diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 94c4552e6a..1f4fd9ceda 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -2242,6 +2242,25 @@ Setting this option to true will prevent this behavior.
 +
 By default false.
 
+[[gerrit.xframeOption]]gerrit.xframeOption::
++
+Add link:https://tools.ietf.org/html/rfc7034[`X-Frame-Options`] header to all HTTP
+responses. The `X-Frame-Options` HTTP response header can be used to indicate
+whether or not a browser should be allowed to render a page in a
+`<frame>`, `<iframe>`, `<embed>` or `<object>`.
++
+Available values:
++
+1. ALLOW - The page can be displayed in a frame.
+2. SAMEORIGIN - The page can only be displayed in a frame on the same origin as the page itself.
++
+If link:#gerrit.canLoadInIFrame is set to false this option is ignored and the
+`X-Frame-Options` header is always set to `DENY`.
+Setting this option to `ALLOW` will cause the `X-Frame-Options` header to be omitted
+the the page can be displayed in a frame.
++
+By default SAMEORIGIN.
+
 [[gerrit.cdnPath]]gerrit.cdnPath::
 +
 Path prefix for PolyGerrit's static resources if using a CDN.
@@ -2976,7 +2995,7 @@ by the JVM. If set to a negative value, defaults to a direct executor.
 [[index.batchThreads]]index.batchThreads::
 +
 Number of threads to use for indexing in background operations, such as
-online schema upgrades.
+online schema upgrades, and also for offline reindexing.
 +
 If not set or set to a zero, defaults to the number of logical CPUs as returned
 by the JVM. If set to a negative value, defaults to a direct executor.
diff --git a/Documentation/note-db.txt b/Documentation/note-db.txt
index 0505dd209d..89758a092c 100644
--- a/Documentation/note-db.txt
+++ b/Documentation/note-db.txt
@@ -110,6 +110,13 @@ Migration requires a heap size comparable to running a Gerrit server. If you
 normally run `gerrit.war daemon` with an `-Xmx` flag, pass that to the migration
 tool as well.
 
+[NOTE]
+Note that by appending `--reindex false` to the above command, you can skip the
+lengthy, implicit reindexing step of the migration. This is useful if you plan
+to perform further Gerrit upgrades while the server is offline and have to
+reindex later anyway (E.g.: a follow-up upgrade to Gerrit 3.2 or newer, which
+requires to reindex changes anyway).
+
 *Advantages*
 
 * Much faster than online; can use all available CPUs, since no live traffic
diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 4006d1dd70..ad543b9e5d 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -6191,7 +6191,7 @@ Number of the parent relative to which the cherry-pick should be considered.
 Notify handling that defines to whom email notifications should be sent
 after the cherry-pick. +
 Allowed values are `NONE`, `OWNER`, `OWNER_REVIEWERS` and `ALL`. +
-If not set, the default is `NONE`.
+If not set, the default is `ALL`.
 |`notify_details`   |optional|
 Additional information about whom to notify about the update as a map
 of recipient type to link:#notify-info[NotifyInfo] entity.
diff --git a/java/com/google/gerrit/extensions/api/changes/CherryPickInput.java b/java/com/google/gerrit/extensions/api/changes/CherryPickInput.java
index 69c1790979..fb03bc5cc3 100644
--- a/java/com/google/gerrit/extensions/api/changes/CherryPickInput.java
+++ b/java/com/google/gerrit/extensions/api/changes/CherryPickInput.java
@@ -24,7 +24,7 @@ public class CherryPickInput {
   public String base;
   public Integer parent;
 
-  public NotifyHandling notify = NotifyHandling.NONE;
+  public NotifyHandling notify = NotifyHandling.ALL;
   public Map<RecipientType, NotifyInfo> notifyDetails;
 
   public boolean keepReviewers;
diff --git a/java/com/google/gerrit/httpd/AllRequestFilter.java b/java/com/google/gerrit/httpd/AllRequestFilter.java
index 9d171d5a54..1c3cafea04 100644
--- a/java/com/google/gerrit/httpd/AllRequestFilter.java
+++ b/java/com/google/gerrit/httpd/AllRequestFilter.java
@@ -18,6 +18,8 @@ import com.google.gerrit.extensions.registration.DynamicSet;
 import com.google.gerrit.server.plugins.Plugin;
 import com.google.gerrit.server.plugins.StopPluginListener;
 import com.google.inject.Inject;
+import com.google.inject.Module;
+import com.google.inject.Scopes;
 import com.google.inject.Singleton;
 import com.google.inject.internal.UniqueAnnotations;
 import com.google.inject.servlet.ServletModule;
@@ -32,11 +34,15 @@ import javax.servlet.ServletResponse;
 
