* response body is [closed][ResponseBody]. The recipient of the callback may consume the response
   * body on another thread.
   *
   * Note that transport-layer success (receiving a HTTP response code, headers and body) does not
   * necessarily indicate application-layer success: `response` may still indicate an unhappy HTTP
   * response code like 404 or 500.
   */
  @Throws(IOException::class)
  fun onResponse(
    call: Call,

A single HTTP call may require follow-up requests to be made to handle authentication challenges, redirects, and HTTP-layer timeouts. In such cases multiple connections, requests, and responses may be attempted. Follow-ups are another reason a single call may trigger multiple events of the same type.

### Threads

#### Application's calling thread

The application-layer must block on writing I/O. We can't return from a write until we've pushed its bytes onto the socket. Otherwise, if the write fails we are unable to deliver its IOException to the application. We would have told the application layer that the write succeeded, but it didn't!

import okhttp3.internal.UTC

/** The last four-digit year: "Fri, 31 Dec 9999 23:59:59 GMT". */
internal const val MAX_DATE = 253402300799999L

/**
 * Most websites serve cookies in the blessed format. Eagerly create the parser to ensure such
 * cookies are on the fast path.
 */
private val STANDARD_DATE_FORMAT =
  object : ThreadLocal<DateFormat>() {
    override fun initialValue(): DateFormat {

   * leaking resources callers must [close the response body][ResponseBody] or the response.
   *
   * Note that transport-layer success (receiving a HTTP response code, headers and body) does not
   * necessarily indicate application-layer success: `response` may still indicate an unhappy HTTP
   * response code like 404 or 500.
   *

# Module okhttp-tls

##### Changes

 * New: Support for `HTTP-draft-09/2.0`.
 * New: Support for `spdy/3.1`. Dropped support for `spdy/3`.
 * New: Use ALPN on Android platforms that support it (4.4+)
 * New: CacheControl model and parser.
 * New: Protocol selection in MockWebServer.
 * Fix: Route selection shouldn't use TLS modes that we know will fail.
 * Fix: Cache SPDY responses even if the response body is closed prematurely.

 *
 * Each test is a line of the file `urltestdata.txt`. The format is informally specified by its
 * JavaScript parser `urltestparser.js` with which this class attempts to be compatible.
 *
 * Each line of the `urltestdata.txt` file specifies a test. Lines look like this:
 *
 * ```

 * [Communicator](https://github.com/Taig/Communicator): An OkHttp wrapper for Scala built with Android in mind.
 * [Cronet Transport for OkHttp](https://github.com/google/cronet-transport-for-okhttp): A HTTP3 ready transport layer for OkHttp on Android, based on Chromium network stack.
 * [CWAC-NetSecurity](https://github.com/commonsguy/cwac-netsecurity): Simplifying Secure Internet Access.

   * callback has returned. Synchronous calls become idle once [execute()][Call.execute] returns.
   * This means that if you are doing synchronous calls the network layer will not truly be idle
   * until every returned [Response] has been closed.
   */
  var idleCallback: Runnable? = null
    get() = this.withLock { field }
    set(value) {
      this.withLock { field = value }
    }