Add ref parameter for triggerring builds with gitlab webhook from other project.

app/helpers/triggers_helper.rb

 module TriggersHelper
-  def builds_trigger_url(project_id)
-    "#{Settings.gitlab.url}/api/v3/projects/#{project_id}/trigger/builds"
+  def builds_trigger_url(project_id, ref: nil)
+    if ref.nil?
+      "#{Settings.gitlab.url}/api/v3/projects/#{project_id}/trigger/builds"
+    else
+      "#{Settings.gitlab.url}/api/v3/projects/#{project_id}/ref/#{ref}/trigger/builds"
+    end
   end
 end

app/views/projects/triggers/index.html.haml

@@ -75,6 +75,16 @@
               stage: deploy
               script:
                 - "curl -X POST -F token=TOKEN -F ref=REF_NAME #{builds_trigger_url(@project.id)}"
+        %h5.prepend-top-default
+          Use webhook
+
+        %p.light
+          Add the following webhook to another project for Push and Tag push events.
+          The project will be rebuilt at the corresponding event.
+
+        %pre
+          :plain
+             #{builds_trigger_url(@project.id, ref: 'REF_NAME')}?token=TOKEN
         %h5.prepend-top-default
           Pass build variables
 
@@ -83,10 +93,18 @@
           %code variables[VARIABLE]=VALUE
           to an API request. Variable values can be used to distinguish between triggered builds and normal builds.
 
-        %pre.append-bottom-0
+          With cURL:
+
+        %pre
           :plain
             curl -X POST \
                  -F token=TOKEN \
                  -F "ref=REF_NAME" \
                  -F "variables[RUN_NIGHTLY_BUILD]=true" \
                  #{builds_trigger_url(@project.id)}
+        %p.light
+          With webhook:
+
+        %pre.append-bottom-0
+          :plain
+             #{builds_trigger_url(@project.id, ref: 'REF_NAME')}?token=TOKEN&variables[RUN_NIGHTLY_BUILD]=true

changelogs/unreleased/23584-triggering-builds-from-webhooks.yml

+---
+title: Make it possible to trigger builds from webhooks
+merge_request: 7022
+author: Dmitry Poray

doc/ci/triggers/README.md

@@ -58,6 +58,22 @@ below.
 See the [Examples](#examples) section for more details on how to actually
 trigger a rebuild.
 
+## Trigger a build from webhook
+
+> Introduced in GitLab 8.14.
+
+To trigger a build from webhook of another project you need to add the following
+webhook url for Push and Tag push events:
+
+```
+https://gitlab.example.com/api/v3/projects/:id/ref/:ref/trigger/builds?token=TOKEN
+```
+
+> **Note**:
+- `ref` should be passed as part of url in order to take precedence over `ref`
+  from webhook body that designates the branchref that fired the trigger in the source repository.
+- `ref` should be url encoded if contains slashes.
+
 ## Pass build variables to a trigger
 
 You can pass any number of arbitrary variables in the trigger API call and they
@@ -169,6 +185,14 @@ curl --request POST \
   https://gitlab.example.com/api/v3/projects/9/trigger/builds
 ```
 
+### Using webhook to trigger builds
+
+You can add the following webhook to another project in order to trigger a build:
+
+```
+https://gitlab.example.com/api/v3/projects/9/ref/master/trigger/builds?token=TOKEN&variables[UPLOAD_TO_S3]=true
+```
+
 ### Using cron to trigger nightly builds
 
 Whether you craft a script or just run cURL directly, you can trigger builds

lib/api/triggers.rb

@@ -12,7 +12,7 @@ module API
         requires :token, type: String, desc: 'The unique token of trigger'
         optional :variables, type: Hash, desc: 'The list of variables to be injected into build'
       end
-      post ":id/trigger/builds" do
+      post ":id/(ref/:ref/)trigger/builds" do
         project = Project.find_with_namespace(params[:id]) || Project.find_by(id: params[:id])
         trigger = Ci::Trigger.find_by_token(params[:token].to_s)
         not_found! unless project && trigger

spec/requests/api/triggers_spec.rb

@@ -54,6 +54,13 @@ describe API::API do
         expect(pipeline.builds.size).to eq(5)
       end
 
+      it 'creates builds on webhook from other gitlab repository and branch' do
+        expect do
+          post api("/projects/#{project.id}/ref/master/trigger/builds?token=#{trigger_token}"), { ref: 'refs/heads/other-branch' }
+        end.to change(project.builds, :count).by(5)
+        expect(response).to have_http_status(201)
+      end
+
       it 'returns bad request with no builds created if there\'s no commit for that ref' do
         post api("/projects/#{project.id}/trigger/builds"), options.merge(ref: 'other-branch')
         expect(response).to have_http_status(400)