[FIXED JENKINS-42836] env expand the script path from SCM.

src/main/java/org/jenkinsci/plugins/workflow/cps/CpsScmFlowDefinition.java

@@ -103,11 +103,12 @@ public boolean isLightweight() {
             throw new IOException("can only check out SCM into a Run");
         }
         Run<?,?> build = (Run<?,?>) _build;
+        String expandedScriptPath = build.getEnvironment(listener).expand(scriptPath);
         if (isLightweight()) {
             try (SCMFileSystem fs = SCMFileSystem.of(build.getParent(), scm)) {
                 if (fs != null) {
-                    String script = fs.child(scriptPath).contentAsString();
-                    listener.getLogger().println("Obtained " + scriptPath + " from " + scm.getKey());
+                    String script = fs.child(expandedScriptPath).contentAsString();
+                    listener.getLogger().println("Obtained " + expandedScriptPath + " from " + scm.getKey());
                     Queue.Executable exec = owner.getExecutable();
                     FlowDurabilityHint hint = (exec instanceof Item) ? DurabilityHintProvider.suggestedFor((Item)exec) : GlobalDefaultFlowDurabilityLevel.getDefaultDurabilityHint();
                     return new CpsFlowExecution(script, true, owner, hint);
@@ -127,7 +128,7 @@ public boolean isLightweight() {
         } else { // should not happen, but just in case:
             dir = new FilePath(owner.getRootDir());
         }
-        listener.getLogger().println("Checking out " + scm.getKey() + " into " + dir + " to read " + scriptPath);
+        listener.getLogger().println("Checking out " + scm.getKey() + " into " + dir + " to read " + expandedScriptPath);
         String script = null;
         Computer computer = node.toComputer();
         if (computer == null) {
@@ -161,8 +162,8 @@ public boolean isLightweight() {
                 listener.getLogger().println("Retrying after 10 seconds");
                 Thread.sleep(10000);
             }
-            
-            FilePath scriptFile = dir.child(scriptPath);
+
+            FilePath scriptFile = dir.child(expandedScriptPath);
             if (!scriptFile.absolutize().getRemote().replace('\\', '/').startsWith(dir.absolutize().getRemote().replace('\\', '/') + '/')) { // TODO JENKINS-26838
                 throw new IOException(scriptFile + " is not inside " + dir);
             }

src/test/java/org/jenkinsci/plugins/workflow/cps/CpsScmFlowDefinitionTest.java

@@ -202,4 +202,22 @@
         r.assertLogContains("version one", r.assertBuildStatusSuccess(p.scheduleBuild2(0, new ParametersAction(new StringParameterValue("VERSION", "one")))));
     }
 
+    @Issue("JENKINS-42836")
+    @Test public void usingParameterInScriptPath() throws Exception {
+        sampleRepo.init();
+        sampleRepo.write("flow.groovy", "echo 'version one'");
+        sampleRepo.git("add", "flow.groovy");
+        sampleRepo.write("otherFlow.groovy", "echo 'version two'");
+        sampleRepo.git("add", "otherFlow.groovy");
+        sampleRepo.git("commit", "--all", "--message=commits");
+        WorkflowJob p = r.jenkins.createProject(WorkflowJob.class, "p");
+        CpsScmFlowDefinition def = new CpsScmFlowDefinition(new GitSCM(Collections.singletonList(new UserRemoteConfig(sampleRepo.fileUrl(), null, null, null)),
+                Collections.singletonList(new BranchSpec("master")),
+                false, Collections.<SubmoduleConfig>emptyList(), null, null, Collections.<GitSCMExtension>emptyList()), "${SCRIPT_PATH}");
+        def.setLightweight(false); // TODO SCMFileSystem.of cannot pick up build parameters
+        p.setDefinition(def);
+        p.addProperty(new ParametersDefinitionProperty(new StringParameterDefinition("SCRIPT_PATH", "flow.groovy")));
+        r.assertLogContains("version one", r.assertBuildStatusSuccess(p.scheduleBuild2(0)));
+        r.assertLogContains("version two", r.assertBuildStatusSuccess(p.scheduleBuild2(0, new ParametersAction(new StringParameterValue("SCRIPT_PATH", "otherFlow.groovy")))));
+    }
 }