feat(tonic-xds): parse A32 circuit breaking config

tonic-xds/src/xds/resource/circuit_breaking.rs

@@ -0,0 +1,127 @@
+//! Validated configuration types for [gRFC A32] circuit breaking.
+//!
+//! gRPC supports only the `max_requests` threshold from Envoy's CDS
+//! `CircuitBreakers` config. Other threshold fields are intentionally ignored
+//! because they are connection-pool or retry specific and do not apply to gRPC's
+//! A32 request limiter.
+//!
+//! This parser intentionally stays detached from `ClusterResource` until
+//! enforcement lands; otherwise cluster validation would advertise support before
+//! requests are actually limited.
+//!
+//! [gRFC A32]: https://github.com/grpc/proposal/blob/master/A32-xds-circuit-breaking.md
+
+use envoy_types::pb::envoy::config::cluster::v3::{CircuitBreakers, circuit_breakers::Thresholds};
+use envoy_types::pb::envoy::config::core::v3::RoutingPriority;
+
+/// Default max concurrent requests per cluster from A32.
+pub(crate) const DEFAULT_MAX_REQUESTS: u32 = 1024;
+
+/// Validated A32 circuit-breaking configuration for a cluster.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct CircuitBreakingConfig {
+    /// Maximum number of in-flight requests allowed for the upstream cluster.
+    ///
+    /// This scaffolds the parsed CDS value only; enforcement is wired in a
+    /// follow-up change so request-lifetime accounting can be handled correctly
+    /// for streaming RPCs.
+    pub(crate) max_requests: u32,
+}
+
+impl CircuitBreakingConfig {
+    /// Build circuit-breaking config from a CDS `CircuitBreakers` message.
+    ///
+    /// A32 uses the first threshold for `RoutingPriority::Default`. If no
+    /// applicable threshold or `max_requests` value is present, the gRPC default
+    /// of 1024 is used.
+    pub(crate) fn from_proto(circuit_breakers: Option<&CircuitBreakers>) -> Self {
+        let max_requests = circuit_breakers
+            .and_then(first_default_threshold)
+            .and_then(|threshold| threshold.max_requests.as_ref())
+            .map(|value| value.value)
+            .unwrap_or(DEFAULT_MAX_REQUESTS);
+
+        Self { max_requests }
+    }
+}
+
+impl Default for CircuitBreakingConfig {
+    fn default() -> Self {
+        Self {
+            max_requests: DEFAULT_MAX_REQUESTS,
+        }
+    }
+}
+
+fn first_default_threshold(circuit_breakers: &CircuitBreakers) -> Option<&Thresholds> {
+    circuit_breakers.thresholds.iter().find(|threshold| {
+        RoutingPriority::try_from(threshold.priority).ok() == Some(RoutingPriority::Default)
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use envoy_types::pb::google::protobuf::UInt32Value;
+
+    fn threshold(priority: RoutingPriority, max_requests: Option<u32>) -> Thresholds {
+        Thresholds {
+            priority: priority as i32,
+            max_requests: max_requests.map(|value| UInt32Value { value }),
+            ..Default::default()
+        }
+    }
+
+    #[test]
+    fn defaults_when_circuit_breakers_absent() {
+        assert_eq!(
+            CircuitBreakingConfig::from_proto(None),
+            CircuitBreakingConfig {
+                max_requests: DEFAULT_MAX_REQUESTS,
+            }
+        );
+    }
+
+    #[test]
+    fn defaults_when_default_threshold_absent() {
+        let circuit_breakers = CircuitBreakers {
+            thresholds: vec![threshold(RoutingPriority::High, Some(7))],
+            ..Default::default()
+        };
+
+        assert_eq!(
+            CircuitBreakingConfig::from_proto(Some(&circuit_breakers)).max_requests,
+            DEFAULT_MAX_REQUESTS
+        );
+    }
+
+    #[test]
+    fn defaults_when_default_threshold_has_no_max_requests() {
+        let circuit_breakers = CircuitBreakers {
+            thresholds: vec![threshold(RoutingPriority::Default, None)],
+            ..Default::default()
+        };
+
+        assert_eq!(
+            CircuitBreakingConfig::from_proto(Some(&circuit_breakers)).max_requests,
+            DEFAULT_MAX_REQUESTS
+        );
+    }
+
+    #[test]
+    fn uses_first_default_threshold_max_requests() {
+        let circuit_breakers = CircuitBreakers {
+            thresholds: vec![
+                threshold(RoutingPriority::High, Some(9)),
+                threshold(RoutingPriority::Default, Some(11)),
+                threshold(RoutingPriority::Default, Some(13)),
+            ],
+            ..Default::default()
+        };
+
+        assert_eq!(
+            CircuitBreakingConfig::from_proto(Some(&circuit_breakers)).max_requests,
+            11
+        );
+    }
+}

tonic-xds/src/xds/resource/mod.rs

@@ -10,6 +10,7 @@
 //!
 //! These are *validated* types containing only the fields relevant to gRPC
 
+pub(crate) mod circuit_breaking;
 pub(crate) mod cluster;
 pub(crate) mod endpoints;
 pub(crate) mod hash_policy;