[antlir2][errors] library to integrate with buck2 action error handler

Summary:
Library to make it easier to integrate with buck2 action error handlers
https://www.internalfb.com/intern/staticdocs/buck2/docs/rule_authors/action_error_handler/

Test Plan: See rest of stack

Reviewed By: sergeyfd

Differential Revision: D59243780

fbshipit-source-id: ed4a5d11e5609900488f2d02a211ff16f831e66b

antlir/antlir2/antlir2_error_handler/BUCK

@@ -0,0 +1,13 @@
+load("//antlir/bzl:build_defs.bzl", "rust_library")
+
+oncall("antlir")
+
+rust_library(
+    name = "antlir2_error_handler",
+    srcs = glob(["src/**/*.rs"]),
+    deps = [
+        "serde",
+        "serde_json",
+        "typed-builder",
+    ],
+)

antlir/antlir2/antlir2_error_handler/handler.bzl

@@ -0,0 +1,25 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+def antlir2_error_handler(ctx: ActionErrorCtx) -> list[ActionSubError]:
+    errors = []
+
+    for line in ctx.stderr.splitlines():
+        if line.startswith("antlir2_error_handler: "):
+            err = line.removeprefix("antlir2_error_handler: ")
+            err = json.decode(err)
+            errors.append(ctx.new_sub_error(
+                category = err["category"],
+                message = err.get("message", None),
+                locations = [
+                    ctx.new_error_location(
+                        file = loc["file"],
+                        line = loc.get("line", None),
+                    )
+                    for loc in err.get("locations", [])
+                ],
+            ))
+
+    return errors

antlir/antlir2/antlir2_error_handler/src/lib.rs

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+//! Rust side of Buck2's action error handlers.
+//!
+//! Provides a standard way to report structured errors from Rust and keep that
+//! structure in Buck2.
+//!
+//! Today, these error details only appear in Buck2 datasets and build reports,
+//! but should appear more prominently in the buck2 cli/ui in the future.
+//!
+//! https://www.internalfb.com/intern/staticdocs/buck2/docs/rule_authors/action_error_handler/
+
+use std::fmt::Display;
+
+use serde::Serialize;
+use typed_builder::TypedBuilder;
+
+#[derive(TypedBuilder, Debug, Clone, Serialize)]
+pub struct SubError {
+    #[builder(setter(transform=|s: impl Display| s.to_string()))]
+    category: String,
+    #[builder(default, setter(transform=|s: impl Display| Some(s.to_string())))]
+    message: Option<String>,
+    #[builder(default)]
+    locations: Vec<Location>,
+}
+
+#[derive(TypedBuilder, Debug, Clone, Serialize)]
+pub struct Location {
+    file: String,
+    #[builder(default, setter(strip_option))]
+    line: Option<u32>,
+}
+
+impl SubError {
+    pub fn from_err(category: impl Display, err: impl std::error::Error) -> Self {
+        Self {
+            category: category.to_string(),
+            message: Some(err.to_string()),
+            locations: Default::default(),
+        }
+    }
+
+    pub fn log(&self) {
+        // Dump this to stderr. Unfortunately the action error handler only gets
+        // access to the stdout/err text and not any other artifacts, so we have
+        // to deal with it being mixed in with other output, hence this prefix.
+        eprintln!(
+            "antlir2_error_handler: {}",
+            serde_json::to_string(self).expect("SubError is always json-serializable")
+        );
+    }
+}