Fix branch protection API.

1. Previously, we were not removing existing access levels before
   creating new ones. This is not a problem for EE, but _is_ for CE,
   since we restrict the number of access levels in CE to 1.

2. The correct approach is:

    CE -> delete all access levels before updating a protected branch
    EE -> delete developer access levels if "developers_can_{merge,push}" is switched off

3. The dispatch is performed by checking if a "length: 1" validation is
   present on the access levels or not.

4. Another source of problems was that we didn't put multiple queries in
   a transaction. If the `destroy_all` passes, but the `update` fails,
   we should have a rollback.

5. Modifying the API to provide users direct access to CRUD access
   levels will make things a lot simpler.

6. Create `create/update` services separately for this API, which
   perform the necessary data translation, before calling the regular
   `create/update` services. The translation code was getting too large
   for the API endpoint itself, so this move makes sense.

app/models/concerns/protected_branch_access.rb

 module ProtectedBranchAccess
   extend ActiveSupport::Concern
 
+  included do
+    scope :master, -> () { where(access_level: Gitlab::Access::MASTER) }
+    scope :developer, -> () { where(access_level: Gitlab::Access::DEVELOPER) }
+  end
+
   def humanize
     self.class.human_access_levels[self.access_level]
   end

app/services/protected_branches/api_create_service.rb

+# The protected branches API still uses the `developers_can_push` and `developers_can_merge`
+# flags for backward compatibility, and so performs translation between that format and the
+# internal data model (separate access levels). The translation code is non-trivial, and so
+# lives in this service.
+module ProtectedBranches
+  class ApiCreateService < BaseService
+    def initialize(project, user, params, developers_can_push:, developers_can_merge:)
+      super(project, user, params)
+      @developers_can_merge = developers_can_merge
+      @developers_can_push = developers_can_push
+    end
+
+    def execute
+      if @developers_can_push
+        @params.merge!(push_access_levels_attributes: [{ access_level: Gitlab::Access::DEVELOPER }])
+      else
+        @params.merge!(push_access_levels_attributes: [{ access_level: Gitlab::Access::MASTER }])
+      end
+
+      if @developers_can_merge
+        @params.merge!(merge_access_levels_attributes: [{ access_level: Gitlab::Access::DEVELOPER }])
+      else
+        @params.merge!(merge_access_levels_attributes: [{ access_level: Gitlab::Access::MASTER }])
+      end
+
+      service = ProtectedBranches::CreateService.new(@project, @current_user, @params)
+      service.execute
+    end
+  end
+end

app/services/protected_branches/api_update_service.rb

+# The protected branches API still uses the `developers_can_push` and `developers_can_merge`
+# flags for backward compatibility, and so performs translation between that format and the
+# internal data model (separate access levels). The translation code is non-trivial, and so
+# lives in this service.
+module ProtectedBranches
+  class ApiUpdateService < BaseService
+    def initialize(project, user, params, developers_can_push:, developers_can_merge:)
+      super(project, user, params)
+      @developers_can_merge = developers_can_merge
+      @developers_can_push = developers_can_push
+    end
+
+    def execute(protected_branch)
+      @protected_branch = protected_branch
+
+      protected_branch.transaction do
+        # If a protected branch can have only a single access level (CE), delete it to
+        # make room for the new access level. If a protected branch can have more than
+        # one access level (EE), only remove the relevant access levels. If we don't do this,
+        # we'll have a failed validation.
+        if protected_branch_restricted_to_single_access_level?
+          delete_redundant_ce_access_levels
+        else
+          delete_redundant_ee_access_levels
+        end
+
+        if @developers_can_push
+          params.merge!(push_access_levels_attributes: [{ access_level: Gitlab::Access::DEVELOPER }])
+        elsif @developers_can_push == false
+          params.merge!(push_access_levels_attributes: [{ access_level: Gitlab::Access::MASTER }])
+        end
+
+        if @developers_can_merge
+          params.merge!(merge_access_levels_attributes: [{ access_level: Gitlab::Access::DEVELOPER }])
+        elsif @developers_can_merge == false
+          params.merge!(merge_access_levels_attributes: [{ access_level: Gitlab::Access::MASTER }])
+        end
+
+        service = ProtectedBranches::UpdateService.new(@project, @current_user, @params)
+        service.execute(protected_branch)
+      end
+    end
+
+    private
+
+    def delete_redundant_ce_access_levels
+      if @developers_can_merge || @developers_can_merge == false
+        @protected_branch.merge_access_levels.destroy_all
+      end
+
+      if @developers_can_push || @developers_can_push == false
+        @protected_branch.push_access_levels.destroy_all
+      end
+    end
+
+    def delete_redundant_ee_access_levels
+      if @developers_can_merge
+        @protected_branch.merge_access_levels.developer.destroy_all
+      elsif @developers_can_merge == false
+        @protected_branch.merge_access_levels.developer.destroy_all
+        @protected_branch.merge_access_levels.master.destroy_all
+      end
+
+      if @developers_can_push
+        @protected_branch.push_access_levels.developer.destroy_all
+      elsif @developers_can_push == false
+        @protected_branch.push_access_levels.developer.destroy_all
+        @protected_branch.push_access_levels.master.destroy_all
+      end
+    end
+
+    def protected_branch_restricted_to_single_access_level?
+      length_validator = ProtectedBranch.validators_on(:push_access_levels).find do |validator|
+        validator.is_a? ActiveModel::Validations::LengthValidator
+      end
+      length_validator.options[:is] == 1 if length_validator
+    end
+  end
+end

