Ship a list of default experiments from the backend and allow disabling

With this commit, we ship a list of default experiments from the backend
to the frontend and allow administrators to disable default experiments
using a config in gerrit.config.

Change-Id: I2b47042475f330b1816195b230c03a3594ebdaa7

Documentation/config-gerrit.txt

@@ -3383,6 +3383,17 @@ removed in the next stable version.
   enabled = ExperimentKey
 ----
 
+[[experiments.disabled]]experiments.disabled::
++
+List of experiments that are currently disabled. The release notes contain currently
+available experiments. This list disables experiments with the given key that are
+either enabled by default or explicitly in the config.
+
+----
+[experiments]
+  disabled = ExperimentKey
+----
+
 [[ldap]]
 === Section ldap
 

java/com/google/gerrit/httpd/raw/IndexHtmlUtil.java

@@ -20,6 +20,7 @@ import static java.util.stream.Collectors.toSet;
 
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
 import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.common.UsedAt;
@@ -48,6 +49,10 @@ import org.eclipse.jgit.lib.Config;
 public class IndexHtmlUtil {
   private static final FluentLogger logger = FluentLogger.forEnclosingClass();
 
+  static final ImmutableSet<String> DEFAULT_EXPERIMENTS =
+      ImmutableSet.of(
+          "UiFeature__patchset_comments", "UiFeature__patchset_choice_for_comment_links");
+
   private static final Gson GSON = OutputFormat.JSON_COMPACT.newGson();
   /**
    * Returns both static and dynamic parameters of {@code index.html}. The result is to be used when
@@ -69,9 +74,13 @@ public class IndexHtmlUtil {
                 canonicalURL, cdnPath, faviconPath, urlParameterMap, urlInScriptTagOrdainer))
         .putAll(dynamicTemplateData(gerritApi, requestedURL));
 
-    Set<String> enabledExperiments = new HashSet<>(experimentData(urlParameterMap));
+    Set<String> enabledExperiments = new HashSet<>();
     Arrays.stream(gerritServerConfig.getStringList("experiments", null, "enabled"))
         .forEach(enabledExperiments::add);
+    DEFAULT_EXPERIMENTS.forEach(enabledExperiments::add);
+    Arrays.stream(gerritServerConfig.getStringList("experiments", null, "disabled"))
+        .forEach(enabledExperiments::remove);
+    experimentData(urlParameterMap).forEach(enabledExperiments::add);
     if (!enabledExperiments.isEmpty()) {
       data.put("enabledExperiments", serializeObject(GSON, enabledExperiments).toString());
     }

javatests/com/google/gerrit/httpd/raw/IndexServletTest.java

@@ -15,6 +15,7 @@
 package com.google.gerrit.httpd.raw;
 
 import static com.google.common.truth.Truth.assertThat;
+import static java.util.stream.Collectors.joining;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
@@ -54,8 +55,12 @@ public class IndexServletTest {
     String testCdnPath = "bar-cdn";
     String testFaviconURL = "zaz-url";
 
+    String disabledDefault = IndexHtmlUtil.DEFAULT_EXPERIMENTS.asList().get(0);
     org.eclipse.jgit.lib.Config serverConfig = new org.eclipse.jgit.lib.Config();
-    serverConfig.setStringList("experiments", null, "enabled", ImmutableList.of("NewFeature"));
+    serverConfig.setStringList(
+        "experiments", null, "enabled", ImmutableList.of("NewFeature", "DisabledFeature"));
+    serverConfig.setStringList(
+        "experiments", null, "disabled", ImmutableList.of("DisabledFeature", disabledDefault));
     IndexServlet servlet =
         new IndexServlet(testCanonicalUrl, testCdnPath, testFaviconURL, gerritApi, serverConfig);
 
@@ -80,8 +85,14 @@ public class IndexServletTest {
                 + "\\x22\\/config\\/server\\/info\\x22: \\x7b\\x22default_theme\\x22:"
                 + "\\x22my-default-theme\\x22\\x7d, \\x22\\/config\\/server\\/top-menus\\x22: "
                 + "\\x5b\\x5d\\x7d');");
+    String enabledDefaults =
+        IndexHtmlUtil.DEFAULT_EXPERIMENTS.stream()
+            .filter(e -> !e.equals(disabledDefault))
+            .collect(joining("\\x22,"));
     assertThat(output)
         .contains(
-            "window.ENABLED_EXPERIMENTS = JSON.parse('\\x5b\\x22NewFeature\\x22\\x5d');</script>");
+            "window.ENABLED_EXPERIMENTS = JSON.parse('\\x5b\\x22NewFeature\\x22,\\x22"
+                + enabledDefaults
+                + "\\x22\\x5d');</script>");
   }
 }