Add Repository#discover_default_branch and add it to PushObserver

app/roles/push_observer.rb

+# Includes methods for handling Git Push events
+#
+# Triggered by PostReceive job
 module PushObserver
   def observe_push(oldrev, newrev, ref, user)
     data = post_receive_data(oldrev, newrev, ref, user)
@@ -84,11 +87,10 @@ module PushObserver
     data
   end
 
-
-  # This method will be called after each post receive
-  # and only if user present in gitlab.
-  # All callbacks for post receive should be placed here
+  # This method will be called after each post receive and only if the provided
+  # user is present in GitLab.
   #
+  # All callbacks for post receive should be placed here.
   def trigger_post_receive(oldrev, newrev, ref, user)
     # Create push event
     self.observe_push(oldrev, newrev, ref, user)
@@ -101,5 +103,11 @@ module PushObserver
 
     # Create satellite
     self.satellite.create unless self.satellite.exists?
+
+    # Discover the default branch, but only if it hasn't already been set to
+    # something else
+    if default_branch.nil?
+      update_attributes(default_branch: discover_default_branch)
+    end
   end
 end

app/roles/repository.rb

@@ -94,6 +94,24 @@ module Repository
     end.sort_by(&:name)
   end
 
+  # Discovers the default branch based on the repository's available branches
+  #
+  # - If no branches are present, returns nil
+  # - If one branch is present, returns its name
+  # - If two or more branches are present, returns the one that has a name
+  #   matching root_ref (default_branch or 'master' if default_branch is nil)
+  def discover_default_branch
+    branches = heads.collect(&:name)
+
+    if branches.length == 0
+      nil
+    elsif branches.length == 1
+      branches.first
+    else
+      branches.select { |v| v == root_ref }.first
+    end
+  end
+
   def has_commits?
     !!commit
   end
@@ -102,7 +120,7 @@ module Repository
     default_branch || "master"
   end
 
-  def root_ref? branch
+  def root_ref?(branch)
     root_ref == branch
   end
 
@@ -111,7 +129,7 @@ module Repository
   # Already packed repo archives stored at
   # app_root/tmp/repositories/project_name/project_name-commit-id.tag.gz
   #
-  def archive_repo ref
+  def archive_repo(ref)
     ref = ref || self.root_ref
     commit = self.commit(ref)
     return nil unless commit
@@ -138,6 +156,6 @@ module Repository
   end
 
   def http_url_to_repo
-    http_url = [Gitlab.config.url, "/", path, ".git"].join()
+    http_url = [Gitlab.config.url, "/", path, ".git"].join('')
   end
 end