Use poll env when checking for ls-remote branches.

src/main/java/hudson/plugins/git/BranchSpec.java

@@ -138,7 +138,7 @@ public List<Branch> filterMatchingBranches(Collection<Branch> branches, EnvVars
         return items;
     }
 
-    private String getExpandedName(EnvVars env) {
+    public String getExpandedName(EnvVars env) {
         String expandedName = env.expand(name);
         if (expandedName.length() == 0) {
             return "**";

src/main/java/hudson/plugins/git/GitSCM.java

@@ -71,6 +71,7 @@
 
 import javax.servlet.ServletException;
 
+import java.util.HashMap;
 import java.io.File;
 import java.io.IOException;
 import java.io.OutputStreamWriter;
@@ -750,21 +751,29 @@ private PollingResult compareRemoteRevisionWithImpl(Job<?, ?> project, Launcher
                         }
                     }
 
+                    Map<BranchSpec, String> headMatches = new HashMap<>();
                     for (BranchSpec branchSpec : getBranches()) {
                         for (Entry<String, ObjectId> entry : heads.entrySet()) {
                             final String head = entry.getKey();
                             // head is "refs/(heads|tags|whatever)/branchName
 
+                            // Use pollEnv here to include Parameters from lastBuild.
                             // first, check the a canonical git reference is configured
-                            if (!branchSpec.matches(head, environment)) {
+                            if (!branchSpec.matches(head, pollEnv)) {
 
                                 // convert head `refs/(heads|tags|whatever)/branch` into shortcut notation `remote/branch`
                                 String name;
                                 Matcher matcher = GIT_REF.matcher(head);
                                 if (matcher.matches()) name = remote + head.substring(matcher.group(1).length());
                                 else name = remote + "/" + head;
 
-                                if (!branchSpec.matches(name, environment)) continue;
+                                // Use pollEnv here to include Parameters from lastBuild.
+                                // Record which branches in the spec we have found a match for so we can alert users when branches are ignored.
+                                if (branchSpec.matches(name, pollEnv)){
+                                    headMatches.put(branchSpec, name);
+                                } else {
+                                    continue;
+                                }
                             }
 
                             final ObjectId sha1 = entry.getValue();
@@ -774,10 +783,22 @@ private PollingResult compareRemoteRevisionWithImpl(Job<?, ?> project, Launcher
                                 continue;
                             }
 
+                            listener.getLogger().println(MessageFormat.format("[poll] pollEnv {0}", pollEnv));
                             listener.getLogger().println("[poll] Latest remote head revision on " + head + " is: " + sha1.getName());
                             return BUILD_NOW;
                         }
                     }
+                    // Tell users if there are branches in the spec that are ignored.
+                    for (BranchSpec branchSpec : getBranches()) {
+                        // If there is a branch in the spec that doesn't exist in the remote, tell the users (as this could means they aren't building something they expect to).
+                        if (!headMatches.containsKey(branchSpec)) {
+                            // If the branchSpec gets expanded using variables in the environment then print the original and expanded versions.
+                            String branchSpecString = branchSpec.toString();
+                            String expandedBranchSpec = branchSpec.getExpandedName(pollEnv);
+                            String branchMessageString = branchSpecString.equals(expandedBranchSpec) ? String.format("'%s'", branchSpecString) : String.format("'%s' (%s)", branchSpecString, expandedBranchSpec);
+                            listener.getLogger().println("[poll] Could not find remote head for branch in spec " + branchMessageString);
+                        }
+                    }
                 }
             }
             return NO_CHANGES;

src/test/java/hudson/plugins/git/GitSCMTest.java

@@ -44,6 +44,7 @@
 import org.jenkinsci.plugins.tokenmacro.TokenMacro;
 import org.jenkinsci.plugins.gitclient.*;
 import org.jenkinsci.plugins.workflow.cps.CpsFlowDefinition;
+import org.jenkinsci.plugins.workflow.cps.CpsScmFlowDefinition;
 import org.jenkinsci.plugins.workflow.job.WorkflowJob;
 import org.jenkinsci.plugins.workflow.job.WorkflowRun;
 import org.junit.Rule;
@@ -2467,6 +2468,41 @@ public void testPolling_environmentValueInBranchSpec() throws Exception {
         assertFalse("No changes to git since last build, thus no new build is expected", project.poll(listener).hasChanges());
     }
 
+    @Test
+    @Issue("JENKINS-20427")
+    public void testPolling_environmentValueInBranchSpecTriggersBuild() throws Exception {
+        assumeTrue("Test class max time " + MAX_SECONDS_FOR_THESE_TESTS + " exceeded", isTimeAvailable());
+        // create parameterized project with environment value in branch specification
+        WorkflowJob project = r.jenkins.createProject(WorkflowJob.class, "pipeline-remote-poll-with-param");
+        List<UserRemoteConfig> remotes = new ArrayList<>();
+        remotes.addAll(testRepo.remoteConfigs());
+        GitSCM scm = new GitSCM(
+                remotes,
+                Collections.singletonList(new BranchSpec("${MY_BRANCH}")),
+                null, null,
+                Collections.emptyList());
+        project.setDefinition(new CpsScmFlowDefinition(scm, "./Jenkinsfile"));
+        project.addProperty(new ParametersDefinitionProperty(new StringParameterDefinition("MY_BRANCH", "master")));
+
+        // commit something in order to create an initial base version in git
+        commit(
+            "Jenkinsfile",
+            "node {\n" +
+            "  echo 'Hello world!'\n" +
+            "}",
+            johnDoe,
+            "Add Jenkinsfile"
+        );
+
+        // build the project
+        Run<?,?> run = r.buildAndAssertStatus(Result.SUCCESS, project);
+
+        // commit something in order to check that polling detects the change.
+        commit("toto/commitFile2", johnDoe, "Commit number 2");
+
+        assertTrue("There are changes to git since last build, thus a new build is expected", project.poll(listener).hasChanges());
+    }
+
     public void baseTestPolling_parentHead(List<GitSCMExtension> extensions) throws Exception {
         // create parameterized project with environment value in branch specification
         FreeStyleProject project = createFreeStyleProject();