Set Gitaly Server feature flags from Rails

Gitaly itself hold very little state, other than the data on disk. This
limits the interfaces to set feature flags. Gitaly now has the ability
to interpret the request metadata to check for feature flags.
https://gitlab.com/gitlab-org/gitaly/merge_requests/704
This allows clients control on the Gitaly server, and given that Rails
has an internal chatops interface to set these values this was chosen as
solution.

Known limitation right now, is that this package doesn't support the opt
out that other Gitaly features do.

lib/gitlab/git/commit.rb

@@ -60,6 +60,11 @@ module Gitlab
           # Some weird thing?
           return nil unless commit_id.is_a?(String)
 
+          # The Go-Git backend Gitaly might use, tries to be nice when resolving
+          # to the commit, and `master:ref` will resolve to the commit that master
+          # resolves to. To keep behaviour the same, we return nil
+          return nil if commit_id.include?(':')
+
           commit = repo.gitaly_migrate(:find_commit) do |is_enabled|
             if is_enabled
               repo.gitaly_commit_client.find_commit(commit_id)

lib/gitlab/gitaly_client.rb

@@ -191,6 +191,8 @@ module Gitlab
       metadata['call_site'] = feature.to_s if feature
       metadata['gitaly-servers'] = address_metadata(remote_storage) if remote_storage
 
+      metadata.merge!(server_feature_flags(feature))
+
       result = { metadata: metadata }
 
       # nil timeout indicates that we should use the default
@@ -209,6 +211,20 @@ module Gitlab
       result
     end
 
+    SERVER_FEATURE_FLAGS = {
+      find_commit: ["gogit-findcommit"]
+    }.freeze
+
+    # Other than data on the disk, Gitaly is stateless. Rails will thus set
+    # feature flags in the request metadata.
+    def self.server_feature_flags(feature)
+      return {} unless SERVER_FEATURE_FLAGS.key?(feature)
+
+      SERVER_FEATURE_FLAGS[feature]
+        .map { |f| ["gitaly-feature-#{f}", feature_enabled?(f).to_s] }
+        .to_h
+    end
+
     def self.token(storage)
       params = Gitlab.config.repositories.storages[storage]
       raise "storage not found: #{storage.inspect}" if params.nil?