RestApiServlet: Allow HEAD requests

Change I3be4aeb009 replaced the "Get file type" REST API endpoint with
the /content endpoint. According to documentation [1], the content type
can be retrieved by making a HEAD request to the endpoint.

This doesn't work because RestApiServlet is only checking for GET, and
HEAD requests result in 404. Modify it to also allow HEAD requests.

Add a test to make sure it works.

[1] https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#get-content

Bug: Issue 4136
Change-Id: If5eaaba2fec94499f332970ce6b8c6db2ac9c571

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/HttpResponse.java

@@ -51,6 +51,16 @@ public class HttpResponse {
     return response.getStatusLine().getStatusCode();
   }
 
+  public String getContentType() {
+    return response.getFirstHeader("X-FYI-Content-Type").getValue();
+  }
+
+  public boolean hasContent() {
+    Preconditions.checkNotNull(response,
+        "Response is not initialized.");
+    return response.getEntity() != null;
+  }
+
   public String getEntityContent() throws IOException {
     Preconditions.checkNotNull(response,
         "Response is not initialized.");

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/RestSession.java

@@ -114,6 +114,9 @@ public class RestSession extends HttpSession {
     return execute(Request.Delete(url + "/a" + endPoint));
   }
 
+  public RestResponse head(String endPoint) throws IOException {
+    return execute(Request.Head(url + "/a" + endPoint));
+  }
 
   public static RawInput newRawInput(String content) {
     return newRawInput(content.getBytes(UTF_8));

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/revision/RevisionIT.java

@@ -19,13 +19,14 @@ import static com.google.gerrit.acceptance.PushOneCommit.FILE_CONTENT;
 import static com.google.gerrit.acceptance.PushOneCommit.FILE_NAME;
 import static com.google.gerrit.acceptance.PushOneCommit.PATCH;
 import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.apache.http.HttpStatus.SC_OK;
 import static org.eclipse.jgit.lib.Constants.HEAD;
 
 import com.google.common.base.Predicate;
 import com.google.common.collect.Iterables;
 import com.google.gerrit.acceptance.AbstractDaemonTest;
-import com.google.gerrit.acceptance.NoHttpd;
 import com.google.gerrit.acceptance.PushOneCommit;
+import com.google.gerrit.acceptance.RestResponse;
 import com.google.gerrit.acceptance.TestAccount;
 import com.google.gerrit.extensions.api.changes.ChangeApi;
 import com.google.gerrit.extensions.api.changes.CherryPickInput;
@@ -63,7 +64,6 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 
-@NoHttpd
 public class RevisionIT extends AbstractDaemonTest {
 
   private TestAccount admin2;
@@ -433,6 +433,20 @@ public class RevisionIT extends AbstractDaemonTest {
     assertThat(res).isEqualTo(FILE_CONTENT);
   }
 
+  @Test
+  public void contentType() throws Exception {
+    PushOneCommit.Result r = createChange();
+
+    String endPoint = "/changes/" + r.getChangeId()
+      + "/revisions/" + r.getCommit().name()
+      + "/files/" + FILE_NAME
+      + "/content";
+    RestResponse response = adminSession.head(endPoint);
+    assertThat(response.getStatusCode()).isEqualTo(SC_OK);
+    assertThat(response.getContentType()).startsWith("text/plain");
+    assertThat(response.hasContent()).isFalse();
+  }
+
   private void assertMergeable(String id, boolean expected) throws Exception {
     MergeableInfo m = gApi.changes().id(id).current().mergeable();
     assertThat(m.mergeable).isEqualTo(expected);

gerrit-httpd/src/main/java/com/google/gerrit/httpd/restapi/RestApiServlet.java

@@ -317,8 +317,7 @@ public class RestApiServlet extends HttpServlet {
         return;
       }
 
-      if (viewData.view instanceof RestReadView<?>
+      if (viewData.view instanceof RestReadView<?> && isGetOrHead(req)) {
-          && "GET".equals(req.getMethod())) {
         result = ((RestReadView<RestResource>) viewData.view).apply(rsrc);
       } else if (viewData.view instanceof RestModifyView<?, ?>) {
         @SuppressWarnings("unchecked")