Add Decision, decide and the runner stub to brokerd

Implemented-By: Laguna S 2.1 (OpenCode)

crates/brokerd/src/policy.rs

@@ -0,0 +1,95 @@
+//! Policy decisions. `Decision` can only be constructed in this module.
+//!
+//! Code outside this module cannot build a `Decision` with a struct literal, because its fields
+//! are private:
+//!
+//! ```compile_fail
+//! let request = proto::ToolRequest {
+//!     session: proto::SessionId::new("s1").unwrap(),
+//!     call: proto::CallId(1),
+//!     tool: "read_file".to_string(),
+//!     arguments: "{}".to_string(),
+//! };
+//! let _ = brokerd::policy::Decision { request, grant: "g".to_string() };
+//! ```
+//!
+//! Nor with the constructor, because it is private to this module:
+//!
+//! ```compile_fail
+//! let request = proto::ToolRequest {
+//!     session: proto::SessionId::new("s1").unwrap(),
+//!     call: proto::CallId(1),
+//!     tool: "read_file".to_string(),
+//!     arguments: "{}".to_string(),
+//! };
+//! let _ = brokerd::policy::Decision::new(request, "g".to_string());
+//! ```
+//!
+//! The same setup compiles when it goes through `decide`, which proves the two examples above
+//! fail because of `Decision` and not because of a mistake in the setup:
+//!
+//! ```
+//! let request = proto::ToolRequest {
+//!     session: proto::SessionId::new("s1").unwrap(),
+//!     call: proto::CallId(1),
+//!     tool: "read_file".to_string(),
+//!     arguments: "{}".to_string(),
+//! };
+//! assert_eq!(brokerd::policy::decide(request).unwrap_err(), proto::DenyReason::NoGrant);
+//! ```
+
+use proto::{DenyReason, ToolRequest};
+
+#[derive(Debug)]
+pub struct Decision {
+    request: ToolRequest,
+    grant: String,
+}
+
+impl Decision {
+    #[cfg_attr(not(test), expect(dead_code, reason = "grant matching arrives in M3"))]
+    fn new(request: ToolRequest, grant: String) -> Self {
+        Decision { request, grant }
+    }
+
+    pub fn request(&self) -> &ToolRequest {
+        &self.request
+    }
+
+    pub fn grant(&self) -> &str {
+        &self.grant
+    }
+}
+
+/// Until M3 there are no grants, so every request is denied with DenyReason::NoGrant.
+pub fn decide(request: ToolRequest) -> Result<Decision, DenyReason> {
+    let _ = request;
+    Err(DenyReason::NoGrant)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use proto::{CallId, SessionId};
+
+    fn request() -> ToolRequest {
+        ToolRequest {
+            session: SessionId::new("s1").unwrap(),
+            call: CallId(1),
+            tool: "read_file".to_string(),
+            arguments: "{}".to_string(),
+        }
+    }
+
+    #[test]
+    fn no_grants_means_deny() {
+        assert_eq!(decide(request()).unwrap_err(), DenyReason::NoGrant);
+    }
+
+    #[test]
+    fn decision_exposes_request_and_grant() {
+        let d = Decision::new(request(), "g1".to_string());
+        assert_eq!(d.request().tool, "read_file");
+        assert_eq!(d.grant(), "g1");
+    }
+}