Add support for overriding replication configuration It is a next step in the direction of modifying the replicationn configuraiton from an external source. Where the external source could be a git repository or third party configuration managemnet system. This adds a `ReplicationConfigOverrides` interface and a default bindng for `DynamicItem<ReplicationConfigOverrides`. The added interface is just an extension of `ConfigResource` as Guice was complaining about duplicated binding for `ConfigResource` when it was used in `DynamicItem`. Additinally `RepilcationConfigOverrides` conveys better its purpose. For now, the default implementation provides no overrides. As the location of overrides files is still to be decided. Finally, the `MergedConfigResource` will take both, the base configuraiton for the file system, and configured overrides and merge both JGit `Config` objects togehter. With this approach, an implementation of `ReplicationConfigOverrides` can provide a single confiuration option or the whole configuration file. Later we may consider, discarding some of the overrides, like `gerrit.autoReload`, as disabling that option will effecively prevent users from updating configuation. Change-Id: I2e401c05571180719df33a4a094fbb4ccfb39f23

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/AutoReloadConfigDecorator.java b/src/main/java/com/googlesource/gerrit/plugins/replication/AutoReloadConfigDecorator.java
index b6532d9..9d0c978 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/AutoReloadConfigDecorator.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/AutoReloadConfigDecorator.java

@@ -42,7 +42,7 @@
   public AutoReloadConfigDecorator(
       @PluginName String pluginName,
       WorkQueue workQueue,
-      FileReplicationConfig replicationConfig,
+      ReplicationConfigImpl replicationConfig,
       AutoReloadRunnable reloadRunner,
       EventBus eventBus) {
     this.currentConfig = replicationConfig;

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/AutoReloadRunnable.java b/src/main/java/com/googlesource/gerrit/plugins/replication/AutoReloadRunnable.java
index 9ed43fe..ae07760 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/AutoReloadRunnable.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/AutoReloadRunnable.java

@@ -27,14 +27,14 @@
   private final Provider<ObservableQueue> queueObserverProvider;
   private final ConfigParser configParser;
   private ReplicationConfig loadedConfig;
-  private Provider<FileReplicationConfig> replicationConfigProvider;
+  private Provider<ReplicationConfigImpl> replicationConfigProvider;
   private String loadedConfigVersion;
   private String lastFailedConfigVersion;
 
   @Inject
   public AutoReloadRunnable(
       ConfigParser configParser,
-      Provider<FileReplicationConfig> replicationConfigProvider,
+      Provider<ReplicationConfigImpl> replicationConfigProvider,
       EventBus eventBus,
       Provider<ObservableQueue> queueObserverProvider) {
     this.replicationConfigProvider = replicationConfigProvider;

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java b/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java
index 9c70aa6..008e50b 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java

@@ -15,7 +15,7 @@
 package com.googlesource.gerrit.plugins.replication;
 
 import static com.google.gerrit.server.project.ProjectCache.noSuchProject;
-import static com.googlesource.gerrit.plugins.replication.FileReplicationConfig.replaceName;
+import static com.googlesource.gerrit.plugins.replication.ReplicationConfigImpl.replaceName;
 import static org.eclipse.jgit.transport.RemoteRefUpdate.Status.NON_EXISTING;
 import static org.eclipse.jgit.transport.RemoteRefUpdate.Status.REJECTED_OTHER_REASON;
 

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/DestinationsCollection.java b/src/main/java/com/googlesource/gerrit/plugins/replication/DestinationsCollection.java
index 1d471fe..08f3477 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/DestinationsCollection.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/DestinationsCollection.java

@@ -17,7 +17,7 @@
 import static com.googlesource.gerrit.plugins.replication.AdminApiFactory.isGerrit;
 import static com.googlesource.gerrit.plugins.replication.AdminApiFactory.isGerritHttp;
 import static com.googlesource.gerrit.plugins.replication.AdminApiFactory.isSSH;
-import static com.googlesource.gerrit.plugins.replication.FileReplicationConfig.replaceName;
+import static com.googlesource.gerrit.plugins.replication.ReplicationConfigImpl.replaceName;
 import static com.googlesource.gerrit.plugins.replication.ReplicationQueue.repLog;
 import static java.util.stream.Collectors.toList;
 

