SubmoduleOp: Introduce a setting to disable all superproject subscriptions

Not every host needs superproject subscriptions. Have an option to turn it
off completely. In fact a quick search reveals people are mostly confused
by this feature in the wild.

Change-Id: I20f699c06f6bf832d5686cd19db31238e1842851
Signed-off-by: Stefan Beller <sbeller@google.com>

Documentation/config-gerrit.txt

@@ -3976,6 +3976,11 @@ the commit message of the superproject update.
 +
 By default this is true.
 
+[[submodule.enableSuperProjectSubscriptions]]submodule.enableSuperProjectSubscriptions
++
+This allows to enable the superproject subscription mechanism.
++
+By default this is true.
 
 [[user]]
 === Section user

Documentation/user-submodules.txt

@@ -20,6 +20,10 @@ When a new commit of a registered submodule is merged, Gerrit
 automatically updates the subscribers to the submodule with a new
 commit having the updated gitlinks.
 
+This feature is enabled by default and can be disabled
+via link:config-gerrit.html#submodule.enableSuperProjectSubscriptions[submodule.enableSuperProjectSubscriptions]
+in the server configuration.
+
 == Git Submodules Overview
 
 Submodules are a git feature that allows an external repository to be

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/git/SubmoduleSubscriptionsIT.java

@@ -28,6 +28,17 @@ import org.junit.Test;
 
 public class SubmoduleSubscriptionsIT extends AbstractSubmoduleSubscription {
 
+  @Test
+  @GerritConfig(name = "submodule.enableSuperProjectSubscriptions", value = "false")
+  public void testSubscriptionWithoutServerSetting() throws Exception {
+    TestRepository<?> superRepo = createProjectWithPush("super-project");
+    TestRepository<?> subRepo = createProjectWithPush("subscribed-to-project");
+
+    createSubmoduleSubscription(superRepo, "master", "subscribed-to-project", "master");
+    pushChangeTo(subRepo, "master");
+    assertThat(hasSubmodule(superRepo, "master", "subscribed-to-project")).isFalse();
+  }
+
   @Test
   public void testSubscriptionToEmptyRepo() throws Exception {
     TestRepository<?> superRepo = createProjectWithPush("super-project");
@@ -76,7 +87,6 @@ public class SubmoduleSubscriptionsIT extends AbstractSubmoduleSubscription {
   }
 
   @Test
-
   public void testSubmoduleCommitMessage() throws Exception {
     TestRepository<?> superRepo = createProjectWithPush("super-project");
     TestRepository<?> subRepo = createProjectWithPush("subscribed-to-project");

gerrit-server/src/main/java/com/google/gerrit/server/git/SubmoduleOp.java

@@ -79,6 +79,7 @@ public class SubmoduleOp {
   private final ChangeHooks changeHooks;
   private final SubmoduleSectionParser.Factory subSecParserFactory;
   private final boolean verboseSuperProject;
+  private final boolean enableSuperProjectSubscriptions;
 
   @Inject
   public SubmoduleOp(
@@ -99,12 +100,17 @@ public class SubmoduleOp {
     this.subSecParserFactory = subSecParserFactory;
     this.verboseSuperProject = cfg.getBoolean("submodule",
         "verboseSuperprojectUpdate", true);
-
+    this.enableSuperProjectSubscriptions = cfg.getBoolean("submodule",
+        "enableSuperProjectSubscriptions", true);
     updatedSubscribers = new HashSet<>();
   }
 
   void updateSubmoduleSubscriptions(ReviewDb db, Set<Branch.NameKey> branches)
       throws SubmoduleException {
+    if (!enableSuperProjectSubscriptions) {
+      return;
+    }
+
     for (Branch.NameKey branch : branches) {
       updateSubmoduleSubscriptions(db, branch);
     }
@@ -112,6 +118,9 @@ public class SubmoduleOp {
 
   void updateSubmoduleSubscriptions(ReviewDb db, Branch.NameKey destBranch)
       throws SubmoduleException {
+    if (!enableSuperProjectSubscriptions) {
+      return;
+    }
     if (urlProvider.get() == null) {
       logAndThrowSubmoduleException("Cannot establish canonical web url used "
           + "to access gerrit. It should be provided in gerrit.config file.");
@@ -185,6 +194,9 @@ public class SubmoduleOp {
 
   protected void updateSuperProjects(ReviewDb db,
       Collection<Branch.NameKey> updatedBranches) throws SubmoduleException {
+    if (!enableSuperProjectSubscriptions) {
+      return;
+    }
     try {
       // These (repo/branch) will be updated later with all the given
       // individual submodule subscriptions