## Version 5.0.0-alpha.4

_2022-02-01_

**This release introduces fast fallback to better support mixed IPv4+IPv6 networks.** Fast fallback
is what we're calling our implementation of Happy Eyeballs, [RFC 8305][rfc_8305]. With this
feature OkHttp will attempt both IPv6 and IPv4 connections concurrently, keeping whichever connects
first. Fast fallback gives IPv6 connections a 250 ms head start so IPv6 is preferred on networks

 * [declaration provider][org.jetbrains.kotlin.analysis.providers.KotlinDeclarationProvider].
 *
 * Since this fallback is not suitable for symbols from binary libraries in Standalone mode, the symbol names provider absolutely needs to
 * work there. This functionality is covered by package scope tests.
 */

        if (cacheResponse.code != HttpURLConnection.HTTP_GATEWAY_TIMEOUT) {
          return cacheResponse
        }
      } catch (ioe: IOException) {
        // Failures are ignored as we can fallback to the network
        // and hopefully repopulate the cache.
      }
    }

    return null
  }

  @Throws(Exception::class)
  private fun readResponse(
    hostname: String,

low/compiler/mlir/tfrt/transforms/mlrt,tensorflow/core/runtime_fallback,tensorflow/core/runtime_fallback/conversion,tensorflow/core/runtime_fallback/kernel,tensorflow/core/runtime_fallback/opdefs,tensorflow/core/runtime_fallback/runtime,tensorflow/core/runtime_fallback/util,tensorflow/core/runtime_fallback/test,tensorflow/core/runtime_fallback/test/gpu,tensorflow/core/runtime_fallback/test/saved_model,tensorflow/core/runtime_fallback/test/testdata,tensorflow/core/tfrt/stubs,tensorflow/core/tfrt/...

    client.newCall(Request.Builder().url(server.url("/a")).build()).enqueue(callback)
    callback.await(server.url("/a")).assertBody("abc")
    client.newCall(Request.Builder().url(server.url("/b")).build()).enqueue(callback)
    callback.await(server.url("/b")).assertBody("def")
    client.newCall(Request.Builder().url(server.url("/c")).build()).enqueue(callback)
    callback.await(server.url("/c")).assertBody("ghi")

Go 1.20 introduced automatic seeding of the
[`math/rand`](/pkg/math/rand) global random number generator,
controlled by the [`randautoseed` setting](/pkg/math/rand/#Seed).

Go 1.20 introduced the concept of fallback roots for use during certificate verification,
controlled by the [`x509usefallbackroots` setting](/pkg/crypto/x509/#SetFallbackRoots).

Go 1.20 removed the preinstalled `.a` files for the standard library

    )

    taskFaker.assertNoMoreTasks()
  }

  /**
   * This test puts several connections in flight that all fail at approximately the same time. It
   * confirms the fast fallback implements these invariants:
   *
   *  * if there's no TCP connect in flight, start one.
   *  * don't start a new TCP connect within 250 ms of the previous TCP connect.
   */
  @Test

# OWNER_LIST: Brian.Norman
# OWNER_LIST: Sergej.Jaskiewicz
# OWNER_LIST: Wojciech.Litewka
# Kotlin Libraries
# OWNER_LIST: A.Qurbonzoda Vsevolod.Tolstopyato Ilya.Gorbunov Sergey.Shanshin Leonid.Startsev Filipp.Zhinkin

# Fallback rule, must always be top
* Kotlin

/.idea/ "Kotlin Build Infrastructure"
/.idea/kotlinTestDataPluginTestDataPaths.xml Dmitriy.Novozhilov Kirill.Rakhman Sergej.Jaskiewicz

/.fleet/ "Kotlin Build Infrastructure"

Now use the parameter `callbacks` in *your API's path operation decorator* to pass the attribute `.routes` (that's actually just a `list` of routes/*path operations*) from that callback router:

```Python hl_lines="35"
{!../../../docs_src/openapi_callbacks/tutorial001.py!}
```

!!! tip

### 添加回调路由

至此，在上文创建的回调路由里就包含了*回调路径操作*（外部开发者要在外部 API 中实现）。

现在使用 API *路径操作装饰器*的参数 `callbacks`，从回调路由传递属性 `.routes`（实际上只是路由/路径操作的**列表**）：

```Python hl_lines="36"
{!../../../docs_src/openapi_callbacks/tutorial001.py!}
```

!!! tip "提示"

    注意，不能把路由本身（`invoices_callback_router`）传递给 `callback=`，要传递 `invoices_callback_router.routes` 中的 `.routes` 属性。

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