feat: Support constraints in pip_compile

[vihangm]

This adds in support to pass in a constraints file to pip-compile.
This is extremly useful when you want to uprade an indirect/intermediate
dependency to pull in security fixes but don't want to add said dependency to
the requirements.in file.

[aignas]

The docs say that the constraints file can be used more than once: https://pip-tools.readthedocs.io/en/latest/cli/pip-compile/

What is more, I think it is possible to do this today without this PR?

1. Add the constraints.txt files to data attribute.
2. Use $(location) in the args.

Am I missing something here?

[vihangm]

The docs say that the constraints file can be used more than once: pip-tools.readthedocs.io/en/latest/cli/pip-compile

Fair point, we could support a list instead of a single file/label instead.

What is more, I think it is possible to do this today without this PR?

1. Add the constraints.txt files to data attribute.
2. Use $(location) in the args.

Am I missing something here?

The data attribute gets popped from the kwargs here,

rules_python/python/private/pypi/pip_compile.bzl

Line 99 in a746b8f

srcs = kwargs.pop("data", []) + [requirements_txt],

and then the .update rule here

rules_python/python/private/pypi/pip_compile.bzl

Lines 160 to 165 in a746b8f

	py_binary(
	name = name + ".update",
	env = env,
	python_version = kwargs.get("python_version", None),
	**attrs
	)

gets the $(location) in the args but the file isn't in it's data deps. So your suggestion doesn't work at the moment.

[aignas]

But then it gets reincluded back in via

rules_python/python/private/pypi/pip_compile.bzl

Line 103 in a746b8f

data = [name, requirements_txt] + srcs + [f for f in (requirements_linux, requirements_darwin, requirements_windows) if f != None]

I think I am missing something crucial here. Just by reading the code, this looks like it should work. What is the minimum example that does not work?

[vihangm]

I think I am missing something crucial here. Just by reading the code, this looks like it should work. What is the minimum example that does not work?

e10255e fails.

$ bazelisk build //:requirements.update
ERROR: rules_python/examples/pip_parse/BUILD.bazel:57:25: in args attribute of py_binary rule //:requirements.update: label '//:constraints.txt' in $(location) expression is not a declared
 prerequisite of this rule. Since this rule was created by the macro 'pip_compile', the error might have been caused by the macro implementation
ERROR: rules_python/examples/pip_parse/BUILD.bazel:57:25: Analysis of target '//:requirements.update' failed
ERROR: Analysis of target '//:requirements.update' failed; build aborted
INFO: Elapsed time: 0.175s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully

102a76a is the minimal change to get it working again. I'm fine with that patch instead if you don't want to support an explicit arg.