bxl: get output artifacts for an Action

- Rename action to bxl.Action so it is findable in docs (I don't know if
  that was what was preventing docs generation, but it's worth a try!)
- Add a method to get the output artifacts for an action in bxl. This
  allows selectively rebuilding the action by ensuring those outputs.

I added a test, but I have no way to run those besides
www.metacareers.com: #1027

This is part of making action graph debugging slightly easier, which I
want to prototype by writing some rather nasty BXLs:
#1217

Unfortunately I *think* there's no way to get the dependencies after the
fact, since I *just* have a RegisteredAction. I believe that I can
probably do something very bad with owner() then rerunning the analysis
to get deps though.

Author: lf-

app/buck2_bxl/src/bxl/starlark_defs/nodes/action.rs

@@ -12,9 +12,11 @@ use std::convert::Infallible;
 use std::sync::Arc;
 
 use allocative::Allocative;
+use buck2_artifact::artifact::artifact_type::Artifact;
 use buck2_build_api::actions::RegisteredAction;
 use buck2_build_api::actions::query::ActionQueryNode;
 use buck2_build_api::actions::query::OwnedActionAttr;
+use buck2_build_api::interpreter::rule_defs::artifact::starlark_artifact::StarlarkArtifact;
 use buck2_core::deferred::base_deferred_key::BaseDeferredKey;
 use buck2_error::buck2_error;
 use buck2_interpreter::types::target_label::StarlarkConfiguredTargetLabel;
@@ -49,7 +51,7 @@ pub(crate) struct StarlarkAction(pub(crate) Arc<RegisteredAction>);
 
 starlark_simple_value!(StarlarkAction);
 
-#[starlark_value(type = "action")]
+#[starlark_value(type = "bxl.Action")]
 impl<'v> StarlarkValue<'v> for StarlarkAction {
     fn get_methods() -> Option<&'static Methods> {
         static RES: MethodsStatic = MethodsStatic::new();
@@ -67,7 +69,7 @@ impl<'a> UnpackValue<'a> for StarlarkAction {
     }
 }
 
-/// Methods for an action.
+/// Methods for an action obtained from [`bxl.AuditContext.output()`](../AuditContext#output).
 #[starlark_module]
 fn action_methods(builder: &mut MethodsBuilder) {
     /// Gets the owning configured target label for an action.
@@ -90,6 +92,17 @@ fn action_methods(builder: &mut MethodsBuilder) {
             .into()),
         }
     }
+
+    /// Gets the artifacts built by this action.
+    fn outputs<'v>(this: StarlarkAction) -> starlark::Result<Vec<StarlarkArtifact>> {
+        Ok(this
+            .0
+            .action()
+            .outputs()
+            .iter()
+            .map(|a| StarlarkArtifact::new(Artifact::from(a.clone())))
+            .collect())
+    }
 }
 
 #[derive(Debug, Display, ProvidesStaticType, Allocative)]

tests/core/bxl/test_audit_data/audit.bxl

@@ -15,6 +15,7 @@ def _audit_output_action_exists_impl(ctx):
     action = ctx.audit().output(buck_out)
 
     asserts.equals(action.owner(), target.label)
+    asserts.equals(actions.outputs()[0].basename, "with_output.txt")
 
 audit_output_action_exists = bxl_main(
     impl = _audit_output_action_exists_impl,

tests/core/query/aquery/test_aquery_data/aquery.bxl

@@ -130,7 +130,7 @@ def _impl_action_query_node(ctx):
     action = result[0]
     analysis = result[1]
 
-    _assert_eq(type(action.action()), "action")
+    _assert_eq(type(action.action()), "bxl.Action")
     _assert_eq(action.rule_type, "copy")
     _assert_eq(str(action.action().owner().raw_target()), "root//:test")