Branches without parent commit cannot be merged.

app/controllers/projects/merge_requests_controller.rb

@@ -95,14 +95,18 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       @note_counts = Note.where(commit_id: @commits.map(&:id)).
           group(:commit_id).count
 
-      @diffs = compare_action.diffs
-      @merge_request.title = @merge_request.source_branch.titleize.humanize
-      @target_project = @merge_request.target_project
-      @target_repo = @target_project.repository
-
-      diff_line_count = Commit::diff_line_count(@diffs)
-      @suppress_diff = Commit::diff_suppress?(@diffs, diff_line_count)
-      @force_suppress_diff = @suppress_diff
+      begin
+        @diffs = compare_action.diffs
+        @merge_request.title = @merge_request.source_branch.titleize.humanize
+        @target_project = @merge_request.target_project
+        @target_repo = @target_project.repository
+
+        diff_line_count = Commit::diff_line_count(@diffs)
+        @suppress_diff = Commit::diff_suppress?(@diffs, diff_line_count)
+        @force_suppress_diff = @suppress_diff
+      rescue Gitlab::Satellite::BranchesWithoutParent
+        @error = "Selected branches have no common commit so they cannot be compared."
+      end
     end
   end
 

app/views/projects/merge_requests/_new_submit.html.haml

@@ -10,72 +10,75 @@
 
   %span.pull-right
     = link_to 'Change branches', new_project_merge_request_path(@project)
+- if @error.present?
+  .centered-light-block
+    %h4 #{@error}
+- else
+  = form_for [@project, @merge_request], html: { class: "merge-request-form" } do |f|
+    .panel.panel-default
 
-= form_for [@project, @merge_request], html: { class: "merge-request-form" } do |f|
-  .panel.panel-default
+      .panel-body
+        .form-group
+          .light
+            = f.label :title do
+              = "Title *"
+          = f.text_field :title, class: "form-control input-lg js-gfm-input", maxlength: 255, rows: 5, required: true
+        .form-group
+          .light
+            = f.label :description, "Description"
+          = f.text_area :description, class: "form-control js-gfm-input markdown-area", rows: 10
+          .clearfix.hint
+            .pull-left Description is parsed with #{link_to "GitLab Flavored Markdown", help_page_path("markdown", "markdown"), target: '_blank'}.
+            .pull-right Attach images (JPG, PNG, GIF) by dragging & dropping or #{link_to "selecting them", '#', class: 'markdown-selector' }.
+          .error-alert
+        .form-group
+          .issue-assignee
+            = f.label :assignee_id do
+              %i.icon-user
+              Assign to
+            %div
+              = project_users_select_tag('merge_request[assignee_id]', placeholder: 'Select a user', class: 'custom-form-control', selected: @merge_request.assignee_id, project_id: @merge_request.target_project_id)
+               
+              = link_to 'Assign to me', '#', class: 'btn assign-to-me-link'
+        .form-group
+          .issue-milestone
+            = f.label :milestone_id do
+              %i.icon-time
+              Milestone
+            %div= f.select(:milestone_id, milestone_options(@merge_request), { include_blank: "Select milestone" }, {class: 'select2'})
+      .panel-footer
+        - if @target_repo.contribution_guide
+          - contribution_guide_url = project_blob_path(@target_project, tree_join(@target_repo.root_ref, @target_repo.contribution_guide.name))
+          %p
+            Please review the
+            %strong #{link_to "guidelines for contribution", contribution_guide_url}
+            to this repository.
+        = f.hidden_field :source_project_id
+        = f.hidden_field :target_project_id
+        = f.hidden_field :target_branch
+        = f.hidden_field :source_branch
+        = f.submit 'Submit merge request', class: "btn btn-create"
 