 /** Filters all HTTP requests passing through the server. */
 public abstract class AllRequestFilter implements Filter {
-  public static ServletModule module() {
+  public static Module module() {
     return new ServletModule() {
       @Override
       protected void configureServlets() {
         DynamicSet.setOf(binder(), AllRequestFilter.class);
+        DynamicSet.bind(binder(), AllRequestFilter.class)
+            .to(AllowRenderInFrameFilter.class)
+            .in(Scopes.SINGLETON);
+
         filter("/*").through(FilterProxy.class);
 
         bind(StopPluginListener.class)
diff --git a/java/com/google/gerrit/httpd/AllowRenderInFrameFilter.java b/java/com/google/gerrit/httpd/AllowRenderInFrameFilter.java
new file mode 100644
index 0000000000..b414aad7c1
--- /dev/null
+++ b/java/com/google/gerrit/httpd/AllowRenderInFrameFilter.java
@@ -0,0 +1,59 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.httpd;
+
+import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.inject.Inject;
+import java.io.IOException;
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import javax.servlet.http.HttpServletResponse;
+import org.eclipse.jgit.lib.Config;
+
+public class AllowRenderInFrameFilter extends AllRequestFilter {
+  static final String X_FRAME_OPTIONS_HEADER_NAME = "X-Frame-Options";
+
+  public static enum XFrameOption {
+    ALLOW,
+    SAMEORIGIN;
+  }
+
+  private final String xframeOptionString;
+  private final boolean skipXFrameOption;
+
+  @Inject
+  public AllowRenderInFrameFilter(@GerritServerConfig Config cfg) {
+    XFrameOption xframeOption =
+        cfg.getEnum("gerrit", null, "xframeOption", XFrameOption.SAMEORIGIN);
+    boolean canLoadInIFrame = cfg.getBoolean("gerrit", "canLoadInIFrame", false);
+    xframeOptionString = canLoadInIFrame ? xframeOption.name() : "DENY";
+
+    skipXFrameOption = xframeOption.equals(XFrameOption.ALLOW) && canLoadInIFrame;
+  }
+
+  @Override
+  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
+      throws IOException, ServletException {
+    if (skipXFrameOption) {
+      chain.doFilter(request, response);
+    } else {
+      HttpServletResponse httpResponse = (HttpServletResponse) response;
+      httpResponse.addHeader(X_FRAME_OPTIONS_HEADER_NAME, xframeOptionString);
+      chain.doFilter(request, httpResponse);
+    }
+  }
+}
diff --git a/java/com/google/gerrit/httpd/raw/IndexHtmlUtil.java b/java/com/google/gerrit/httpd/raw/IndexHtmlUtil.java
index 41d2f83975..cddaea4aaf 100644
--- a/java/com/google/gerrit/httpd/raw/IndexHtmlUtil.java
+++ b/java/com/google/gerrit/httpd/raw/IndexHtmlUtil.java
@@ -153,8 +153,7 @@ public class IndexHtmlUtil {
           serializeObject(GSON, accountApi.getEditPreferences()));
       data.put("userIsAuthenticated", true);
     } catch (AuthException e) {
-      logger.atFine().withCause(e).log(
-          "Can't inline account-related data because user is unauthenticated");
+      logger.atFine().log("Can't inline account-related data because user is unauthenticated");
       // Don't render data
     }
 
diff --git a/java/com/google/gerrit/index/Schema.java b/java/com/google/gerrit/index/Schema.java
index 0aa374b0e0..3aa9de0614 100644
--- a/java/com/google/gerrit/index/Schema.java
+++ b/java/com/google/gerrit/index/Schema.java
@@ -22,6 +22,7 @@ import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.flogger.FluentLogger;
+import com.google.gerrit.exceptions.StorageException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -176,6 +177,33 @@ public class Schema<T> {
     return true;
   }
 
