Make diff preferences defaults configurable

Read diff preferences from All-Users refs/users/default and update
the default values with those preferences.

Change-Id: I813ad9976f7e73190fb68f373e77f596dcd6537c

gerrit-server/src/main/java/com/google/gerrit/server/account/GetDiffPreferences.java

@@ -15,6 +15,7 @@
 package com.google.gerrit.server.account;
 
 import static com.google.gerrit.server.config.ConfigUtil.loadSection;
+import static com.google.gerrit.server.config.ConfigUtil.skipField;
 
 import com.google.gerrit.extensions.client.DiffPreferencesInfo;
 import com.google.gerrit.extensions.restapi.AuthException;
@@ -31,11 +32,17 @@ import com.google.inject.Singleton;
 import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.errors.RepositoryNotFoundException;
 import org.eclipse.jgit.lib.Repository;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
+import java.lang.reflect.Field;
 
 @Singleton
 public class GetDiffPreferences implements RestReadView<AccountResource> {
+  private static final Logger log =
+      LoggerFactory.getLogger(GetDiffPreferences.class);
+
   private final Provider<CurrentUser> self;
   private final Provider<AllUsersName> allUsersName;
   private final GitRepositoryManager gitMgr;
@@ -66,13 +73,41 @@ public class GetDiffPreferences implements RestReadView<AccountResource> {
       DiffPreferencesInfo in)
       throws IOException, ConfigInvalidException, RepositoryNotFoundException {
     try (Repository git = gitMgr.openRepository(allUsersName)) {
+      // Load all users prefs.
+      VersionedAccountPreferences dp =
+          VersionedAccountPreferences.forDefault();
+      dp.load(git);
+      DiffPreferencesInfo allUserPrefs = new DiffPreferencesInfo();
+      loadSection(dp.getConfig(), UserConfigSections.DIFF, null, allUserPrefs,
+          DiffPreferencesInfo.defaults(), in);
+
+      // Load user prefs
       VersionedAccountPreferences p =
           VersionedAccountPreferences.forUser(id);
       p.load(git);
       DiffPreferencesInfo prefs = new DiffPreferencesInfo();
       loadSection(p.getConfig(), UserConfigSections.DIFF, null, prefs,
-          DiffPreferencesInfo.defaults(), in);
+          updateDefaults(allUserPrefs), in);
       return prefs;
     }
   }
+
+  private static DiffPreferencesInfo updateDefaults(DiffPreferencesInfo input) {
+    DiffPreferencesInfo result = DiffPreferencesInfo.defaults();
+    try {
+      for (Field field : input.getClass().getDeclaredFields()) {
+        if (skipField(field)) {
+          continue;
+        }
+        Object newVal = field.get(input);
+        if (newVal != null) {
+          field.set(result, newVal);
+        }
+      }
+    } catch (IllegalAccessException e) {
+      log.warn("Cannot get default diff preferences from All-Users", e);
+      return DiffPreferencesInfo.defaults();
+    }
+    return result;
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/server/config/ConfigUtil.java

@@ -382,16 +382,16 @@ public class ConfigUtil {
     return s;
   }
 
+  public static boolean skipField(Field field) {
+    int modifiers = field.getModifiers();
+    return Modifier.isFinal(modifiers) || Modifier.isTransient(modifiers);
+  }
+
   private static boolean isCollectionOrMap(Class<?> t) {
     return Collection.class.isAssignableFrom(t)
         || Map.class.isAssignableFrom(t);
   }
 
-  private static boolean skipField(Field field) {
-    int modifiers = field.getModifiers();
-    return Modifier.isFinal(modifiers) || Modifier.isTransient(modifiers);
-  }
-
   private static boolean isString(Class<?> t) {
     return String.class == t;
   }