* UnsupportedOperationException.
   *
   * <p>The implementation deviates from the {@code ExecutorService} specification with regards to
   * the {@code shutdownNow} method. First, "best-effort" with regards to canceling running tasks is
   * implemented as "no-effort". No interrupts or other attempts are made to stop threads executing
   * tasks. Second, the returned list will always be empty, as any submitted task is considered to

### Canceling a Call ([.kt][CancelCallKotlin], [.java][CancelCallJava])

    `MockWebServer`.
 *  Fix: Honor the `MockResponse` header delay in MockWebServer.
 *  Fix: Don't release HTTP/2 connections that have multiple canceled calls. We had a bug where
    canceling calls would cause the shared HTTP/2 connection to be unnecessarily released. This
    harmed connection reuse.
 *  Fix: Ensure canceled and discarded HTTP/2 data is not permanently counted against the limited

    includes potential security problems (newline characters) as well as simple
    non-ASCII characters (including international characters and emoji).

 *  **Call canceling is more reliable.**  We had a bug where a socket being
     connected wasn't being closed when the application used `Call.cancel()`.

 *  **Changing a HttpUrl’s scheme now tracks the default port.** We had a bug

 *  Fix: Don't crash with a `InaccessibleObjectException` when detecting the platform trust manager
    on Java 17+.
 *  Fix: Don't crash if a cookie's value is a lone double quote character.
 *  Fix: Don't crash when canceling an event source created by `EventSources.processResponse()`.
 *  New: `Cache` now has a public constructor that takes an [okio.FileSystem]. This should make it
    possible to implement decorators for cache encryption or compression.

	}

	if z.SinglePool() {
		return errInvalidArgument
	}

	z.poolMetaMutex.Lock()
	defer z.poolMetaMutex.Unlock()

	fn := z.decommissionCancelers[idx]
	if fn == nil {
		// canceling a decommission before it started return an error.
		return errDecommissionNotStarted
	}

	defer fn() // cancel any active thread.

	if z.poolMeta.DecommissionCancel(idx) {

      responseDequeuedLatches[responseIndex].countDown()
      requestCanceledLatches[responseIndex].await()
      responseIndex++
      return response
    }
  }

  /** Make a call and canceling it as soon as it's accepted by the server.  */
  private fun callAndCancel(
    expectedSequenceNumber: Int,
    responseDequeuedLatch: CountDownLatch?,
    requestCanceledLatch: CountDownLatch?,
  ) {

    assertArrayEquals("fghi".toByteArray(), data2.data)
  }

  /**
   * Confirm that we account for discarded data frames. It's possible that data frames are in-flight
   * just prior to us canceling a stream.
   */
  @Test fun discardedDataFramesAreCounted() {
    // Write the mocking script.
    peer.sendFrame().settings(Settings())
    peer.acceptFrame() // ACK
    peer.acceptFrame() // SYN_STREAM 3