[HTTP2] Improve performance of backoff timer done event (#464)

Author: dnadoba

Diff for: Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP2Connections.swift

@@ -49,6 +49,15 @@ extension HTTPConnectionPool {
             }
         }
 
+        var isStartingOrActive: Bool {
+            switch self.state {
+            case .starting, .active:
+                return true
+            case .draining, .backingOff, .closed:
+                return false
+            }
+        }
+
         /// A connection is established and can potentially execute requests if not all streams are leased
         var isActive: Bool {
             switch self.state {
@@ -439,24 +448,26 @@ extension HTTPConnectionPool {
             }
         }
 
-        /// used after backoff is done to determine if we need to create a new connection
-        func hasStartingOrActiveConnection() -> Bool {
-            self.connections.contains { connection in
-                connection.canOrWillBeAbleToExecuteRequests
-            }
-        }
-
         /// used after backoff is done to determine if we need to create a new connection
         /// - Parameters:
         ///   - eventLoop: connection `EventLoop` to search for
-        /// - Returns: true if at least one connection is starting or active for the given `eventLoop`
-        func hasStartingOrActiveConnection(
+        /// - Returns: if we have a starting or active general purpose connection and if we have also one for the given `eventLoop`
+        func backingOffTimerDone(
             for eventLoop: EventLoop
-        ) -> Bool {
-            self.connections.contains { connection in
-                connection.eventLoop === eventLoop &&
-                    connection.canOrWillBeAbleToExecuteRequests
+        ) -> RetryConnectionCreationContext {
+            var hasGeneralPurposeConnection: Bool = false
+            var hasConnectionOnSpecifiedEventLoop: Bool = false
+            for connection in self.connections {
+                guard connection.isStartingOrActive else { continue }
+                hasGeneralPurposeConnection = true
+                guard connection.eventLoop === eventLoop else { continue }
+                hasConnectionOnSpecifiedEventLoop = true
+                break
             }
+            return RetryConnectionCreationContext(
+                hasGeneralPurposeConnection: hasGeneralPurposeConnection,
+                hasConnectionOnSpecifiedEventLoop: hasConnectionOnSpecifiedEventLoop
+            )
         }
 
         mutating func createNewConnection(on eventLoop: EventLoop) -> Connection.ID {
@@ -675,6 +686,14 @@ extension HTTPConnectionPool {
 
         // MARK: Result structs
 
+        struct RetryConnectionCreationContext {
+            /// true if at least one connection is starting or active regardless of the event loop.
+            let hasGeneralPurposeConnection: Bool
+
+            ///  true if at least one connection is starting or active for the given `eventLoop`
+            let hasConnectionOnSpecifiedEventLoop: Bool
+        }
+
         /// Information around a connection which is either in the .active or .draining state.
         struct EstablishedConnectionContext {
             /// number of streams which can be leased

Diff for: Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP2StateMachine.swift

@@ -321,18 +321,21 @@ extension HTTPConnectionPool {
                 // per event loop for required event loop requests and only need one connection for
                 // general purpose requests.
 
+                // precompute if we have starting or active connections to only iterate once over `self.connections`
+                let context = self.connections.backingOffTimerDone(for: eventLoop)
+
                 // we need to start a new on connection in two cases:
                 let needGeneralPurposeConnection =
                     // 1. if we have general purpose requests
                     !self.requests.isEmpty(for: nil) &&
                     // and no connection starting or active
-                    !self.connections.hasStartingOrActiveConnection()
+                    !context.hasGeneralPurposeConnection
 
                 let needRequiredEventLoopConnection =
                     // 2. or if we have requests for a required event loop
                     !self.requests.isEmpty(for: eventLoop) &&
                     // and no connection starting or active for the given event loop
-                    !self.connections.hasStartingOrActiveConnection(for: eventLoop)
+                    !context.hasConnectionOnSpecifiedEventLoop
 
                 guard needGeneralPurposeConnection || needRequiredEventLoopConnection else {
                     // otherwise we can remove the connection

Diff for: Tests/AsyncHTTPClientTests/HTTPConnectionPool+HTTP2StateMachineTests+XCTest.swift

@@ -39,6 +39,7 @@ extension HTTPConnectionPool_HTTP2StateMachineTests {
             ("testMigrationFromHTTP1ToHTTP2", testMigrationFromHTTP1ToHTTP2),
             ("testMigrationFromHTTP1ToHTTP2WithAlreadyStartedHTTP1Connections", testMigrationFromHTTP1ToHTTP2WithAlreadyStartedHTTP1Connections),
             ("testHTTP2toHTTP1Migration", testHTTP2toHTTP1Migration),
+            ("testConnectionIsImmediatelyCreatedAfterBackoffTimerFires", testConnectionIsImmediatelyCreatedAfterBackoffTimerFires),
         ]
     }
 }

