### EventListener.Factory

In the preceding example we used a field, `callStartNanos`, to track the elapsed time of each event. This is handy, but it won’t work if multiple calls are executing concurrently. To accommodate this, use a `Factory` to create a new `EventListener` instance for each `Call`. This allows each listener to keep call-specific state.

import java.lang.ref.WeakReference
import java.net.Socket
import java.util.concurrent.CopyOnWriteArrayList
import java.util.concurrent.ExecutorService
import java.util.concurrent.RejectedExecutionException
import java.util.concurrent.TimeUnit.MILLISECONDS
import java.util.concurrent.atomic.AtomicBoolean
import java.util.concurrent.atomic.AtomicInteger
import java.util.concurrent.locks.ReentrantLock
import okhttp3.Call
import okhttp3.Callback

 * decide not to by returning -1L.
 *
 * Task Queues
 * -----------
 *
 * Tasks are bound to the [TaskQueue] they are scheduled in. Each queue is sequential and the tasks
 * within it never execute concurrently. It is an error to use a task in multiple queues.
 */
abstract class Task(
  val name: String,
  val cancelable: Boolean = true,
) {
  // Guarded by the TaskRunner.
  internal var queue: TaskQueue? = null

 * limitations under the License.
 */
package okhttp3.internal.concurrent

import assertk.assertThat
import assertk.assertions.isEqualTo
import java.io.Closeable
import java.util.AbstractQueue
import java.util.concurrent.BlockingQueue
import java.util.concurrent.Executors
import java.util.concurrent.TimeUnit
import java.util.logging.Logger
import kotlin.concurrent.withLock
import okhttp3.OkHttpClient

 */
package okhttp3

import java.util.ArrayDeque
import java.util.Collections
import java.util.Deque
import java.util.concurrent.ExecutorService
import java.util.concurrent.SynchronousQueue
import java.util.concurrent.ThreadPoolExecutor
import java.util.concurrent.TimeUnit
import java.util.concurrent.locks.ReentrantLock
import okhttp3.internal.assertNotHeld
import okhttp3.internal.connection.Locks.withLock

    strictly ASCII. This issue is tracked as [CVE-2021-0341].


## Version 4.9.1

_2021-01-30_

 *  Fix: Work around a crash in Android 10 and 11 that may be triggered when two threads
    concurrently close an SSL socket. This would have appeared in crash logs as
    `NullPointerException: bio == null`.


## Version 4.9.0

_2020-09-11_

   dropped in 2.0.

## Version 1.5.4

_2014-04-14_

 * Drop ALPN support in Android. There's a concurrency bug in all
   currently-shipping versions.
 * Support asynchronous disconnects by breaking the socket only. This should
   prevent flakiness from multiple threads concurrently accessing a stream.

## Version 1.5.3

_2014-03-29_

 * Fix bug where the Content-Length header was not always dropped when

Since version 5.0, `OkHttpClient` supports fast fallback, which is our implementation of Happy Eyeballs [RFC 6555](https://datatracker.ietf.org/doc/html/rfc6555).

With fast fallback, OkHttp attempts to connect to multiple web servers concurrently. It keeps whichever route connects first and cancels all of the others. Its rules are:

 * Prefer to alternate IP addresses from different address families, (IPv6 / IPv4), starting with IPv6.

 *  Fix: Don't start the clock on response timeouts until the request body is fully transmitted.
    This is only relevant for duplex request bodies, because they are written concurrently when
    reading the response body.

 *  New: `MockResponse.inTunnel()` is a new `mockwebserver3` API to configure responses that are

  /**
   * A plan holds either an immediately-usable connection, or one that must be connected first.
   * These steps are split so callers can call [connectTcp] on a background thread if attempting
   * multiple plans concurrently.
   */
  interface Plan {
    val isReady: Boolean

    fun connectTcp(): ConnectResult

    fun connectTlsEtc(): ConnectResult

    fun handleSuccess(): RealConnection

    fun cancel()