Add start_sha to commits API

When passing start_branch on committing from the WebIDE, it's possible
that the branch has changed since editing started, which results in the
change being applied on top of the latest commit in the branch and
overwriting the new changes.

By passing the start_sha instead we can make sure that the change is
applied on top of the commit which the user started editing from.

5 files changed, 191 insertions, 46 deletions

diff --git a/spec/lib/gitlab/git_spec.rb b/spec/lib/gitlab/git_spec.rb
index ce15057dd7d..6515be85ae3 100644
--- a/spec/lib/gitlab/git_spec.rb
+++ b/spec/lib/gitlab/git_spec.rb
@@ -39,6 +39,26 @@ describe Gitlab::Git do
     end
   end
 
+  describe '.commit_id?' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:sha, :result) do
+      ''                                         | false
+      'foobar'                                   | false
+      '4b825dc'                                  | false
+      'zzz25dc642cb6eb9a060e54bf8d69288fbee4904' | false
+
+      '4b825dc642cb6eb9a060e54bf8d69288fbee4904' | true
+      Gitlab::Git::BLANK_SHA                     | true
+    end
+
+    with_them do
+      it 'returns the expected result' do
+        expect(described_class.commit_id?(sha)).to eq(result)
+      end
+    end
+  end
+
   describe '.shas_eql?' do
     using RSpec::Parameterized::TableSyntax
 
diff --git a/spec/requests/api/commits_spec.rb b/spec/requests/api/commits_spec.rb
index 204e378f7be..311ef3ee99a 100644
--- a/spec/requests/api/commits_spec.rb
+++ b/spec/requests/api/commits_spec.rb
@@ -320,67 +320,132 @@ describe API::Commits do
         end
       end
 
-      context 'when the API user is a guest' do
+      context 'when committing to a new branch' do
         def last_commit_id(project, branch_name)
           project.repository.find_branch(branch_name)&.dereferenced_target&.id
         end
 
-        let(:public_project) { create(:project, :public, :repository) }
-        let!(:url) { "/projects/#{public_project.id}/repository/commits" }
-        let(:guest) { create(:user).tap { |u| public_project.add_guest(u) } }
+        before do
+          valid_c_params[:start_branch] = 'master'
+          valid_c_params[:branch] = 'patch'
+        end
 
-        it 'returns a 403' do
-          post api(url, guest), params: valid_c_params
+        context 'when the API user is a guest' do
+          let(:public_project) { create(:project, :public, :repository) }
+          let(:url) { "/projects/#{public_project.id}/repository/commits" }
+          let(:guest) { create(:user).tap { |u| public_project.add_guest(u) } }
 
-          expect(response).to have_gitlab_http_status(403)
-        end
+          it 'returns a 403' do
+            post api(url, guest), params: valid_c_params
 
-        context 'when start_project is provided' do
-          context 'when posting to a forked project the user owns' do
-            let!(:forked_project) { fork_project(public_project, guest, namespace: guest.namespace, repository: true) }
-            let!(:url) { "/projects/#{forked_project.id}/repository/commits" }
+            expect(response).to have_gitlab_http_status(403)
+          end
 
-            before do
-              valid_c_params[:start_branch]  = "master"
-              valid_c_params[:branch]        = "patch"
-            end
+          context 'when start_project is provided' do
+            context 'when posting to a forked project the user owns' do
+              let(:forked_project) { fork_project(public_project, guest, namespace: guest.namespace, repository: true) }
+              let(:url) { "/projects/#{forked_project.id}/repository/commits" }
+
+              context 'identified by Integer (id)' do
+                before do
+                  valid_c_params[:start_project] = public_project.id
+                end
+
+                it 'adds a new commit to forked_project and returns a 201' do
+                  expect_request_with_status(201) { post api(url, guest), params: valid_c_params }
+                    .to change { last_commit_id(forked_project, valid_c_params[:branch]) }
+                    .and not_change { last_commit_id(public_project, valid_c_params[:start_branch]) }
+                end
+              end
 
-            context 'identified by Integer (id)' do
-              before do
-                valid_c_params[:start_project] = public_project.id
+              context 'identified by String (full_path)' do
+                before do
+                  valid_c_params[:start_project] = public_project.full_path
+                end
+
+                it 'adds a new commit to forked_project and returns a 201' do
+                  expect_request_with_status(201) { post api(url, guest), params: valid_c_params }
+                    .to change { last_commit_id(forked_project, valid_c_params[:branch]) }
+                    .and not_change { last_commit_id(public_project, valid_c_params[:start_branch]) }
+                end
               end
 