Diff for: Tests/AsyncHTTPClientTests/HTTPConnectionPool+HTTP2StateMachineTests.swift

@@ -869,4 +869,78 @@ class HTTPConnectionPool_HTTP2StateMachineTests: XCTestCase {
         XCTAssertEqual(releaseAction.request, .none)
         XCTAssertNoThrow(try connections.closeConnection(http2Conn))
     }
+
+    func testConnectionIsImmediatelyCreatedAfterBackoffTimerFires() {
+        let elg = EmbeddedEventLoopGroup(loops: 2)
+        let el1 = elg.next()
+        let el2 = elg.next()
+        var connections = MockConnectionPool()
+        var queuer = MockRequestQueuer()
+        var state = HTTPConnectionPool.StateMachine(idGenerator: .init(), maximumConcurrentHTTP1Connections: 8)
+
+        var connectionIDs: [HTTPConnectionPool.Connection.ID] = []
+        for el in [el1, el2, el2] {
+            let mockRequest = MockHTTPRequest(eventLoop: el, requiresEventLoopForChannel: true)
+            let request = HTTPConnectionPool.Request(mockRequest)
+            let action = state.executeRequest(request)
+            guard case .createConnection(let connID, let eventLoop) = action.connection else {
+                return XCTFail("Unexpected connection action \(action.connection)")
+            }
+            connectionIDs.append(connID)
+            XCTAssertTrue(eventLoop === el)
+            XCTAssertEqual(action.request, .scheduleRequestTimeout(for: request, on: mockRequest.eventLoop))
+            XCTAssertNoThrow(try connections.createConnection(connID, on: el))
+            XCTAssertNoThrow(try queuer.queue(mockRequest, id: request.id))
+        }
+
+        // fail the two connections for el2
+        for connectionID in connectionIDs.dropFirst() {
+            struct SomeError: Error {}
+            XCTAssertNoThrow(try connections.failConnectionCreation(connectionID))
+            let action = state.failedToCreateNewConnection(SomeError(), connectionID: connectionID)
+            XCTAssertEqual(action.request, .none)
+            guard case .scheduleBackoffTimer(connectionID, backoff: _, on: _) = action.connection else {
+                return XCTFail("unexpected connection action \(connectionID)")
+            }
+            XCTAssertNoThrow(try connections.startConnectionBackoffTimer(connectionID))
+        }
+        let http2ConnID1 = connectionIDs[0]
+        let http2ConnID2 = connectionIDs[1]
+        let http2ConnID3 = connectionIDs[2]
+
+        // let the first connection on el1 succeed as a http2 connection
+        let http2Conn1: HTTPConnectionPool.Connection = .__testOnly_connection(id: http2ConnID1, eventLoop: el1)
+        XCTAssertNoThrow(try connections.succeedConnectionCreationHTTP2(http2ConnID1, maxConcurrentStreams: 10))
+        let migrationAction1 = state.newHTTP2ConnectionCreated(http2Conn1, maxConcurrentStreams: 10)
+        guard case .executeRequestsAndCancelTimeouts(let requests, http2Conn1) = migrationAction1.request else {
+            return XCTFail("unexpected request action \(migrationAction1.request)")
+        }
+        XCTAssertEqual(migrationAction1.connection, .migration(createConnections: [], closeConnections: [], scheduleTimeout: nil))
+        XCTAssertEqual(requests.count, 1)
+        for request in requests {
+            XCTAssertNoThrow(try queuer.get(request.id, request: request.__testOnly_wrapped_request()))
+            XCTAssertNoThrow(try connections.execute(request.__testOnly_wrapped_request(), on: http2Conn1))
+        }
+
+        // we now have 1 active connection on el1 and 2 backing off connections on el2
+        // with 2 queued requests with a requirement to be executed on el2
+
+        // if the backoff timer fires for a connection on el2, we should immediately start a new connection
+        XCTAssertNoThrow(try connections.connectionBackoffTimerDone(http2ConnID2))
+        let action2 = state.connectionCreationBackoffDone(http2ConnID2)
+        XCTAssertEqual(action2.request, .none)
+        guard case .createConnection(let newHttp2ConnID2, let eventLoop2) = action2.connection else {
+            return XCTFail("Unexpected connection action \(action2.connection)")
+        }
+        XCTAssertTrue(eventLoop2 === el2)
+        XCTAssertNoThrow(try connections.createConnection(newHttp2ConnID2, on: el2))
+
+        // we now have a starting connection for el2 and another one backing off
+
+        // if the backoff timer fires now for a connection on el2, we should *not* start a new connection
+        XCTAssertNoThrow(try connections.connectionBackoffTimerDone(http2ConnID3))
+        let action3 = state.connectionCreationBackoffDone(http2ConnID3)
+        XCTAssertEqual(action3.request, .none)
+        XCTAssertEqual(action3.connection, .none)
+    }
 }