+  private Values<T> fieldValues(T obj, FieldDef<T, ?> f, ImmutableSet<String> skipFields) {
+    if (skipFields.contains(f.getName())) {
+      return null;
+    }
+
+    Object v;
+    try {
+      v = f.get(obj);
+    } catch (StorageException e) {
+      // StorageException is thrown when the object is not found. On this case,
+      // it is pointless to make further attempts for each field, so propagate
+      // the exception to return an empty list.
+      logger.atSevere().withCause(e).log("error getting field %s of %s", f.getName(), obj);
+      throw e;
+    } catch (RuntimeException e) {
+      logger.atSevere().withCause(e).log("error getting field %s of %s", f.getName(), obj);
+      return null;
+    }
+    if (v == null) {
+      return null;
+    } else if (f.isRepeatable()) {
+      return new Values<>(f, (Iterable<?>) v);
+    } else {
+      return new Values<>(f, Collections.singleton(v));
+    }
+  }
+
   /**
    * Build all fields in the schema from an input object.
    *
@@ -186,31 +214,14 @@ public class Schema<T> {
    * @return all non-null field values from the object.
    */
   public final Iterable<Values<T>> buildFields(T obj, ImmutableSet<String> skipFields) {
-    return fields.values().stream()
-        .map(
-            f -> {
-              if (skipFields.contains(f.getName())) {
-                return null;
-              }
-
-              Object v;
-              try {
-                v = f.get(obj);
-              } catch (RuntimeException e) {
-                logger.atSevere().withCause(e).log(
-                    "error getting field %s of %s", f.getName(), obj);
-                return null;
-              }
-              if (v == null) {
-                return null;
-              } else if (f.isRepeatable()) {
-                return new Values<>(f, (Iterable<?>) v);
-              } else {
-                return new Values<>(f, Collections.singleton(v));
-              }
-            })
-        .filter(Objects::nonNull)
-        .collect(toImmutableList());
+    try {
+      return fields.values().stream()
+          .map(f -> fieldValues(obj, f, skipFields))
+          .filter(Objects::nonNull)
+          .collect(toImmutableList());
+    } catch (StorageException e) {
+      return ImmutableList.of();
+    }
   }
 
   @Override
diff --git a/java/com/google/gerrit/index/SiteIndexer.java b/java/com/google/gerrit/index/SiteIndexer.java
index ecfc7bdf9f..32b4b21b99 100644
--- a/java/com/google/gerrit/index/SiteIndexer.java
+++ b/java/com/google/gerrit/index/SiteIndexer.java
@@ -83,7 +83,7 @@ public abstract class SiteIndexer<K, V, I extends Index<K, V>> {
   }
 
   protected PrintWriter newPrintWriter(OutputStream out) {
-    return new PrintWriter(new OutputStreamWriter(out, UTF_8));
+    return new PrintWriter(new OutputStreamWriter(out, UTF_8), true);
   }
 
   private static class ErrorListener implements Runnable {
diff --git a/java/com/google/gerrit/pgm/Reindex.java b/java/com/google/gerrit/pgm/Reindex.java
index 2e526bb454..966801f382 100644
--- a/java/com/google/gerrit/pgm/Reindex.java
+++ b/java/com/google/gerrit/pgm/Reindex.java
@@ -202,6 +202,9 @@ public class Reindex extends SiteProgram {
     if (result.success()) {
       index.markReady(true);
     }
+    System.out.format(
+        "Index %s in version %d is %sready\n",
+        def.getName(), index.getSchema().getVersion(), result.success() ? "" : "NOT ");
     return result.success();
   }
 }
diff --git a/java/com/google/gerrit/server/index/change/AllChangesIndexer.java b/java/com/google/gerrit/server/index/change/AllChangesIndexer.java
index 2a8a5baff2..3c40c4fd45 100644
--- a/java/com/google/gerrit/server/index/change/AllChangesIndexer.java
+++ b/java/com/google/gerrit/server/index/change/AllChangesIndexer.java
@@ -244,7 +244,8 @@ public class AllChangesIndexer extends SiteIndexer<Change.Id, ChangeData, Change
       try {
         indexer.index(changeDataFactory.create(r.notes()));
         done.update(1);
-        verboseWriter.println("Reindexed change " + r.id());
+        verboseWriter.format(
+            "Reindexed change %d (project: %s)\n", r.id().get(), r.notes().getProjectName().get());
       } catch (RejectedExecutionException e) {
         // Server shutdown, don't spam the logs.
         failSilently();
diff --git a/java/com/google/gerrit/server/project/ProjectJson.java b/java/com/google/gerrit/server/project/ProjectJson.java
index f2254d6bd7..f00df537f6 100644
--- a/java/com/google/gerrit/server/project/ProjectJson.java
+++ b/java/com/google/gerrit/server/project/ProjectJson.java
@@ -18,6 +18,7 @@ import static java.util.stream.Collectors.toMap;
 
