* server.
 *
 * ## Preemptive Authentication
 *
 * To make HTTPS calls using an HTTP proxy server OkHttp must first negotiate a connection with
 * the proxy. This proxy connection is called a "TLS Tunnel" and is specified by
 * [RFC 2817][1]. The HTTP CONNECT request that creates this tunnel connection is special: it
 * does not participate in any [interceptors][Interceptor] or [event listeners][EventListener]. It

import okio.IOException

interface Callback {
  /**
   * Called when the request could not be executed due to cancellation, a connectivity problem or
   * timeout. Because networks can fail during an exchange, it is possible that the remote server
   * accepted the request before the failure.
   */
  fun onFailure(
    call: Call,
    e: IOException,
  )

  /**

 *
 * Other exception types cancel the current call:
 *
 *  * For synchronous calls made with [Call.execute], the exception is propagated to the caller.
 *
 *  * For asynchronous calls made with [Call.enqueue], an [IOException] is propagated to the caller
 *    indicating that the call was canceled. The interceptor's exception is delivered to the current

    }
  }

  private fun logEvent(e: ConnectionEvent) {
    if (e.connection != null) {
      assertThat(Thread.holdsLock(e.connection), "Called with lock $${e.connection}")
        .isFalse()
    }
    for (lock in forbiddenLocks) {
      assertThat(Thread.holdsLock(lock), "Called with lock $lock")
        .isFalse()
    }

    if (enforceOrder) {
      checkForStartEvent(e)
    }

    /**
     * Creates a new event source and immediately returns it. Creating an event source initiates an
     * asynchronous process to connect the socket. Once that succeeds or fails, `listener` will be
     * notified. The caller must cancel the returned event source when it is no longer in use.
     */
    fun newEventSource(
      request: Request,
      listener: EventSourceListener,
    ): EventSource
  }

      } else {
        return EventListener.NONE;
      }
    }
  };

  ...
}
```

### Events with Failures

When an operation fails, a failure method is called. This is `connectFailed()` for failures while building a connection to the server, and `callFailed()` when the HTTP call fails permanently. When a failure happens it is possible that a `start` event won’t have a corresponding `end` event.

        String::class.java,
        String::class.java,
      )
    } catch (_: NoSuchMethodException) {
      null
    }

  /** Android method to clean and sort certificates, called via reflection. */
  @Suppress("unused", "UNCHECKED_CAST")
  fun checkServerTrusted(
    chain: Array<out X509Certificate>,
    authType: String,
    host: String,
  ): List<Certificate> {

 */
interface CookieJar {
  /**
   * Saves [cookies] from an HTTP response to this store according to this jar's policy.
   *
   * Note that this method may be called a second time for a single HTTP response if the response
   * includes a trailer. For this obscure HTTP feature, [cookies] contains only the trailer's
   * cookies.
   */
  fun saveFromResponse(
    url: HttpUrl,

import okhttp3.internal.connection.RealCall

/**
 * A concrete interceptor chain that carries the entire interceptor chain: all application
 * interceptors, the OkHttp core, all network interceptors, and finally the network caller.
 *
 * If the chain is for an application interceptor then [exchange] must be null. Otherwise it is for
 * a network interceptor and [exchange] must be non-null.
 */
class RealInterceptorChain(

   *
   * ```java
   * // ensure the response (and underlying response body) is closed
   * try (Response response = client.newCall(request).execute()) {
   *   ...
   * }
   * ```
   *
   * The caller may read the response body with the response's [Response.body] method. To avoid
   * leaking resources callers must [close the response body][ResponseBody] or the response.
   *