PexProcess and VenvPexProcess do not use append_only_caches.

[jsirois]

Right now they avoid the plumbing needed for this by using an absolute named_caches based path for the PEX_ROOT. See comments here: #11040 (comment)

pants/src/python/pants/backend/python/util_rules/pex_environment.py

Lines 224 to 234 in 60916e1

	return PexEnvironment(
	path=pex_runtime_env.path(pex_relevant_environment),
	interpreter_search_paths=tuple(
	python_setup.interpreter_search_paths(pex_relevant_environment)
	),
	subprocess_environment_dict=subprocess_env_vars.vars,
	# TODO: This path normalization is duplicated with `engine_initializer.py`. How can we do
	# the normalization only once, via the options system?
	named_caches_dir=Path(global_options.options.named_caches_dir).resolve().as_posix(),
	bootstrap_python=first_python_binary(),
	)

pants/src/python/pants/backend/python/util_rules/pex_environment.py

Lines 122 to 132 in 60916e1

	def environment_dict(self, *, python_configured: bool) -> Mapping[str, str]:
	"""The environment to use for running anything with PEX.
	If the Process is run with a pre-selected Python interpreter, set `python_configured=True`
	to avoid PEX from trying to find a new interpreter.
	"""
	d = dict(
	PATH=create_path_env_var(self.path),
	PEX_INHERIT_PATH="false",
	PEX_IGNORE_RCFILES="true",
	PEX_ROOT=os.path.join(self.named_caches_dir, "pex_root"),

pants/src/python/pants/backend/python/util_rules/pex.py

Lines 968 to 988 in 60916e1

	env = {
	**pex_environment.environment_dict(python_configured=pex.python is not None),
	**(request.extra_env or {}),
	}
	input_digest = (
	await Get(Digest, MergeDigests((pex.digest, request.input_digest)))
	if request.input_digest
	else pex.digest
	)
	return Process(
	argv,
	description=request.description,
	level=request.level,
	input_digest=input_digest,
	env=env,
	output_files=request.output_files,
	output_directories=request.output_directories,
	timeout_seconds=request.timeout_seconds,
	execution_slot_variable=request.execution_slot_variable,
	cache_scope=request.cache_scope,
	)

This breaks remote execution where a PEX might be built in a container with a different named caches dir absolute path than where it is run.

The VenvPexProcess setup will additionally need a fix for its venv_dir which is likewise absolute and should be relativized against the append_only_caches dir:

pants/src/python/pants/backend/python/util_rules/pex.py

Lines 841 to 848 in 60916e1

	pex_request = request.pex_request
	seeded_venv_request = dataclasses.replace(
	pex_request, additional_args=pex_request.additional_args + ("--venv", "--seed")
	)
	result = await Get(BuildPexResult, PexRequest, seeded_venv_request)
	# Pex --seed mode outputs the path of the PEX executable. In the --venv case this is the `pex`
	# script in the venv root directory.
	venv_dir = PurePath(result.result.stdout.decode().strip()).parent

[jsirois]

As discovered in #11742.