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/MergedConfigResource.java b/src/main/java/com/googlesource/gerrit/plugins/replication/MergedConfigResource.java
new file mode 100644
index 0000000..4cd0af0
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/MergedConfigResource.java

@@ -0,0 +1,75 @@
+// Copyright (C) 2024 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.googlesource.gerrit.plugins.replication;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.flogger.FluentLogger;
+import com.google.gerrit.common.Nullable;
+import com.google.gerrit.extensions.registration.DynamicItem;
+import com.google.inject.Inject;
+import com.google.inject.Provider;
+import com.google.inject.util.Providers;
+import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.lib.Config;
+
+public class MergedConfigResource {
+  @VisibleForTesting
+  static MergedConfigResource withBaseOnly(ConfigResource base) {
+    return new MergedConfigResource(Providers.of(base), null);
+  }
+
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
+  private final Provider<ConfigResource> base;
+  @Nullable private final DynamicItem<ReplicationConfigOverrides> overrides;
+
+  @Inject
+  MergedConfigResource(
+      Provider<ConfigResource> base, @Nullable DynamicItem<ReplicationConfigOverrides> overrides) {
+    this.base = base;
+    this.overrides = overrides;
+  }
+
+  public Config getConfig() {
+    Config config = base.get().getConfig();
+    if (noOverrides()) {
+      return config;
+    }
+
+    String overridesText = overrides.get().getConfig().toText();
+    if (!overridesText.isEmpty()) {
+      try {
+        config.fromText(overridesText);
+      } catch (ConfigInvalidException e) {
+        logger.atWarning().withCause(e).log("Failed to merge replication config overrides");
+      }
+    }
+
+    return config;
+  }
+
+  public String getVersion() {
+    String baseVersion = base.get().getVersion();
+    if (noOverrides()) {
+      return baseVersion;
+    }
+
+    return baseVersion + overrides.get().getVersion();
+  }
+
+  private boolean noOverrides() {
+    return overrides == null || overrides.get() == null;
+  }
+}

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/FileReplicationConfig.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigImpl.java
similarity index 89%
rename from src/main/java/com/googlesource/gerrit/plugins/replication/FileReplicationConfig.java
rename to src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigImpl.java
index b34ea54..59c6bb7 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/FileReplicationConfig.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigImpl.java

@@ -13,7 +13,6 @@
 // limitations under the License.
 package com.googlesource.gerrit.plugins.replication;
 
-import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Strings;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.annotations.PluginData;
@@ -22,7 +21,7 @@
 import java.nio.file.Path;
 import org.eclipse.jgit.lib.Config;
 
-public class FileReplicationConfig implements ReplicationConfig {
+public class ReplicationConfigImpl implements ReplicationConfig {
   private static final int DEFAULT_SSH_CONNECTION_TIMEOUT_MS = 2 * 60 * 1000; // 2 minutes
 
   private final SitePaths site;
@@ -32,19 +31,18 @@
   private final int maxRefsToShow;
   private int sshCommandTimeout;
   private int sshConnectionTimeout = DEFAULT_SSH_CONNECTION_TIMEOUT_MS;
-  private final ConfigResource configResource;
+  private final MergedConfigResource configResource;
   private final Path pluginDataDir;
 
   // TODO: remove in follow-up change, as this was added to reduce the diff size
   // for change Ic6a5c5b8ab5
-  @VisibleForTesting
-  public FileReplicationConfig(SitePaths paths, @PluginData Path pluginDataDir) {
-    this(new FileConfigResource(paths), paths, pluginDataDir);
+  public ReplicationConfigImpl(SitePaths paths, @PluginData Path pluginDataDir) {
+    this(MergedConfigResource.withBaseOnly(new FileConfigResource(paths)), paths, pluginDataDir);
   }
 
   @Inject
-  public FileReplicationConfig(
-      ConfigResource configResource, SitePaths site, @PluginData Path pluginDataDir) {
+  public ReplicationConfigImpl(
+      MergedConfigResource configResource, SitePaths site, @PluginData Path pluginDataDir) {
     this.site = site;
     this.configResource = configResource;
     Config config = configResource.getConfig();