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
+import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.common.data.LabelType;
 import com.google.gerrit.common.data.LabelValue;
 import com.google.gerrit.entities.Project;
@@ -34,6 +35,7 @@ import java.util.HashMap;
 /** Collection of routines to populate {@link ProjectInfo}. */
 @Singleton
 public class ProjectJson {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
 
   private final AllProjectsName allProjects;
   private final WebLinks webLinks;
@@ -50,7 +52,17 @@ public class ProjectJson {
     for (LabelType t : projectState.getLabelTypes().getLabelTypes()) {
       LabelTypeInfo labelInfo = new LabelTypeInfo();
       labelInfo.values =
-          t.getValues().stream().collect(toMap(LabelValue::formatValue, LabelValue::getText));
+          t.getValues().stream()
+              .collect(
+                  toMap(
+                      LabelValue::formatValue,
+                      LabelValue::getText,
+                      (v1, v2) -> {
+                        logger.atSevere().log(
+                            "Duplicate values for project: %s, label: %s found: '%s':'%s'",
+                            projectState.getName(), t.getName(), v1, v2);
+                        return v1;
+                      }));
       labelInfo.defaultValue = t.getDefaultValue();
       info.labels.put(t.getName(), labelInfo);
     }
diff --git a/javatests/com/google/gerrit/httpd/AllowRenderInFrameFilterTest.java b/javatests/com/google/gerrit/httpd/AllowRenderInFrameFilterTest.java
new file mode 100644
index 0000000000..26798290c0
--- /dev/null
+++ b/javatests/com/google/gerrit/httpd/AllowRenderInFrameFilterTest.java
@@ -0,0 +1,136 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.httpd;
+
+import static com.google.common.truth.Truth.assertThat;
+import static com.google.gerrit.httpd.AllowRenderInFrameFilter.X_FRAME_OPTIONS_HEADER_NAME;
+import static com.google.gerrit.testing.GerritJUnit.assertThrows;
+import static org.mockito.Mockito.anyString;
+import static org.mockito.Mockito.eq;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+import com.google.gerrit.httpd.AllowRenderInFrameFilter.XFrameOption;
+import java.io.IOException;
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import org.eclipse.jgit.lib.Config;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+
+@RunWith(MockitoJUnitRunner.class)
+public class AllowRenderInFrameFilterTest {
+
+  private Config cfg = new Config();
+  @Mock ServletRequest request;
+  @Mock HttpServletResponse response;
+  @Mock FilterChain filterChain;
+
+  @Test
+  public void shouldDenyInFrameRenderingWhenCanRenderInFrameIsFalse()
+      throws IOException, ServletException {
+    cfg.setBoolean("gerrit", null, "canLoadInIFrame", false);
+
+    AllowRenderInFrameFilter objectUnderTest = new AllowRenderInFrameFilter(cfg);
+    objectUnderTest.doFilter(request, response, filterChain);
+
+    verify(response, times(1)).addHeader(X_FRAME_OPTIONS_HEADER_NAME, "DENY");
+  }
+
+  @Test
+  public void shouldDenyInFrameRenderingWhenCanRenderInFrameIsFalseAndXFormOptionIsSAMEORIGIN()
+      throws IOException, ServletException {
+    cfg.setBoolean("gerrit", null, "canLoadInIFrame", false);
+    cfg.setEnum("gerrit", null, "xframeOption", XFrameOption.SAMEORIGIN);
+
+    AllowRenderInFrameFilter objectUnderTest = new AllowRenderInFrameFilter(cfg);
+    objectUnderTest.doFilter(request, response, filterChain);
+
+    verify(response, times(1)).addHeader(X_FRAME_OPTIONS_HEADER_NAME, "DENY");
+  }
+
+  @Test
+  public void shouldDenyInFrameRenderingWhenCanRenderInFrameIsFalseAndXFormOptionIsALLOW()
+      throws IOException, ServletException {
+    cfg.setBoolean("gerrit", null, "canLoadInIFrame", false);
+    cfg.setEnum("gerrit", null, "xframeOption", XFrameOption.ALLOW);
+