-              it 'adds a new commit to forked_project and returns a 201' do
-                expect { post api(url, guest), params: valid_c_params }
-                  .to change { last_commit_id(forked_project, valid_c_params[:branch]) }
-                  .and not_change { last_commit_id(public_project, valid_c_params[:start_branch]) }
+              context 'when branch already exists' do
+                before do
+                  valid_c_params.delete(:start_branch)
+                  valid_c_params[:branch] = 'master'
+                  valid_c_params[:start_project] = public_project.id
+                end
+
+                it 'returns a 400' do
+                  post api(url, guest), params: valid_c_params
+
+                  expect(response).to have_gitlab_http_status(400)
+                  expect(json_response['message']).to eq("A branch called 'master' already exists. Switch to that branch in order to make changes")
+                end
+
+                context 'when force is set to true' do
+                  before do
+                    valid_c_params[:force] = true
+                  end
+
+                  it 'adds a new commit to forked_project and returns a 201' do
+                    expect_request_with_status(201) { post api(url, guest), params: valid_c_params }
+                      .to change { last_commit_id(forked_project, valid_c_params[:branch]) }
+                      .and not_change { last_commit_id(public_project, valid_c_params[:branch]) }
+                  end
+                end
+              end
+
+              context 'when start_sha is also provided' do
+                let(:forked_project) { fork_project(public_project, guest, namespace: guest.namespace, repository: false) }
+                let(:start_sha) { public_project.repository.commit.parent.sha }
+
+                before do
+                  # initialize an empty repository to force fetching from the original project
+                  forked_project.repository.create_if_not_exists
 
-                expect(response).to have_gitlab_http_status(201)
+                  valid_c_params[:start_project] = public_project.id
+                  valid_c_params[:start_sha] = start_sha
+                  valid_c_params.delete(:start_branch)
+                end
+
+                it 'fetches the start_sha from the original project to use as parent commit and returns a 201' do
+                  expect_request_with_status(201) { post api(url, guest), params: valid_c_params }
+                    .to change { last_commit_id(forked_project, valid_c_params[:branch]) }
+                    .and not_change { last_commit_id(forked_project, 'master') }
+
+                  last_commit = forked_project.repository.find_branch(valid_c_params[:branch]).dereferenced_target
+                  expect(last_commit.parent_id).to eq(start_sha)
+                end
               end
             end
 
-            context 'identified by String (full_path)' do
+            context 'when the target project is not part of the fork network of start_project' do
+              let(:unrelated_project) { create(:project, :public, :repository, creator: guest) }
+              let(:url) { "/projects/#{unrelated_project.id}/repository/commits" }
+
               before do
-                valid_c_params[:start_project] = public_project.full_path
+                valid_c_params[:start_branch] = 'master'
+                valid_c_params[:branch] = 'patch'
+                valid_c_params[:start_project] = public_project.id
               end
 
-              it 'adds a new commit to forked_project and returns a 201' do
-                expect { post api(url, guest), params: valid_c_params }
-                  .to change { last_commit_id(forked_project, valid_c_params[:branch]) }
-                  .and not_change { last_commit_id(public_project, valid_c_params[:start_branch]) }
+              it 'returns a 403' do
+                post api(url, guest), params: valid_c_params
 
-                expect(response).to have_gitlab_http_status(201)
+                expect(response).to have_gitlab_http_status(403)
               end
             end
           end
 
-          context 'when the target project is not part of the fork network of start_project' do
-            let(:unrelated_project) { create(:project, :public, :repository, creator: guest) }
-            let!(:url) { "/projects/#{unrelated_project.id}/repository/commits" }
+          context 'when posting to a forked project the user does not have write access' do
+            let(:forked_project) { fork_project(public_project, user, namespace: user.namespace, repository: true) }
+            let(:url) { "/projects/#{forked_project.id}/repository/commits" }
 
             before do
-              valid_c_params[:start_branch]  = "master"
-              valid_c_params[:branch]        = "patch"
+              valid_c_params[:start_branch] = 'master'
+              valid_c_params[:branch] = 'patch'
               valid_c_params[:start_project] = public_project.id
             end
 
