy_file_range) python/cpython#81338
  • GH-73991: Add pathlib.Path.copy() python/cpython#119058
    • unlike the other mentioned mentioned tools / languages in python this was implemented with a new API instead of transparently making the old API call copy_file_range etc.

[eli-schwartz]

• python3

  • shutil: add reflink=False to file copy functions to control clone/CoW copies (use copy_file_range) python/cpython#81338

  • GH-73991: Add pathlib.Path.copy() python/cpython#119058

    • unlike the other mentioned mentioned tools / languages in python this was implemented with a new API instead of transparently making the old API call copy_file_range etc.

This is an open python bug, not a closed one. It is not solved. Pathlib adding support doesn't mean anything since that isn't the standard file copying routine.

Python needs to fix this -- the changes will then propagate to the entire Python software ecosystem without further effort, like with how other languages do it.

In addition to the problems I already mentioned, pathlib's new API isn't even in a single released version of python -- it is exclusive to Python 3.14 -- which means nobody can use it at all until it is released. Furthermore, meson supports more than just the latest version of Python, so we don't have the option of moving to pathlib even if we wanted to (I very much don't like the idea of relying on pathlib at all for concrete path ops) until October 2029, when support for Python 3.13 is dropped.

I'm sorry. :(