+    AllowRenderInFrameFilter objectUnderTest = new AllowRenderInFrameFilter(cfg);
+    objectUnderTest.doFilter(request, response, filterChain);
+
+    verify(response, times(1)).addHeader(X_FRAME_OPTIONS_HEADER_NAME, "DENY");
+  }
+
+  @Test
+  public void shouldRestrictAccessToSAMEORIGINWhenCanRenderInFrameIsTrue()
+      throws IOException, ServletException {
+    cfg.setBoolean("gerrit", null, "canLoadInIFrame", true);
+
+    AllowRenderInFrameFilter objectUnderTest = new AllowRenderInFrameFilter(cfg);
+    objectUnderTest.doFilter(request, response, filterChain);
+
+    verify(response, times(1)).addHeader(X_FRAME_OPTIONS_HEADER_NAME, "SAMEORIGIN");
+  }
+
+  @Test
+  public void shouldSkipHeaderWhenCanRenderInFrameIsTrueAndXFormOptionIsALLOW()
+      throws IOException, ServletException {
+    cfg.setBoolean("gerrit", null, "canLoadInIFrame", true);
+    cfg.setEnum("gerrit", null, "xframeOption", XFrameOption.ALLOW);
+
+    AllowRenderInFrameFilter objectUnderTest = new AllowRenderInFrameFilter(cfg);
+    objectUnderTest.doFilter(request, response, filterChain);
+
+    verify(response, never()).addHeader(eq(X_FRAME_OPTIONS_HEADER_NAME), anyString());
+  }
+
+  @Test
+  public void shouldRestrictAccessToSAMEORIGINWhenCanRenderInFrameIsTrueAndXFormOptionIsSAMEORIGIN()
+      throws IOException, ServletException {
+    cfg.setBoolean("gerrit", null, "canLoadInIFrame", true);
+    cfg.setEnum("gerrit", null, "xframeOption", XFrameOption.SAMEORIGIN);
+
+    AllowRenderInFrameFilter objectUnderTest = new AllowRenderInFrameFilter(cfg);
+    objectUnderTest.doFilter(request, response, filterChain);
+
+    verify(response, times(1)).addHeader(X_FRAME_OPTIONS_HEADER_NAME, "SAMEORIGIN");
+  }
+
+  @Test
+  public void shouldIgnoreXFrameOriginCaseSensitivity() throws IOException, ServletException {
+    cfg.setBoolean("gerrit", null, "canLoadInIFrame", true);
+    cfg.setString("gerrit", null, "xframeOption", "sameOrigin");
+
+    AllowRenderInFrameFilter objectUnderTest = new AllowRenderInFrameFilter(cfg);
+    objectUnderTest.doFilter(request, response, filterChain);
+
+    verify(response, times(1)).addHeader(X_FRAME_OPTIONS_HEADER_NAME, "SAMEORIGIN");
+  }
+
+  @Test
+  public void shouldThrowExceptionWhenUnknownXFormOptionValue() {
+    cfg.setBoolean("gerrit", null, "canLoadInIFrame", true);
+    cfg.setString("gerrit", null, "xframeOption", "unsupported value");
+
+    IllegalArgumentException e =
+        assertThrows(IllegalArgumentException.class, () -> new AllowRenderInFrameFilter(cfg));
+    assertThat(e).hasMessageThat().contains("gerrit.xframeOption=unsupported value");
+  }
+}
diff --git a/resources/com/google/gerrit/httpd/raw/HostPage.html b/resources/com/google/gerrit/httpd/raw/HostPage.html
deleted file mode 100644
index c0d84468d9..0000000000
--- a/resources/com/google/gerrit/httpd/raw/HostPage.html
+++ /dev/null
@@ -1,26 +0,0 @@
-<html>
-  <head>
-    <title>Gerrit Code Review</title>
-    <meta name="gwt:property" content="locale=en_US" />
-    <script id="gerrit_hostpagedata"></script>
-    <style id="gerrit_sitecss" type="text/css"></style>
-    <link rel="shortcut icon" type="image/x-icon" href="favicon.ico" />
-  </head>
-  <body>
-    <div id="gerrit_topmenu"></div>
-    <div id="gerrit_header"></div>
-    <div id="gerrit_startinggerrit" style="margin-left: 10px;">
-      <p>Loading <a href="https://www.gerritcodereview.com/" target="_blank">Gerrit Code Review</a> ...</p>
-      <noscript>
-        <p>Gerrit requires a JavaScript enabled browser.</p>
-      </noscript>
-    </div>
-    <div id="gerrit_body"></div>
-    <div style="clear: both">
-      <div id="gerrit_footer"></div>
-      <div id="gerrit_btmmenu"></div>
-    </div>
-    <iframe src="javascript:''" id="__gwt_historyFrame" tabIndex='-1' style="position:absolute;width:0;height:0;border:0"></iframe>
-    <script id="gerrit_module"></script>
-  </body>
-</html>