fix: harden bucket routing review follow-up

src/cluster/bucketed_service.rs

@@ -1,3 +1,5 @@
+use anyhow::Result;
+
 use super::cluster::{AdmissionStats, Cluster};
 use crate::config::{BucketConfig, Config, ModelConfig};
 use crate::instance::Instance;
@@ -46,17 +48,17 @@ impl BucketedService {
         &self.buckets[bucket_id as usize]
     }
 
-    pub fn route_and_admit(&mut self, req: &RequestRecord, now: f64) -> AdmissionStats {
+    pub fn route_and_admit(&mut self, req: &RequestRecord, now: f64) -> Result<AdmissionStats> {
         let bucket_views = self
             .buckets
             .iter()
             .map(|bucket| bucket.cluster.bucket_view(bucket.id, &bucket.cfg))
             .collect::<Vec<_>>();
-        let global = self.global_router.route(req, &bucket_views, now);
+        let global = self.global_router.route(req, &bucket_views, now)?;
         let bucket = &mut self.buckets[global.chosen_bucket as usize];
-        bucket
+        Ok(bucket
             .cluster
-            .route_and_admit_with_global(req, now, &global)
+            .route_and_admit_with_global(req, now, &global))
     }
 }
 
@@ -167,9 +169,19 @@ mod tests {
     fn strict_input_length_routes_into_matching_bucket() {
         let cfg = test_config();
         let mut service = BucketedService::new(&cfg, &cfg.model);
-        let stats = service.route_and_admit(&req(1, 24, &[10, 11]), 0.0);
+        let stats = service
+            .route_and_admit(&req(1, 24, &[10, 11]), 0.0)
+            .unwrap();
         assert_eq!(stats.bucket, 0);
         assert_eq!(stats.decision.chosen_bucket, 0);
+        assert_eq!(
+            stats.decision.global_reason,
+            "unique bucket range contains input_length"
+        );
+        assert_eq!(
+            stats.decision.local_reason,
+            "argmin(kv_used + alpha * queue_len)"
+        );
         assert_eq!(service.bucket(0).instances().len(), 2);
     }
 
@@ -177,10 +189,45 @@ mod tests {
     fn bucket_meta_store_is_isolated() {
         let cfg = test_config();
         let mut service = BucketedService::new(&cfg, &cfg.model);
-        let _ = service.route_and_admit(&req(1, 24, &[10, 11]), 0.0);
-        let long_stats = service.route_and_admit(&req(2, 64, &[10, 11, 12, 13]), 1.0);
+        let _ = service
+            .route_and_admit(&req(1, 24, &[10, 11]), 0.0)
+            .unwrap();
+        let long_stats = service
+            .route_and_admit(&req(2, 64, &[10, 11, 12, 13]), 1.0)
+            .unwrap();
         assert_eq!(long_stats.bucket, 1);
         assert_eq!(long_stats.remote_hit_blocks, 0);
         assert_eq!(long_stats.l1_hit_blocks, 0);
     }
+
+    #[test]
+    fn unmatched_input_length_returns_recoverable_error() {
+        let mut cfg = test_config();
+        cfg.cluster.buckets[1].input_length_min = 40;
+        let mut service = BucketedService::new(&cfg, &cfg.model);
+
+        let err = service
+            .route_and_admit(&req(3, 36, &[20, 21, 22]), 0.0)
+            .unwrap_err();
+
+        assert!(err.to_string().contains("no bucket"));
+        assert!(err.to_string().contains("input_length=36"));
+    }
+
+    #[test]
+    fn bucket_score_placeholder_reports_strict_fallback() {
+        let mut cfg = test_config();
+        cfg.cluster.global_router.mode = GlobalRouterMode::BucketScore;
+        let mut service = BucketedService::new(&cfg, &cfg.model);
+
+        let stats = service
+            .route_and_admit(&req(4, 24, &[30, 31]), 0.0)
+            .unwrap();
+
+        assert_eq!(stats.decision.global_mode, "strict_input_length");
+        assert!(stats
+            .decision
+            .global_reason
+            .contains("bucket_score is not implemented"));
+    }
 }