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigModule.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigModule.java
index 220ec50..bcea543 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigModule.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigModule.java

@@ -18,6 +18,7 @@
 import static com.googlesource.gerrit.plugins.replication.FileConfigResource.CONFIG_NAME;
 
 import com.google.gerrit.extensions.events.LifecycleListener;
+import com.google.gerrit.extensions.registration.DynamicItem;
 import com.google.gerrit.server.config.SitePaths;
 import com.google.inject.AbstractModule;
 import com.google.inject.Inject;
@@ -46,6 +47,7 @@
   @Override
   protected void configure() {
     bind(ConfigResource.class).to(getConfigResourceClass());
+    DynamicItem.itemOf(binder(), ReplicationConfigOverrides.class);
 
     if (getReplicationConfig().getBoolean("gerrit", "autoReload", false)) {
       bind(ReplicationConfig.class).to(AutoReloadConfigDecorator.class).in(Scopes.SINGLETON);
@@ -53,7 +55,7 @@
           .annotatedWith(UniqueAnnotations.create())
           .to(AutoReloadConfigDecorator.class);
     } else {
-      bind(ReplicationConfig.class).to(FileReplicationConfig.class).in(Scopes.SINGLETON);
+      bind(ReplicationConfig.class).to(ReplicationConfigImpl.class).in(Scopes.SINGLETON);
     }
   }
 

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigOverrides.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigOverrides.java
new file mode 100644
index 0000000..97a77bf
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationConfigOverrides.java

@@ -0,0 +1,21 @@
+// Copyright (C) 2024 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.googlesource.gerrit.plugins.replication;
+
+/**
+ * Provide a way to override or extend replication configuration from other sources, like git
+ * repository or external configuration management tool.
+ */
+public interface ReplicationConfigOverrides extends ConfigResource {}

diff --git a/src/test/java/com/googlesource/gerrit/plugins/replication/AbstractConfigTest.java b/src/test/java/com/googlesource/gerrit/plugins/replication/AbstractConfigTest.java
index 4c43cb0..342d394 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/replication/AbstractConfigTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/replication/AbstractConfigTest.java

@@ -131,7 +131,8 @@
   protected DestinationsCollection newDestinationsCollections(ConfigResource configResource)
       throws ConfigInvalidException {
     return newDestinationsCollections(
-        new FileReplicationConfig(configResource, sitePaths, pluginDataPath));
+        new ReplicationConfigImpl(
+            MergedConfigResource.withBaseOnly(configResource), sitePaths, pluginDataPath));
   }
 
   protected DestinationsCollection newDestinationsCollections(ReplicationConfig replicationConfig)
@@ -144,7 +145,7 @@
         eventBus);
   }
 
-  protected FileReplicationConfig newReplicationFileBasedConfig() {
-    return new FileReplicationConfig(sitePaths, pluginDataPath);
+  protected ReplicationConfigImpl newReplicationFileBasedConfig() {
+    return new ReplicationConfigImpl(sitePaths, pluginDataPath);
   }
 }

diff --git a/src/test/java/com/googlesource/gerrit/plugins/replication/AutoReloadConfigDecoratorTest.java b/src/test/java/com/googlesource/gerrit/plugins/replication/AutoReloadConfigDecoratorTest.java
index bb08830..aa0d5f3 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/replication/AutoReloadConfigDecoratorTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/replication/AutoReloadConfigDecoratorTest.java