@@ -392,20 +457,68 @@ describe API::Commits do
           end
         end
 
-        context 'when posting to a forked project the user does not have write access' do
-          let!(:forked_project) { fork_project(public_project, user, namespace: user.namespace, repository: true) }
-          let!(:url) { "/projects/#{forked_project.id}/repository/commits" }
+        context 'when start_sha is provided' do
+          let(:start_sha) { project.repository.commit.parent.sha }
 
           before do
-            valid_c_params[:start_branch]  = "master"
-            valid_c_params[:branch]        = "patch"
-            valid_c_params[:start_project] = public_project.id
+            valid_c_params[:start_sha] = start_sha
+            valid_c_params.delete(:start_branch)
           end
 
-          it 'returns a 403' do
-            post api(url, guest), params: valid_c_params
+          it 'returns a 400 if start_branch is also provided' do
+            valid_c_params[:start_branch] = 'master'
+            post api(url, user), params: valid_c_params
 
-            expect(response).to have_gitlab_http_status(403)
+            expect(response).to have_gitlab_http_status(400)
+            expect(json_response['error']).to eq('start_branch, start_sha are mutually exclusive')
+          end
+
+          it 'returns a 400 if branch already exists' do
+            valid_c_params[:branch] = 'master'
+            post api(url, user), params: valid_c_params
+
+            expect(response).to have_gitlab_http_status(400)
+            expect(json_response['message']).to eq("A branch called 'master' already exists. Switch to that branch in order to make changes")
+          end
+
+          it 'returns a 400 if start_sha does not exist' do
+            valid_c_params[:start_sha] = '1' * 40
+            post api(url, user), params: valid_c_params
+
+            expect(response).to have_gitlab_http_status(400)
+            expect(json_response['message']).to eq("Cannot find start_sha '#{valid_c_params[:start_sha]}'")
+          end
+
+          it 'returns a 400 if start_sha is not a full SHA' do
+            valid_c_params[:start_sha] = start_sha.slice(0, 7)
+            post api(url, user), params: valid_c_params
+
+            expect(response).to have_gitlab_http_status(400)
+            expect(json_response['message']).to eq("Invalid start_sha '#{valid_c_params[:start_sha]}'")
+          end
+
+          it 'uses the start_sha as parent commit and returns a 201' do
+            expect_request_with_status(201) { post api(url, user), params: valid_c_params }
+              .to change { last_commit_id(project, valid_c_params[:branch]) }
+              .and not_change { last_commit_id(project, 'master') }
+
+            last_commit = project.repository.find_branch(valid_c_params[:branch]).dereferenced_target
+            expect(last_commit.parent_id).to eq(start_sha)
+          end
+
+          context 'when force is set to true and branch already exists' do
+            before do
+              valid_c_params[:force] = true
+              valid_c_params[:branch] = 'master'
+            end
+
+            it 'uses the start_sha as parent commit and returns a 201' do
+              expect_request_with_status(201) { post api(url, user), params: valid_c_params }
+                .to change { last_commit_id(project, valid_c_params[:branch]) }
+
+              last_commit = project.repository.find_branch(valid_c_params[:branch]).dereferenced_target
+              expect(last_commit.parent_id).to eq(start_sha)
+            end
           end
         end
       end
diff --git a/spec/services/submodules/update_service_spec.rb b/spec/services/submodules/update_service_spec.rb
index cf92350c1b2..47b31d4bcbf 100644
--- a/spec/services/submodules/update_service_spec.rb
+++ b/spec/services/submodules/update_service_spec.rb
@@ -142,7 +142,7 @@ describe Submodules::UpdateService do
             let(:branch_name) { nil }
 
             it_behaves_like 'returns error result' do
-              let(:error_message) { 'You can only create or edit files when you are on a branch' }
+              let(:error_message) { 'Invalid parameters' }
             end
           end
 
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 95e0d8858b9..fb9e3b295f8 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -104,6 +104,7 @@ RSpec.configure do |config|
   config.include Rails.application.routes.url_helpers, type: :routing
   config.include PolicyHelpers, type: :policy
   config.include MemoryUsageHelper
+  config.include ExpectRequestWithStatus, type: :request
 
   if ENV['CI']
     # This includes the first try, i.e. tests will be run 4 times before failing.
diff --git a/spec/support/helpers/expect_request_with_status.rb b/spec/support/helpers/expect_request_with_status.rb
new file mode 100644
index 00000000000..0469a94e336
--- /dev/null
+++ b/spec/support/helpers/expect_request_with_status.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module ExpectRequestWithStatus
+  def expect_request_with_status(status)
+    expect do
+      yield
+
+      expect(response).to have_gitlab_http_status(status)
+    end
+  end
+end