-    .panel-body
-      .form-group
-        .light
-          = f.label :title do
-            = "Title *"
-        = f.text_field :title, class: "form-control input-lg js-gfm-input", maxlength: 255, rows: 5, required: true
-      .form-group
-        .light
-          = f.label :description, "Description"
-        = f.text_area :description, class: "form-control js-gfm-input markdown-area", rows: 10
-        .clearfix.hint
-          .pull-left Description is parsed with #{link_to "GitLab Flavored Markdown", help_page_path("markdown", "markdown"), target: '_blank'}.
-          .pull-right Attach images (JPG, PNG, GIF) by dragging & dropping or #{link_to "selecting them", '#', class: 'markdown-selector' }.
-        .error-alert
-      .form-group
-        .issue-assignee
-          = f.label :assignee_id do
-            %i.icon-user
-            Assign to
-          %div
-            = project_users_select_tag('merge_request[assignee_id]', placeholder: 'Select a user', class: 'custom-form-control', selected: @merge_request.assignee_id, project_id: @merge_request.target_project_id)
-             
-            = link_to 'Assign to me', '#', class: 'btn assign-to-me-link'
-      .form-group
-        .issue-milestone
-          = f.label :milestone_id do
-            %i.icon-time
-            Milestone
-          %div= f.select(:milestone_id, milestone_options(@merge_request), { include_blank: "Select milestone" }, {class: 'select2'})
-    .panel-footer
-      - if @target_repo.contribution_guide
-        - contribution_guide_url = project_blob_path(@target_project, tree_join(@target_repo.root_ref, @target_repo.contribution_guide.name))
-        %p
-          Please review the
-          %strong #{link_to "guidelines for contribution", contribution_guide_url}
-          to this repository.
-      = f.hidden_field :source_project_id
-      = f.hidden_field :target_project_id
-      = f.hidden_field :target_branch
-      = f.hidden_field :source_branch
-      = f.submit 'Submit merge request', class: "btn btn-create"
+  .mr-compare
+    %div.panel.panel-default
+      .panel-heading
+        Commits (#{@commits.count})
+      - if @commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
+        %ul.well-list
+          - Commit.decorate(@commits.first(MergeRequestDiff::COMMITS_SAFE_SIZE)).each do |commit|
+            = render "projects/commits/inline_commit", commit: commit, project: @project
+          %li.warning-row.unstyled
+            other #{@commits.size - MergeRequestDiff::COMMITS_SAFE_SIZE} commits hidden to prevent performance issues.
+      - else
+        %ul.well-list= render Commit.decorate(@commits), project: @project
 
-.mr-compare
-  %div.panel.panel-default
-    .panel-heading
-      Commits (#{@commits.count})
-    - if @commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
-      %ul.well-list
-        - Commit.decorate(@commits.first(MergeRequestDiff::COMMITS_SAFE_SIZE)).each do |commit|
-          = render "projects/commits/inline_commit", commit: commit, project: @project
-        %li.warning-row.unstyled
-          other #{@commits.size - MergeRequestDiff::COMMITS_SAFE_SIZE} commits hidden to prevent performance issues.
-    - else
-      %ul.well-list= render Commit.decorate(@commits), project: @project
-
-  %h4 Changes
-  - if @diffs.present?
-    = render "projects/commits/diffs", diffs: @diffs, project: @project
-  - elsif @commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
-    .bs-callout.bs-callout-danger
-      %h4 This comparison includes more than #{MergeRequestDiff::COMMITS_SAFE_SIZE} commits.
-      %p To preserve performance the line changes are not shown.
+    %h4 Changes
+    - if @diffs.present?
+      = render "projects/commits/diffs", diffs: @diffs, project: @project
+    - elsif @commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
+      .bs-callout.bs-callout-danger
+        %h4 This comparison includes more than #{MergeRequestDiff::COMMITS_SAFE_SIZE} commits.
+        %p To preserve performance the line changes are not shown.
 
 
 :javascript

lib/gitlab/satellite/compare_action.rb

 module Gitlab
   module Satellite
+    class BranchesWithoutParent < StandardError; end
+
     class CompareAction < Action
       def initialize(user, target_project, target_branch, source_project, source_branch)
         super user, target_project
@@ -22,7 +24,7 @@ module Gitlab
           diffs
         end
       rescue Grit::Git::CommandFailed => ex
-        handle_exception(ex)
+        raise BranchesWithoutParent
       end
 
       # Retrieve an array of commits between the source and the target