src/router/global_bucket.rs

@@ -1,3 +1,4 @@
+use anyhow::{anyhow, Result};
 use serde::Serialize;
 
 use crate::config::{Config, GlobalRouterMode};
@@ -54,7 +55,7 @@ pub trait GlobalRouter: Send {
         req: &RequestRecord,
         buckets: &[BucketView],
         now: f64,
-    ) -> GlobalRouteDecision;
+    ) -> Result<GlobalRouteDecision>;
 }
 
 struct StrictInputLengthRouter {
@@ -81,7 +82,7 @@ impl GlobalRouter for StrictInputLengthRouter {
         req: &RequestRecord,
         buckets: &[BucketView],
         _now: f64,
-    ) -> GlobalRouteDecision {
+    ) -> Result<GlobalRouteDecision> {
         let candidates = buckets
             .iter()
             .map(|view| BucketCandidate {
@@ -102,20 +103,31 @@ impl GlobalRouter for StrictInputLengthRouter {
             .map(|candidate| candidate.bucket)
             .collect::<Vec<_>>();
 
-        assert_eq!(
-            matches.len(),
-            1,
-            "global bucket routing requires exactly one matching bucket for input_len={}",
-            req.input_len
-        );
+        let chosen_bucket = match matches.as_slice() {
+            [bucket] => *bucket,
+            [] => {
+                return Err(anyhow!(
+                    "cluster.global_router.mode={} has no bucket for input_length={}",
+                    self.reported_mode,
+                    req.input_len
+                ));
+            }
+            _ => {
+                return Err(anyhow!(
+                    "cluster.global_router.mode={} matched multiple buckets for input_length={}",
+                    self.reported_mode,
+                    req.input_len
+                ));
+            }
+        };
 
-        GlobalRouteDecision {
+        Ok(GlobalRouteDecision {
             req_id: req.req_id,
             mode: self.reported_mode,
-            chosen_bucket: matches[0],
+            chosen_bucket,
             candidates,
             reason: self.reason,
-        }
+        })
     }
 }
 
@@ -126,8 +138,8 @@ pub fn build(full: &Config) -> Box<dyn GlobalRouter> {
             "unique bucket range contains input_length",
         )) as Box<dyn GlobalRouter>,
         GlobalRouterMode::BucketScore => Box::new(StrictInputLengthRouter::new(
-            "bucket_score",
-            "bucket_score placeholder falls back to strict_input_length",
+            "strict_input_length",
+            "bucket_score is not implemented in Task 2; falling back to strict_input_length",
         )) as Box<dyn GlobalRouter>,
     }
 }

src/router/mod.rs

@@ -39,17 +39,19 @@ pub struct RouteDecision {
     pub req_id: u64,
     pub global_mode: &'static str,
     pub mode: &'static str,
+    pub global_reason: &'static str,
+    pub local_reason: &'static str,
     pub chosen_bucket: BucketId,
     pub chosen: InstanceId,
     pub probe_overhead_s: f64,
     pub bucket_candidates: Vec<BucketCandidate>,
     pub candidates: Vec<CandidateInfo>,
-    pub reason: &'static str,
 }
 
 impl RouteDecision {
     pub fn with_global(mut self, decision: &GlobalRouteDecision) -> Self {
         self.global_mode = decision.mode;
+        self.global_reason = decision.reason;
         self.chosen_bucket = decision.chosen_bucket;
         self.bucket_candidates = decision.candidates.clone();
         self
@@ -90,12 +92,13 @@ pub fn local_route_decision(
         req_id,
         global_mode: "single_pool",
         mode,
+        global_reason: "single pool uses bucket 0",
+        local_reason: reason,
         chosen_bucket: 0,
         chosen,
         probe_overhead_s,
         bucket_candidates: Vec::new(),
         candidates,
-        reason,
     }
 }
 
@@ -393,7 +396,7 @@ mod tests {
         let mut router = PrefixAffinityRouter::new(&cfg);
         let decision = router.route(&req, &instances, &meta, 0.0);
 
-        assert_eq!(decision.reason, "affinity fallback: min(drain+fetch)");
+        assert_eq!(decision.local_reason, "affinity fallback: min(drain+fetch)");
         assert_eq!(decision.chosen, 1);
     }