[4.x] Add test for 103 handling

mockwebserver/src/main/kotlin/okhttp3/mockwebserver/MockResponse.kt

@@ -29,6 +29,8 @@ class MockResponse : Cloneable {
   @set:JvmName("status")
   var status: String = ""
 
+  internal var informationalResponses: List<MockResponse> = listOf()
+
   private var headersBuilder = Headers.Builder()
   private var trailersBuilder = Headers.Builder()
 
@@ -347,6 +349,10 @@ class MockResponse : Cloneable {
 
   override fun toString() = status
 
+  fun addInformationalResponse(informationalResponse: MockResponse): MockResponse = apply {
+    informationalResponses += informationalResponse
+  }
+
   companion object {
     private const val CHUNKED_BODY_HEADER = "Transfer-encoding: chunked"
   }

mockwebserver/src/main/kotlin/okhttp3/mockwebserver/MockWebServer.kt

@@ -1026,6 +1026,13 @@ class MockWebServer : ExternalResource(), Closeable {
         readBody = true
       }
 
+      peek.informationalResponses.forEach {
+        val informationalHeader =
+          listOf(Header(Header.RESPONSE_STATUS, it.status.replace("HTTP/1.1 ", "")))
+        stream.writeHeaders(informationalHeader, outFinished = false, flushHeaders = true)
+        stream.connection.flush()
+      }
+
       val body = Buffer()
       val requestLine = "$method $path HTTP/1.1"
       var exception: IOException? = null

okhttp/src/test/java/okhttp3/CallTest.java

@@ -2812,6 +2812,43 @@ public void cancelWhileRequestHeadersAreSent_HTTP_2() throws Exception {
     assertThat(recordedRequest.getBody().readUtf8()).isEqualTo("abc");
   }
 
+  @Test
+  public void serverRespondsWithEarlyHintsHttp2() throws Exception {
+    enableProtocol(Protocol.HTTP_2);
+    server.enqueue(
+            new MockResponse()
+                    .addInformationalResponse(
+                            new MockResponse().setResponseCode(103).addHeader("Link", "</style.css>; rel=preload; as=style")
+                    )
+    );
+    Request request = new Request.Builder()
+            .url(server.url("/"))
+            .post(RequestBody.create("abc", MediaType.get("text/plain")))
+            .build();
+    executeSynchronously(request)
+            .assertFailure(SocketTimeoutException.class);
+  }
+
+  @Test
+  public void serverRespondsWithEarlyHintsHttp1() throws Exception {
+    server.enqueue(
+            new MockResponse()
+                    .addInformationalResponse(
+                            new MockResponse().setResponseCode(103).addHeader("Link", "</style.css>; rel=preload; as=style")
+                    )
+    );
+    Request request = new Request.Builder()
+            .url(server.url("/"))
+            .post(RequestBody.create("abc", MediaType.get("text/plain")))
+            .build();
+    executeSynchronously(request)
+            .assertSuccessful()
+            .assertCode(200);
+    RecordedRequest recordedRequest = server.takeRequest();
+    assertThat(recordedRequest.getBody().readUtf8()).isEqualTo("abc");
+    assertThat(recordedRequest.getHeader("Link")).isNull();
+  }
+
   @Test public void serverRespondsWithUnsolicited100Continue_HTTP2() throws Exception {
     enableProtocol(Protocol.HTTP_2);
     serverRespondsWithUnsolicited100Continue();