3 files changed, 82 insertions, 3 deletions
diff --git a/lib/api/repositories.rb b/lib/api/repositories.rb
index 33a9646ac3b..79736107bbb 100644
--- a/lib/api/repositories.rb
+++ b/lib/api/repositories.rb
@@ -123,6 +123,39 @@ module API
           not_found!
         end
       end
+
+      desc 'Get the common ancestor between commits' do
+        success Entities::Commit
+      end
+      params do
+        # For now we just support 2 refs passed, but `merge-base` supports
+        # multiple defining this as an Array instead of 2 separate params will
+        # make sure we don't need to deprecate this API in favor of one
+        # supporting multiple commits when this functionality gets added to
+        # Gitaly
+        requires :refs, type: Array[String]
+      end
+      get ':id/repository/merge_base' do
+        refs = params[:refs]
+
+        unless refs.size == 2
+          render_api_error!('Provide exactly 2 refs', 400)
+        end
+
+        merge_base = Gitlab::Git::MergeBase.new(user_project.repository, refs)
+
+        if merge_base.unknown_refs.any?
+          ref_noun = 'ref'.pluralize(merge_base.unknown_refs.size)
+          message = "Could not find #{ref_noun}: #{merge_base.unknown_refs.join(', ')}"
+          render_api_error!(message, 400)
+        end
+
+        if merge_base.commit
+          present merge_base.commit, with: Entities::Commit
+        else
+          not_found!("Merge Base")
+        end
+      end
     end
   end
 end
diff --git a/lib/gitlab/git.rb b/lib/gitlab/git.rb
index 55236a1122f..2913a3e416d 100644
--- a/lib/gitlab/git.rb
+++ b/lib/gitlab/git.rb
@@ -10,9 +10,11 @@ module Gitlab
     TAG_REF_PREFIX = "refs/tags/".freeze
     BRANCH_REF_PREFIX = "refs/heads/".freeze
 
-    CommandError = Class.new(StandardError)
-    CommitError = Class.new(StandardError)
-    OSError = Class.new(StandardError)
+    BaseError = Class.new(StandardError)
+    CommandError = Class.new(BaseError)
+    CommitError = Class.new(BaseError)
+    OSError = Class.new(BaseError)
+    UnknownRef = Class.new(BaseError)
 
     class << self
       include Gitlab::EncodingHelper
diff --git a/lib/gitlab/git/merge_base.rb b/lib/gitlab/git/merge_base.rb
new file mode 100644
index 00000000000..b27f7038c26
--- /dev/null
+++ b/lib/gitlab/git/merge_base.rb
@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    class MergeBase
+      include Gitlab::Utils::StrongMemoize
+
+      def initialize(repository, refs)
+        @repository, @refs = repository, refs
+      end
+
+      # Returns the SHA of the first common ancestor
+      def sha
+        if unknown_refs.any?
+          raise UnknownRef, "Can't find merge base for unknown refs: #{unknown_refs.inspect}"
+        end
+
+        strong_memoize(:sha) do
+          @repository.merge_base(*commits_for_refs)
+        end
+      end
+
+      # Returns the merge base as a Gitlab::Git::Commit
+      def commit
+        return unless sha
+
+        @commit ||= @repository.commit_by(oid: sha)
+      end
+
+      # Returns the refs passed on initialization that aren't found in
+      # the repository, and thus cannot be used to find a merge base.
+      def unknown_refs
+        @unknown_refs ||= Hash[@refs.zip(commits_for_refs)]
+                            .select { |ref, commit| commit.nil? }.keys
+      end
+
+      private
+
+      def commits_for_refs
+        @commits_for_refs ||= @repository.commits_by(oids: @refs)
+      end
+    end
+  end
+end