Allow to configure the number of hook execution workers

Allow to configure the number of workers with hooks.executorThreads.

Default to 1 if not set, keeping the same behavior as before.

Bug: Issue 10823
Change-Id: I688b88651822e7eee4a81414ffb594a915b382fd

src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java
index 1b2a307..86c457d 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java
@@ -15,11 +15,13 @@
 package com.googlesource.gerrit.plugins.hooks;
 
 import com.google.gerrit.extensions.events.LifecycleListener;
+import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.git.WorkQueue;
 import com.google.inject.Inject;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.concurrent.ScheduledExecutorService;
+import org.eclipse.jgit.lib.Config;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -27,11 +29,13 @@
   private static final Logger log = LoggerFactory.getLogger(HookQueue.class);
 
   private final WorkQueue workQueue;
+  private final int executorThreads;
 
   private ScheduledExecutorService queue;
 
   @Inject
-  HookQueue(WorkQueue workQueue) {
+  HookQueue(WorkQueue workQueue, @GerritServerConfig Config config) {
+    this.executorThreads = config.getInt("hooks", "executorThreads", 1);
     this.workQueue = workQueue;
   }
 
@@ -49,7 +53,7 @@
 
   @Override
   public void start() {
-    queue = workQueue.createQueue(1, "HookQueue");
+    queue = workQueue.createQueue(executorThreads, "HookQueue");
   }
 
   @Override

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 9303587..2aea566 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -37,6 +37,9 @@
 :	Timeout value in seconds for synchronous hooks. If not set, defaults
 to 30 seconds.
 
+hooks.executorThreads
+:	Number of workers to use for hook execution. If not set, defaults to 1.
+
 hooks.changeAbandonedHook
 :	Filename for the change abandoned hook. If not set, defaults to `change-abandoned`.