testing piper forks (#3420)

* developer doku update on how to test with forked repos
Co-authored-by: rosemarieB <45030247+rosemarieB@users.noreply.github.com>

DEVELOPMENT.md

@@ -478,6 +478,78 @@ func TestMethod(t *testing.T) {
 }
 ```
 
+### Test pipeline for your fork (Jenkins)
+
+Piper is ececuting the steps of each stage within a container. If you want to test your developments you have to ensure they are part of the image which is used in your test pipeline.
+
+#### Testing Pipeline or Stage Definition changes (Jenkins)
+
+As the pipeline and stage definitions (e.g. \*Pipeline\*Stage\*.groovy files in the vars folder) are directly executed you can easily test them just by referencing to your repo/branch/tag in the jenkinsfile.
+
+```groovy
+@Library('my-piper-lib-os-fork@MyTest') _
+
+abapEnvironmentPipeline script: this
+```
+
+#### Testing changes on Step Level (Jenkins)
+
+To trigger the creation of a "custom" container with your changes you can reuse a feature in piper which is originally meant for executing the integration tests. If the environment variables 'REPOSITORY_UNDER_TEST' (pointing to your forked repo) and 'LIBRARY_VERSION_UNDER_TEST' (pointing to a tag in your forked repo) are set a corresponding container gets created on the fly upon first usage in the pipeline. The drawback is that this takes extra time (1-2 minutes) you have to spend for every execution of the pipeline.
+
+```groovy
+@Library('piper-lib-os') _
+
+env.REPOSITORY_UNDER_TEST       = 'myfork' // e.g. 'myUser/jenkins-library'
+env.LIBRARY_VERSION_UNDER_TEST  = 'MyTag'
+
+abapEnvironmentPipeline script: this
+```
+
+#### Using Parameterized Pipelines (Jenkins)
+
+For test purpose it can be useful to utilize a parameterized pipeline. E.g. to toggle creation of the custom container:
+
+```groovy
+@Library('my-piper-lib-os-fork@MyTest') _
+
+properties([
+    parameters([
+        booleanParam(name: 'toggleSomething', defaultValue: false, description: 'dito'),
+        booleanParam(name: 'testPiperFork', defaultValue: false, description: 'dito'),
+        string(name: 'repoUnderTest', defaultValue: '<MyUser>/jenkins-library', description: 'dito'),
+        string(name: 'tag', defaultValue: 'MyTest', description: 'dito')
+    ])
+])
+
+if (params.testPiperFork == true) {
+    env.REPOSITORY_UNDER_TEST       = params.repoUnderTest
+    env.LIBRARY_VERSION_UNDER_TEST  = params.tag
+}
+
+abapEnvironmentPipeline script: this
+```
+
+or skipping steps/stages with the help of extensions:
+
+```groovy
+void call(Map piperParams) {
+  echo "Start - Extension for stage: ${piperParams.stageName}"
+
+  if (params.toggleSomething == true) {
+    // do something
+    echo "now execute original stage as defined in the template"
+    piperParams.originalStage()
+  } else {
+    // do something else
+    // e.g. only this singele step of the stage
+    somePiperStep( script: piperParams.script, someConfigParameter: '<...>' )
+  }
+  
+  echo "End - Extension for stage: ${piperParams.stageName}"
+}
+return this
+```
+
 ## Debugging
 
 Debugging can be initiated with VS code fairly easily. Compile the binary with specific compiler flags to turn off optimizations `go build -gcflags "all=-N -l" -o piper.exe`.