@@ -76,13 +76,18 @@
     remoteConfig.save();
 
     replicationConfig =
-        new FileReplicationConfig(new FanoutConfigResource(sitePaths), sitePaths, pluginDataPath);
+        new ReplicationConfigImpl(
+            MergedConfigResource.withBaseOnly(new FanoutConfigResource(sitePaths)),
+            sitePaths,
+            pluginDataPath);
 
     newAutoReloadConfig(
             () -> {
               try {
-                return new FileReplicationConfig(
-                    new FanoutConfigResource(sitePaths), sitePaths, pluginDataPath);
+                return new ReplicationConfigImpl(
+                    MergedConfigResource.withBaseOnly(new FanoutConfigResource(sitePaths)),
+                    sitePaths,
+                    pluginDataPath);
               } catch (IOException | ConfigInvalidException e) {
                 throw new RuntimeException(e);
               }
@@ -125,13 +130,18 @@
     remoteConfig.save();
 
     replicationConfig =
-        new FileReplicationConfig(new FanoutConfigResource(sitePaths), sitePaths, pluginDataPath);
+        new ReplicationConfigImpl(
+            MergedConfigResource.withBaseOnly(new FanoutConfigResource(sitePaths)),
+            sitePaths,
+            pluginDataPath);
 
     newAutoReloadConfig(
             () -> {
               try {
-                return new FileReplicationConfig(
-                    new FanoutConfigResource(sitePaths), sitePaths, pluginDataPath);
+                return new ReplicationConfigImpl(
+                    MergedConfigResource.withBaseOnly(new FanoutConfigResource(sitePaths)),
+                    sitePaths,
+                    pluginDataPath);
               } catch (IOException | ConfigInvalidException e) {
                 throw new RuntimeException(e);
               }
@@ -173,13 +183,18 @@
     remoteConfig.save();
 
     replicationConfig =
-        new FileReplicationConfig(new FanoutConfigResource(sitePaths), sitePaths, pluginDataPath);
+        new ReplicationConfigImpl(
+            MergedConfigResource.withBaseOnly(new FanoutConfigResource(sitePaths)),
+            sitePaths,
+            pluginDataPath);
 
     newAutoReloadConfig(
             () -> {
               try {
-                return new FileReplicationConfig(
-                    new FanoutConfigResource(sitePaths), sitePaths, pluginDataPath);
+                return new ReplicationConfigImpl(
+                    MergedConfigResource.withBaseOnly(new FanoutConfigResource(sitePaths)),
+                    sitePaths,
+                    pluginDataPath);
               } catch (IOException | ConfigInvalidException e) {
                 throw new RuntimeException(e);
               }
@@ -234,14 +249,14 @@
   }
 
   private AutoReloadConfigDecorator newAutoReloadConfig(
-      Supplier<FileReplicationConfig> configSupplier) {
+      Supplier<ReplicationConfigImpl> configSupplier) {
     AutoReloadRunnable autoReloadRunnable =
         new AutoReloadRunnable(
             configParser,
-            new Provider<FileReplicationConfig>() {
+            new Provider<ReplicationConfigImpl>() {
 
               @Override
-              public FileReplicationConfig get() {
+              public ReplicationConfigImpl get() {
                 return configSupplier.get();
               }
             },

diff --git a/src/test/java/com/googlesource/gerrit/plugins/replication/AutoReloadRunnableTest.java b/src/test/java/com/googlesource/gerrit/plugins/replication/AutoReloadRunnableTest.java
index 14307f8..8a1b85d 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/replication/AutoReloadRunnableTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/replication/AutoReloadRunnableTest.java

@@ -81,11 +81,11 @@
     autoReloadRunnable.run();
   }
 
-  private Provider<FileReplicationConfig> newVersionConfigProvider() {
+  private Provider<ReplicationConfigImpl> newVersionConfigProvider() {
     return new Provider<>() {
       @Override
-      public FileReplicationConfig get() {
-        return new FileReplicationConfig(sitePaths, sitePaths.data_dir) {
+      public ReplicationConfigImpl get() {
+        return new ReplicationConfigImpl(sitePaths, sitePaths.data_dir) {
           @Override
           public String getVersion() {
             return String.format("%s", System.nanoTime());

diff --git a/src/test/java/com/googlesource/gerrit/plugins/replication/FanoutConfigResourceTest.java b/src/test/java/com/googlesource/gerrit/plugins/replication/FanoutConfigResourceTest.java
index 34712a7..9a5ece9 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/replication/FanoutConfigResourceTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/replication/FanoutConfigResourceTest.java

@@ -266,7 +266,7 @@
     FanoutConfigResource objectUnderTest = new FanoutConfigResource(sitePaths);
 
