Type mismatches in await Gets give unclear error message

[illicitonion]

If you do something like:
snapshot = yield Get(Snapshot, PathGlobs, "foo")
where there is a type mismatch between the declared parameter type, and the actual parameter type, you get an error:

Exception: WithDeps(Inner(InnerEntry { params: {Console, OptionsBootstrapper, Specs}, rule: Task(Task { product: List, clause: [Select { product: Console }, Select { product: _Options }, Select { product: Specs }], gets: [Get { product: Snapshot, subject: PathGlobs }, Get { product: HydratedTargets, subject: Specs }, Get { product: BuildFileAddresses, subject: Specs }], func: list_targets(), cacheable: false }) })) did not declare a dependency on JustGet(Get { product: Snapshot, subject: str })

Talking about "did not declare a dependency" is really misleading; this error message should read more like:

"Did not have a set of rules installed to transform from a str to a Snapshot, which you tried to do in file X on line Y"

and maybe even a hint along the lines of "You declared that the argument to be of type PathGlobs but it was actually of type str".

[stuhood]

Slightly grouchy comment, but: #7502 (comment) ... relates to #7509.

[stuhood]

Oh! There is a much, much easier way to fix 90% of cases of this one though: the constructor for Get should verify that the types match for the three arg form, here:

pants/src/python/pants/engine/selectors.py

Lines 86 to 87 in 44d4ab5

	elif len(args) == 3:
	product, subject_declared_type, subject = args

[Eric-Arellano]

Oh! There is a much, much easier way to fix 90% of cases of this one though: the constructor for Get should verify that the types match for the three arg form, here:

I tried doing this a few weeks ago. It fails because of union types :/ I couldn't come up with a clean fix. Inspecting union_membership was clunky iirc (and maybe not even possible? I don't recall).

--

@TansyArron and I came up with this error message:

Exception: await Get[Digest](InputFilesContent) expected InputFilesContent but got str instead. See the @rule construct_coverage_config() from pants.backend.python.rules.pytest_coverage line 100.

Rust already has all the information needed to render this, including line numbers. We only need to implement it now.

[Eric-Arellano]

@herman5 said he's interested in taking this on 🎉

To formalize the problem a little more, the error happens when you do:

await Get[A](B, z)

where z is not an instance of the class B. A is the "product". B is the "subject" type, and z is the "subject" instance.

--

The goal is to say something like this:

Exception: await Get[Digest](InputFilesContent) expected InputFilesContent but got str instead. See the @rule construct_coverage_config() from pants.backend.python.rules.pytest_coverage line 100.

Please feel free to improve this!

--

You'll need to change this:

pants/src/rust/engine/src/nodes.rs

Lines 765 to 813 in a154c84

	fn gen_get(
	context: &Context,
	params: &Params,
	entry: &Arc<rule_graph::Entry<Rule>>,
	gets: Vec<externs::Get>,
	) -> NodeFuture<Vec<Value>> {
	let get_futures = gets
	.into_iter()
	.map(|get| {
	let context = context.clone();
	let mut params = params.clone();
	let entry = entry.clone();
	let dependency_key = selectors::DependencyKey::JustGet(selectors::Get {
	product: get.product,
	subject: *get.subject.type_id(),
	});
	let entry = context
	.core
	.rule_graph
	.edges_for_inner(&entry)
	.ok_or_else(|| throw(&format!("no edges for task {:?} exist!", entry)))
	.and_then(|edges| {
	edges
	.entry_for(&dependency_key)
	.cloned()
	.ok_or_else(|| match get.declared_subject {
	Some(ty) if externs::is_union(ty) => {
	let value = externs::get_value_from_type_id(ty);
	match externs::call_method(
	&value,
	"non_member_error_message",
	&[externs::val_for(&get.subject)],
	) {
	Ok(err_msg) => throw(&externs::val_to_str(&err_msg)),
	// If the non_member_error_message() call failed for any reason,
	// fall back to a generic message.
	Err(_e) => throw(&format!(
	"Type {} is not a member of the {} @union",
	get.subject.type_id(),
	ty
	)),
	}
	}
	_ => throw(&format!(
	"{:?} did not declare a dependency on {:?}",
	entry, dependency_key
	)),
	})
	});

Line 809 is what you'll want to change. You can get the declared product type (A) and the actual bad subject type (z) from lines 778-779:

pants/src/rust/engine/src/nodes.rs

Lines 778 to 779 in a154c84

	product: get.product,
	subject: *get.subject.type_id(),

I'm not totally sure how to get the declared subject type (B)? It's definitely possible - if you look at the original error message posted by Daniel, we have access to all of the rule's gets. Possibly, you need to iterate through the rule's gets to filter by the product type, then use that to find the declared subject type?

I think entry should have the rule information you need, specifically the rule name, the line number, and the module name. This is where we generate the representation of the rule function:

pants/src/rust/engine/src/core.rs

Lines 166 to 182 in a154c84

	impl Function {
	pub fn name(&self) -> String {
	let Function(key) = self;
	let module = externs::project_str(&externs::val_for(&key), "__module__");
	let name = externs::project_str(&externs::val_for(&key), "__name__");
	// NB: this is a custom dunder method that Python code should populate before sending the
	// function (e.g. an `@rule`) through FFI.
	let line_number = externs::project_str(&externs::val_for(&key), "__line_number__");
	format!("{}:{}:{}", module, line_number, name)
	}
	}
	impl fmt::Display for Function {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	write!(f, "{}()", self.name())
	}
	}

This generates a value like pants_test.engine.rules:84:a_from_b_noop(). Feel free to stick with that representation of a rule, as opposed to something like construct_coverage_config() from pants.backend.python.rules.pytest_coverage line 100.. Otherwise, you'd need to generalize that above code so that you can request the distinct parts like module, lineno, and name. Maybe that's a good change and how we should have done it from the start?

You might also find this https://github.com/pantsbuild/pants/blob/master/src/rust/engine/src/tasks.rs helpful to see how we generally represent a rule in graph error messages. See the type FormattedTaskRuleElements and impl fmt::Display for Rule. Generally, this has more information than we want for this particular error message, but this could help see how to work with the type.

--

Finally, you'll want to make sure that ./v2 test --debug tests/python/pants_test/engine/test_scheduler.py --pytest-args='-k test_get_type_match_failure' passes.