lib/api/branches.rb

@@ -57,40 +57,25 @@ module API
         developers_can_merge = to_boolean(params[:developers_can_merge])
         developers_can_push = to_boolean(params[:developers_can_push])
 
-        protected_branch_params = {
-          name: @branch.name
+        params = {
+          name: @branch.name,
         }
 
-        # If `developers_can_merge` is switched off, _all_ `DEVELOPER`
-        # merge_access_levels need to be deleted.
-        if developers_can_merge == false
-          protected_branch.merge_access_levels.where(access_level: Gitlab::Access::DEVELOPER).destroy_all
-        end
+        service_args = [user_project, current_user, params,
+                        developers_can_push: developers_can_push,
+                        developers_can_merge: developers_can_merge]
 
-        # If `developers_can_push` is switched off, _all_ `DEVELOPER`
-        # push_access_levels need to be deleted.
-        if developers_can_push == false
-          protected_branch.push_access_levels.where(access_level: Gitlab::Access::DEVELOPER).destroy_all
-        end
+        protected_branch = if protected_branch
+                             ProtectedBranches::ApiUpdateService.new(*service_args).execute(protected_branch)
+                           else
+                             ProtectedBranches::ApiCreateService.new(*service_args).execute
+                           end
 
-        protected_branch_params.merge!(
-          merge_access_levels_attributes: [{
-            access_level: developers_can_merge ? Gitlab::Access::DEVELOPER : Gitlab::Access::MASTER
-          }],
-          push_access_levels_attributes: [{
-            access_level: developers_can_push ? Gitlab::Access::DEVELOPER : Gitlab::Access::MASTER
-          }]
-        )
-
-        if protected_branch
-          service = ProtectedBranches::UpdateService.new(user_project, current_user, protected_branch_params)
-          service.execute(protected_branch)
+        if protected_branch.valid?
+          present @branch, with: Entities::RepoBranch, project: user_project
         else
-          service = ProtectedBranches::CreateService.new(user_project, current_user, protected_branch_params)
-          service.execute
+          render_api_error!(protected_branch.errors.full_messages, 422)
         end
-
-        present @branch, with: Entities::RepoBranch, project: user_project
       end
 
       # Unprotect a single branch
@@ -123,7 +108,7 @@ module API
       post ":id/repository/branches" do
         authorize_push_project
         result = CreateBranchService.new(user_project, current_user).
-          execute(params[:branch_name], params[:ref])
+                 execute(params[:branch_name], params[:ref])
 
         if result[:status] == :success
           present result[:branch],
@@ -142,10 +127,10 @@ module API
       # Example Request:
       #   DELETE /projects/:id/repository/branches/:branch
       delete ":id/repository/branches/:branch",
-          requirements: { branch: /.+/ } do
+             requirements: { branch: /.+/ } do
         authorize_push_project
         result = DeleteBranchService.new(user_project, current_user).
-          execute(params[:branch])
+                 execute(params[:branch])
 
         if result[:status] == :success
           {