Decouple plugins from their "jar" external form

Until now all the server-side plugins have been associated
to a single jar file in the /plugin directory.

As first step to allow different forms of plugins
(e.g. script files, directories or anything else that
can provide classes and resources) we need to de-couple
the underlying Jar file from the server side plugin.

We introduce the concept of "plugin-scanner" as the interface
to scan the external form to discover:
- plugin classes
- plugin resources
- plugin meta-data (i.e. Manifest)

Change-Id: I769595a030545a5f272f453c3cf435b74719e1e7

gerrit-server/src/main/java/com/google/gerrit/server/plugins/ServerPlugin.java

@@ -35,12 +35,13 @@ import com.google.inject.ProvisionException;
 import org.eclipse.jgit.internal.storage.file.FileSnapshot;
 
 import java.io.File;
+import java.io.IOException;
 import java.util.List;
 import java.util.jar.Attributes;
 import java.util.jar.JarFile;
 import java.util.jar.Manifest;
 
-class ServerPlugin extends Plugin {
+public class ServerPlugin extends Plugin {
 
   /** Unique key that changes whenever a plugin reloads. */
   public static final class CacheKey {
@@ -59,6 +60,7 @@ class ServerPlugin extends Plugin {
 
   private final JarFile jarFile;
   private final Manifest manifest;
+  private final PluginContentScanner scanner;
   private final File dataDir;
   private final String pluginCanonicalWebUrl;
   private final ClassLoader classLoader;
@@ -78,28 +80,39 @@ class ServerPlugin extends Plugin {
       File srcJar,
       FileSnapshot snapshot,
       JarFile jarFile,
-      Manifest manifest,
+      PluginContentScanner scanner,
       File dataDir,
       ApiType apiType,
       ClassLoader classLoader,
       @Nullable Class<? extends Module> sysModule,
       @Nullable Class<? extends Module> sshModule,
-      @Nullable Class<? extends Module> httpModule) {
+      @Nullable Class<? extends Module> httpModule)
+      throws InvalidPluginException {
     super(name, srcJar, pluginUser, snapshot, apiType);
     this.pluginCanonicalWebUrl = pluginCanonicalWebUrl;
     this.jarFile = jarFile;
-    this.manifest = manifest;
+    this.scanner = scanner;
     this.dataDir = dataDir;
     this.classLoader = classLoader;
     this.sysModule = sysModule;
     this.sshModule = sshModule;
     this.httpModule = httpModule;
+    this.manifest = getPluginManifest(scanner);
   }
 
   File getSrcJar() {
     return getSrcFile();
   }
 
+  private Manifest getPluginManifest(PluginContentScanner scanner)
+      throws InvalidPluginException {
+    try {
+       return scanner.getManifest();
+    } catch (IOException e) {
+      throw new InvalidPluginException("Cannot get plugin manifest", e);
+    }
+  }
+
   @Nullable
   public String getVersion() {
     Attributes main = manifest.getMainAttributes();
@@ -136,7 +149,7 @@ class ServerPlugin extends Plugin {
 
     AutoRegisterModules auto = null;
     if (sysModule == null && sshModule == null && httpModule == null) {
-      auto = new AutoRegisterModules(getName(), env, jarFile, classLoader);
+      auto = new AutoRegisterModules(getName(), env, scanner, classLoader);
       auto.discover();
     }
 
@@ -270,4 +283,9 @@ class ServerPlugin extends Plugin {
       manager.add(handle);
     }
   }
+
+  @Override
+  public PluginContentScanner getContentScanner() {
+    return scanner;
+  }
 }