proxy: reintroduce dynamic limiter for compute lock (#7737)

## Problem

Computes that are healthy can manage many connection attempts at a time.
Unhealthy computes cannot. We initially handled this with a fixed
concurrency limit, but it seems this inhibits pgbench.

## Summary of changes

Support AIMD for connect_to_compute lock to allow varying the
concurrency limit based on compute health

proxy/src/rate_limiter/limit_algorithm/aimd.rs

@@ -0,0 +1,184 @@
+use std::usize;
+
+use super::{LimitAlgorithm, Outcome, Sample};
+
+/// Loss-based congestion avoidance.
+///
+/// Additive-increase, multiplicative decrease.
+///
+/// Adds available currency when:
+/// 1. no load-based errors are observed, and
+/// 2. the utilisation of the current limit is high.
+///
+/// Reduces available concurrency by a factor when load-based errors are detected.
+#[derive(Clone, Copy, Debug, serde::Deserialize, PartialEq)]
+pub struct Aimd {
+    /// Minimum limit for AIMD algorithm.
+    pub min: usize,
+    /// Maximum limit for AIMD algorithm.
+    pub max: usize,
+    /// Decrease AIMD decrease by value in case of error.
+    pub dec: f32,
+    /// Increase AIMD increase by value in case of success.
+    pub inc: usize,
+    /// A threshold below which the limit won't be increased.
+    pub utilisation: f32,
+}
+
+impl LimitAlgorithm for Aimd {
+    fn update(&self, old_limit: usize, sample: Sample) -> usize {
+        use Outcome::*;
+        match sample.outcome {
+            Success => {
+                let utilisation = sample.in_flight as f32 / old_limit as f32;
+
+                if utilisation > self.utilisation {
+                    let limit = old_limit + self.inc;
+                    let increased_limit = limit.clamp(self.min, self.max);
+                    if increased_limit > old_limit {
+                        tracing::info!(increased_limit, "limit increased");
+                    }
+
+                    increased_limit
+                } else {
+                    old_limit
+                }
+            }
+            Overload => {
+                let limit = old_limit as f32 * self.dec;
+
+                // Floor instead of round, so the limit reduces even with small numbers.
+                // E.g. round(2 * 0.9) = 2, but floor(2 * 0.9) = 1
+                let limit = limit.floor() as usize;
+
+                limit.clamp(self.min, self.max)
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::time::Duration;
+
+    use crate::rate_limiter::limit_algorithm::{
+        DynamicLimiter, RateLimitAlgorithm, RateLimiterConfig,
+    };
+
+    use super::*;
+
+    #[tokio::test(start_paused = true)]
+    async fn should_decrease_limit_on_overload() {
+        let config = RateLimiterConfig {
+            initial_limit: 10,
+            algorithm: RateLimitAlgorithm::Aimd {
+                conf: Aimd {
+                    min: 1,
+                    max: 1500,
+                    inc: 10,
+                    dec: 0.5,
+                    utilisation: 0.8,
+                },
+            },
+        };
+
+        let limiter = DynamicLimiter::new(config);
+
+        let token = limiter
+            .acquire_timeout(Duration::from_millis(1))
+            .await
+            .unwrap();
+        token.release(Outcome::Overload);
+
+        assert_eq!(limiter.state().limit(), 5, "overload: decrease");
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn should_increase_limit_on_success_when_using_gt_util_threshold() {
+        let config = RateLimiterConfig {
+            initial_limit: 4,
+            algorithm: RateLimitAlgorithm::Aimd {
+                conf: Aimd {
+                    min: 1,
+                    max: 1500,
+                    inc: 1,
+                    dec: 0.5,
+                    utilisation: 0.5,
+                },
+            },
+        };
+
+        let limiter = DynamicLimiter::new(config);
+
+        let token = limiter
+            .acquire_timeout(Duration::from_millis(1))
+            .await
+            .unwrap();
+        let _token = limiter
+            .acquire_timeout(Duration::from_millis(1))
+            .await
+            .unwrap();
+        let _token = limiter
+            .acquire_timeout(Duration::from_millis(1))
+            .await
+            .unwrap();
+
+        token.release(Outcome::Success);
+        assert_eq!(limiter.state().limit(), 5, "success: increase");
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn should_not_change_limit_on_success_when_using_lt_util_threshold() {
+        let config = RateLimiterConfig {
+            initial_limit: 4,
+            algorithm: RateLimitAlgorithm::Aimd {
+                conf: Aimd {
+                    min: 1,
+                    max: 1500,
+                    inc: 10,
+                    dec: 0.5,
+                    utilisation: 0.5,
+                },
+            },
+        };
+
+        let limiter = DynamicLimiter::new(config);
+
+        let token = limiter
+            .acquire_timeout(Duration::from_millis(1))
+            .await
+            .unwrap();
+
+        token.release(Outcome::Success);
+        assert_eq!(
+            limiter.state().limit(),
+            4,
+            "success: ignore when < half limit"
+        );
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn should_not_change_limit_when_no_outcome() {
+        let config = RateLimiterConfig {
+            initial_limit: 10,
+            algorithm: RateLimitAlgorithm::Aimd {
+                conf: Aimd {
+                    min: 1,
+                    max: 1500,
+                    inc: 10,
+                    dec: 0.5,
+                    utilisation: 0.5,
+                },
+            },
+        };
+
+        let limiter = DynamicLimiter::new(config);
+
+        let token = limiter
+            .acquire_timeout(Duration::from_millis(1))
+            .await
+            .unwrap();
+        drop(token);
+        assert_eq!(limiter.state().limit(), 10, "ignore");
+    }
+}