Redirect to an already forked project if it exists

Before the we would try to `POST` to the project path, which would
result in a 404, because that `POST` is not supported.

By changing this back to a normal link, not not handled in JS, we can
immeadiatly redirect.

app/views/projects/forks/_fork_button.html.haml

+- avatar = namespace_icon(namespace, 100)
+- can_create_project = current_user.can?(:create_projects, namespace)
+
+- if forked_project = namespace.find_fork_of(@project)
+  .bordered-box.fork-thumbnail.text-center.prepend-left-default.append-right-default.prepend-top-default.append-bottom-default.forked
+    = link_to project_path(forked_project) do
+      - if /no_((\w*)_)*avatar/.match(avatar)
+        = project_identicon(namespace, class: "avatar s100 identicon")
+      - else
+        .avatar-container.s100
+          = image_tag(avatar, class: "avatar s100")
+      %h5.prepend-top-default
+        = namespace.human_name
+- else
+  .bordered-box.fork-thumbnail.text-center.prepend-left-default.append-right-default.prepend-top-default.append-bottom-default{ class: ("disabled" unless can_create_project) }
+    = link_to project_forks_path(@project, namespace_key: namespace.id),
+        method: "POST",
+        class: ("disabled has-tooltip" unless can_create_project),
+        title: (_('You have reached your project limit') unless can_create_project) do
+      - if /no_((\w*)_)*avatar/.match(avatar)
+        = project_identicon(namespace, class: "avatar s100 identicon")
+      - else
+        .avatar-container.s100
+          = image_tag(avatar, class: "avatar s100")
+      %h5.prepend-top-default
+        = namespace.human_name

app/views/projects/forks/new.html.haml

@@ -14,22 +14,7 @@
         %h5.prepend-top-0.append-bottom-0.prepend-left-default.append-right-default
           Click to fork the project
         - @namespaces.each do |namespace|
-          - avatar = namespace_icon(namespace, 100)
-          - can_create_project = current_user.can?(:create_projects, namespace)
-          - forked_project = namespace.find_fork_of(@project)
-          - fork_path = forked_project ? project_path(forked_project) : project_forks_path(@project, namespace_key: namespace.id)
-          .bordered-box.fork-thumbnail.text-center.prepend-left-default.append-right-default.prepend-top-default.append-bottom-default{ class: [("disabled" unless can_create_project), ("forked" if forked_project)] }
-            = link_to fork_path,
-                method: "POST",
-                class: [("js-fork-thumbnail" unless forked_project), ("disabled has-tooltip" unless can_create_project)],
-                title: (_('You have reached your project limit') unless can_create_project) do
-              - if /no_((\w*)_)*avatar/.match(avatar)
-                = project_identicon(namespace, class: "avatar s100 identicon")
-              - else
-                .avatar-container.s100
-                  = image_tag(avatar, class: "avatar s100")
-              %h5.prepend-top-default
-                = namespace.human_name
+          = render 'fork_button', namespace: namespace
     - else
       %strong
         No available namespaces to fork the project.

changelogs/unreleased/bvl-double-fork.yml

+---
+title: Correctly link to a forked project from the new fork page.
+merge_request: 15653
+author:
+type: fixed

spec/features/projects/fork_spec.rb

 require 'spec_helper'
 
 describe 'Project fork' do
+  include ProjectForksHelper
+
   let(:user) { create(:user) }
   let(:project) { create(:project, :public, :repository) }
 
@@ -24,8 +26,9 @@ describe 'Project fork' do
   end
 
   context 'master in group' do
+    let(:group) { create(:group) }
+
     before do
-      group = create(:group)
       group.add_master(user)
     end
 
@@ -53,5 +56,17 @@ describe 'Project fork' do
       expect(page).to have_css('.fork-thumbnail', count: 2)
       expect(page).to have_css('.fork-thumbnail.disabled')
     end
+
+    it 'links to the fork if the project was already forked within that namespace' do
+      forked_project = fork_project(project, user, namespace: group, repository: true)
+
+      visit new_project_fork_path(project)
+
+      expect(page).to have_css('div.forked', text: group.full_name)
+
+      click_link group.full_name
+
+      expect(current_path).to eq(project_path(forked_project))
+    end
   end
 end