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A Unix Socket Path Is Shorter Than tmp_path

Problem

Every test that starts a real tmux server errors at fixture setup on macOS:

error connecting to /private/var/folders/bs/c6nd19gd0g947t2sj57bskth0000gn/T/pytest-of-chris/pytest-322/popen-gw1/test_a_real_hand_made_pane_is_0/tmux.sock (File name too long)

A unix socket address is not a path the kernel resolves lazily. It is a fixed sun_path buffer inside struct sockaddr_un — 104 bytes on macOS, 108 on Linux. The path above is 135. Nothing is wrong with the directory, and ls on it works.

The length comes from pytest's temp root, and that root differs by platform:

macOS   /private/var/folders/<2>/<28>/T/pytest-of-<user>/pytest-<n>/   ~90 before the test
Linux   /tmp/pytest-of-<user>/pytest-<n>/                              ~30 before the test

So tmp_path / 'tmux.sock' fits on Linux with room to spare and cannot fit on macOS. Under pytest-xdist a popen-gw<n>/ segment adds ten more bytes, but the macOS path is already over the limit single-threaded — -n0 does not rescue it.

The failure is invisible until the suite runs on a Mac for the first time. A test written and run on Linux passes there forever.

Solution

Put the socket somewhere short and keep everything else in tmp_path. The tmux_socket fixture in tests/conftest.py is that place: tempfile.mkdtemp under /tmp, which is short on both platforms and still one directory per test and per parallel worker.

Uniqueness is why this is not tmux -L <name>. A name has to be invented, and two xdist workers can invent the same one; mkdtemp invents it atomically.

Key Learnings

  • A path length limit belongs to the socket, not to the filesystem. PATH_MAX is 1024 and sun_path is 104, so a path that works everywhere else fails here.
  • Quote the byte count when measuring one. len(path) against 104 settles it in one command, and the error names neither number.
  • tmp_path is the right home for a test's files and the wrong one for its sockets. The same applies to anything else with an address limit shorter than a path — abstract sockets, some IPC names.
  • A test suite that has only ever run on one platform has not been run. These three files held 291 passing tests that had never executed on macOS.
  • Testing — the tiers and what each may touch