Refactor provider shutdown and pending flows (#240)

Author: artemredkin

Diff for: Sources/AsyncHTTPClient/ConnectionPool.swift

@@ -80,12 +80,13 @@ final class ConnectionPool {
             if let existing = self.providers[key], existing.enqueue() {
                 return existing
             } else {
-                // Connection provider will be created with `pending = 1`
                 let provider = HTTP1ConnectionProvider(key: key,
                                                        eventLoop: taskEventLoop,
                                                        configuration: self.configuration,
                                                        pool: self,
                                                        backgroundActivityLogger: self.backgroundActivityLogger)
+                let enqueued = provider.enqueue()
+                assert(enqueued)
                 self.providers[key] = provider
                 return provider
             }
@@ -263,8 +264,6 @@ struct ConnectionKey: Hashable {
 /// of concurrent requests as it has built-in politeness regarding the maximum number
 /// of concurrent requests to the server.
 class HTTP1ConnectionProvider {
-    struct ProviderClosedError: Error {}
-
     /// The client configuration used to bootstrap new requests
     private let configuration: HTTPClient.Configuration
 
@@ -318,7 +317,7 @@ class HTTP1ConnectionProvider {
         self.state.assertInvariants()
     }
 
-    private func execute(_ action: Action, logger: Logger) {
+    func execute(_ action: Action, logger: Logger) {
         switch action {
         case .lease(let connection, let waiter):
             // if connection is became inactive, we create a new one.
@@ -494,6 +493,11 @@ class HTTP1ConnectionProvider {
                 $0.promise.fail(HTTPClientError.cancelled)
             }
 
+            if available.isEmpty, leased.isEmpty {
+                self.closePromise.succeed(())
+                return self.closePromise.futureResult.map { clean }
+            }
+
             EventLoopFuture.andAllComplete(leased.map { $0.cancel() }, on: self.eventLoop).flatMap { _ in
                 EventLoopFuture.andAllComplete(available.map { $0.close() }, on: self.eventLoop)
             }.whenFailure { error in

Diff for: Sources/AsyncHTTPClient/ConnectionsState.swift

@@ -64,7 +64,7 @@ extension HTTP1ConnectionProvider {
         private var openedConnectionsCount: Int = 0
 
         /// Number of enqueued requests, used to track if it is safe to delete the provider.
-        private var pending: Int = 1
+        private var pending: Int = 0
 
         init(maximumConcurrentConnections: Int = 8, eventLoop: EventLoop) {
             self.maximumConcurrentConnections = maximumConcurrentConnections
@@ -148,7 +148,7 @@ extension HTTP1ConnectionProvider {
                     return .none
                 }
             case .closed:
-                return .fail(waiter, ProviderClosedError())
+                return .fail(waiter, HTTPClientError.alreadyShutdown)
             }
         }