Only crash in debug mode, if HTTPClient was not shutdown (#478)

### Motivation

Generally we want to inform users that they need to shutdown their HTTPClient. Until `1.6.0` we did this with an assert in HTTPClient's deinit. With `1.6.0` this behavior was raised to a precondition. Because of this adopters might suddenly crash in production where they didn't before.

### Changes

- This pr reverts the current behavior back to something pre `1.6.0`

### Result

- HTTPClient doesn't crash in production anymore.

Sources/AsyncHTTPClient/HTTPClient.swift

@@ -68,7 +68,7 @@ public class HTTPClient {
  let eventLoopGroupProvider: EventLoopGroupProvider
  let configuration: Configuration
  let poolManager: HTTPConnectionPool.Manager
- var state: State
+ private var state: State
  private let stateLock = Lock()
 
  internal static let loggingDisabled = Logger(label: "AHC-do-not-log", factory: { _ in SwiftLogNoOpLogHandler() })
@@ -118,8 +118,22 @@ public class HTTPClient {
  }
 
  deinit {
- guard case .shutDown = self.state else {
- preconditionFailure("Client not shut down before the deinit. Please call client.syncShutdown() when no longer needed.")
+ debugOnly {
+ // We want to crash only in debug mode.
+ switch self.state {
+ case .shutDown:
+ break
+ case .shuttingDown:
+ preconditionFailure("""
+ This state should be totally unreachable. While the HTTPClient is shutting down a \
+ reference cycle should exist, that prevents it from deinit.
+ """)
+ case .upAndRunning:
+ preconditionFailure("""
+ Client not shut down before the deinit. Please call client.syncShutdown() when no \
+ longer needed. Otherwise memory will leak.
+ """)
+ }
  }
  }
 

Sources/AsyncHTTPClient/Utils.swift

@@ -32,3 +32,13 @@ public final class HTTPClientCopyingDelegate: HTTPClientResponseDelegate {
  return ()
  }
 }
+
+/// A utility function that runs the body code only in debug builds, without
+/// emitting compiler warnings.
+///
+/// This is currently the only way to do this in Swift: see
+/// https://forums.swift.org/t/support-debug-only-code/11037 for a discussion.
+@inlinable
+internal func debugOnly(_ body: () -> Void) {
+ assert({ body(); return true }())
+}