Fix data race with RedisMetrics.activeConnectionCount

Motivation:

After enabling the thread sanitizer during testing, a data race with the `activeConnectionCount` in `RedisMetrics` was caught due to changing a primitive `Int` across threads.

Modifications:

Add a specialized class `ActiveConnectionGauge` to wrap the semantics of the `activeConnectionCount` with an `Atomic<Int>` property.

Result:

No data races should occur with the `RedisMetrics` subsystem.

Author: Mordil

Sources/RediStack/RedisConnection.swift

@@ -148,7 +148,7 @@ public final class RedisConnection: RedisClient {
         self.logger[metadataKey: loggingKeyID] = "\(UUID())"
         self._state = .open
         self.logger.debug("Connection created.")
-        RedisMetrics.activeConnectionCount += 1
+        RedisMetrics.activeConnectionCount.increment()
         RedisMetrics.totalConnectionCount.increment()
 
         // attach a callback to the channel to capture situations where the channel might be closed out from under
@@ -160,7 +160,7 @@ public final class RedisConnection: RedisClient {
 
             self.state = .closed
             self.logger.warning("Channel was closed unexpectedly.")
-            RedisMetrics.activeConnectionCount -= 1
+            RedisMetrics.activeConnectionCount.decrement()
         }
     }
 
@@ -243,7 +243,7 @@ extension RedisConnection {
         notification.whenSuccess {
             self.state = .closed
             self.logger.debug("Connection closed.")
-            RedisMetrics.activeConnectionCount -= 1
+            RedisMetrics.activeConnectionCount.decrement()
         }
 
         return notification

Sources/RediStack/RedisMetrics.swift

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 import Metrics
+import NIOConcurrencyHelpers
 
 /// The system funnel for all `Metrics` interactions from the Redis library.
 ///
@@ -38,14 +39,8 @@ public struct RedisMetrics {
         }
     }
 
-    private static let activeConnectionCountGauge = Gauge(label: .activeConnectionCount)
-    /// The current number of connections this library has active.
-    /// - Note: Changing this number will update the `Metrics.Gauge` stored for recording the new value.
-    public static var activeConnectionCount: Int = 0 {
-        didSet {
-            activeConnectionCountGauge.record(activeConnectionCount)
-        }
-    }
+    /// The wrapped `Metrics.Gauge` maintaining the current number of connections this library has active.
+    public static var activeConnectionCount = ActiveConnectionGauge()
     /// The `Metrics.Counter` that retains the number of connections made since application startup.
     public static let totalConnectionCount = Counter(label: .totalConnectionCount)
     /// The `Metrics.Counter` that retains the number of commands that successfully returned from Redis
@@ -61,6 +56,31 @@ public struct RedisMetrics {
     private init() { }
 }
 
+/// A specialized wrapper class for working with `Metrics.Gauge` objects for the purpose of an incrementing or decrementing count of active Redis connections.
+public class ActiveConnectionGauge {
+    private let gauge = Gauge(label: .activeConnectionCount)
+    private let count = Atomic<Int>(value: 0)
+    
+    /// The number of the connections that are currently reported as active.
+    var currentCount: Int { return count.load() }
+    
+    internal init() { }
+    
+    /// Increments the current count by the amount specified.
+    /// - Parameter amount: The number to increase the current count by. Default is `1`.
+    public func increment(by amount: Int = 1) {
+        _ = self.count.add(amount)
+        self.gauge.record(self.count.load())
+    }
+    
+    /// Decrements the current count by the amount specified.
+    /// - Parameter amount: The number to decrease the current count by. Default is `1`.
+    public func decrement(by amount: Int = 1) {
+        _ = self.count.sub(amount)
+        self.gauge.record(self.count.load())
+    }
+}
+
 extension Metrics.Counter {
     @inline(__always)
     convenience init(label: RedisMetrics.Label) {