     String replicationConfigVersion =
-        new FileReplicationConfig(sitePaths, pluginDataPath).getVersion();
+        new ReplicationConfigImpl(sitePaths, pluginDataPath).getVersion();
 
     MoreFiles.deleteRecursively(sitePaths.etc_dir.resolve("replication"), ALLOW_INSECURE);
 

diff --git a/src/test/java/com/googlesource/gerrit/plugins/replication/MergedConfigResourceTest.java b/src/test/java/com/googlesource/gerrit/plugins/replication/MergedConfigResourceTest.java
new file mode 100644
index 0000000..80ad449
--- /dev/null
+++ b/src/test/java/com/googlesource/gerrit/plugins/replication/MergedConfigResourceTest.java

@@ -0,0 +1,112 @@
+// Copyright (C) 2024 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.googlesource.gerrit.plugins.replication;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.gerrit.extensions.registration.DynamicItem;
+import com.google.inject.AbstractModule;
+import com.google.inject.Guice;
+import org.eclipse.jgit.lib.Config;
+import org.junit.Test;
+
+public class MergedConfigResourceTest {
+  private static final int BASE_CONFIG_MAX_RETIRES = 10;
+  private static final int OVERRIDDEN_CONFIG_MAX_RETIRES = 5;
+
+  @Test
+  public void onlyUseBaseConfig() {
+    final MergedConfigResource configResource = newMergedConfigResource();
+
+    assertThat(configResource.getVersion()).isEqualTo("base");
+    assertThat(getMaxRetires(configResource)).isEqualTo(BASE_CONFIG_MAX_RETIRES);
+  }
+
+  @Test
+  public void overrideBaseConfig() {
+    final MergedConfigResource configResource =
+        newMergedConfigResource(TestReplicationConfigOverrides.class);
+
+    assertThat(configResource.getVersion()).isEqualTo("baseoverride");
+    assertThat(getMaxRetires(configResource)).isEqualTo(OVERRIDDEN_CONFIG_MAX_RETIRES);
+    assertThat(getUseGcClient(configResource)).isTrue();
+  }
+
+  private MergedConfigResource newMergedConfigResource() {
+    return newMergedConfigResource(null);
+  }
+
+  private MergedConfigResource newMergedConfigResource(
+      Class<? extends ReplicationConfigOverrides> overrides) {
+    return Guice.createInjector(
+            new AbstractModule() {
+              @Override
+              protected void configure() {
+                bind(ConfigResource.class).to(TestBaseConfigResource.class);
+
+                DynamicItem.itemOf(binder(), ReplicationConfigOverrides.class);
+                if (overrides != null) {
+                  DynamicItem.bind(binder(), ReplicationConfigOverrides.class).to(overrides);
+                }
+              }
+            })
+        .getInstance(MergedConfigResource.class);
+  }
+
+  private static class TestBaseConfigResource implements ConfigResource {
+    @Override
+    public Config getConfig() {
+      Config config = new Config();
+      setMaxRetires(config, BASE_CONFIG_MAX_RETIRES);
+      return config;
+    }
+
+    @Override
+    public String getVersion() {
+      return "base";
+    }
+  }
+
+  private static class TestReplicationConfigOverrides implements ReplicationConfigOverrides {
+    @Override
+    public Config getConfig() {
+      Config config = new Config();
+      setMaxRetires(config, OVERRIDDEN_CONFIG_MAX_RETIRES);
+      setUseGcClient(config, true);
+      return config;
+    }
+
+    @Override
+    public String getVersion() {
+      return "override";
+    }
+  }
+
+  private static void setMaxRetires(Config config, int value) {
+    config.setInt("replication", null, "maxRetries", value);
+  }
+
+  private static void setUseGcClient(Config config, boolean value) {
+    config.setBoolean("replication", null, "useGcClient", value);
+  }
+
+  private static int getMaxRetires(MergedConfigResource resource) {
+    return resource.getConfig().getInt("replication", null, "maxRetries", -1);
+  }
+
+  private static boolean getUseGcClient(MergedConfigResource resource) {
+    return resource.getConfig().getBoolean("replication", null, "useGcClient", false);
+  }
+}