[JENKINS-68030] Optionally report most recent commit as changelog of a new build

README.adoc

@@ -1220,6 +1220,23 @@ Inverse::
   This is useful, for example, when you have jobs building your master and various release branches and you want a second job which builds all new feature branches.
   For example, branches which do not match these patterns without redundantly building master and the release branches again each time they change.
 
+[#first-build-changelog]
+==== First build changelog
+
+image:/images/git-extension-for-first-build.png[First build changelog]
+
+The Jenkins git plugin provides an option to trigger a Pipeline build on the first commit on a branch.
+By default, no changelog is generated for the first build because the first build has no predecessor build for comparison.
+When the first build changelog option is enabled, the most recent commit will be used as the changelog of the first build.
+
+[source,groovy]
+----
+checkout scmGit(
+    branches: [[name: 'master']],
+    extensions: [ firstBuildChangelog() ],
+    userRemoteConfigs: [[url: 'https://github.com/jenkinsci/git-plugin.git']])
+----
+
 [#merge-extensions]
 === Merge Extensions
 

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

@@ -28,6 +28,7 @@
 import hudson.plugins.git.extensions.impl.BuildSingleRevisionOnly;
 import hudson.plugins.git.extensions.impl.ChangelogToBranch;
 import hudson.plugins.git.extensions.impl.CloneOption;
+import hudson.plugins.git.extensions.impl.FirstBuildChangelog;
 import hudson.plugins.git.extensions.impl.PathRestriction;
 import hudson.plugins.git.extensions.impl.LocalBranch;
 import hudson.plugins.git.extensions.impl.RelativeTargetDirectory;
@@ -1493,9 +1494,14 @@
             }
 
             if (!exclusion) {
-                // this is the first time we are building this branch, so there's no base line to compare against.
-                // if we force the changelog, it'll contain all the changes in the repo, which is not what we want.
-                listener.getLogger().println("First time build. Skipping changelog.");
+                FirstBuildChangelog firstBuildChangelog = getExtensions().get(FirstBuildChangelog.class);
+                if (firstBuildChangelog != null && firstBuildChangelog.isMakeChangelog()) {
+                    changelog.to(out).max(1).execute();
+                    executed = true;
+                    listener.getLogger().println("First time build. Latest changes added to changelog.");
+                } else {
+                    listener.getLogger().println("First time build. Skipping changelog.");
+                }
             } else {
                 changelog.to(out).max(MAX_CHANGELOG).execute();
                 executed = true;

src/main/java/hudson/plugins/git/extensions/impl/FirstBuildChangelog.java

@@ -0,0 +1,76 @@
+package hudson.plugins.git.extensions.impl;
+
+import hudson.Extension;
+import hudson.plugins.git.extensions.GitSCMExtension;
+import hudson.plugins.git.extensions.GitSCMExtensionDescriptor;
+import java.util.Objects;
+import org.jenkinsci.Symbol;
+import org.kohsuke.stapler.DataBoundConstructor;
+import org.kohsuke.stapler.DataBoundSetter;
+
+/**
+ * First Build generates a changelog.
+ *
+ * @author Derek Inskeep
+ */
+public class FirstBuildChangelog extends GitSCMExtension {
+    private boolean makeChangelog;
+
+    @DataBoundConstructor
+    public FirstBuildChangelog() {
+        makeChangelog = true;
+    }
+
+    public boolean isMakeChangelog() {
+        return makeChangelog;
+    }
+
+    @DataBoundSetter
+    public void setMakeChangelog(boolean makeChangelog) {
+        this.makeChangelog = makeChangelog;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        FirstBuildChangelog that = (FirstBuildChangelog) o;
+        return makeChangelog == that.makeChangelog;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public int hashCode() {
+        return Objects.hash(makeChangelog);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public String toString() {
+        return "FirstBuildChangelog{" + "makeChangelog=" + makeChangelog + '}';
+    }
+
+    @Extension
+    @Symbol("firstBuildChangelog")
+    public static class DescriptorImpl extends GitSCMExtensionDescriptor {
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public String getDisplayName() {
+            return "First build changelog";
+        }
+    }
+}

src/main/java/jenkins/plugins/git/traits/FirstBuildChangelogTrait.java

@@ -0,0 +1,49 @@
+package jenkins.plugins.git.traits;
+
+import edu.umd.cs.findbugs.annotations.CheckForNull;
+import hudson.Extension;
+import hudson.plugins.git.extensions.impl.FirstBuildChangelog;
+import jenkins.scm.api.trait.SCMSourceTrait;
+import org.jenkinsci.Symbol;
+import org.kohsuke.stapler.DataBoundConstructor;
+
+/**
+ * Exposes {@link FirstBuildChangelog} as a {@link SCMSourceTrait}.
+ *
+ * @since 5.3.0
+ */
+public class FirstBuildChangelogTrait extends GitSCMExtensionTrait<FirstBuildChangelog> {
+
+    /**
+     * @deprecated Use constructor that accepts extension instead.
+     */
+    @Deprecated
+    public FirstBuildChangelogTrait() {
+        this(null);
+    }
+
+    /**
+     * Stapler constructor.
+     *
+     * @param extension the option to force first build to have a non-empty changelog.
+     */
+    @DataBoundConstructor
+    public FirstBuildChangelogTrait(@CheckForNull FirstBuildChangelog extension) {
+        super(extension == null ? new FirstBuildChangelog() : extension);
+    }
+
+    /**
+     * Our {@link hudson.model.Descriptor}
+     */
+    @Extension
+    @Symbol("firstBuildChangelog")
+    public static class DescriptorImpl extends GitSCMExtensionTraitDescriptor {
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public String getDisplayName() {
+            return "First Build Changelog";
+        }
+    }
+}

src/main/resources/hudson/plugins/git/extensions/impl/FirstBuildChangelog/help.html

@@ -0,0 +1,5 @@
+<div>
+  First builds will populate the changelog with the latest commit, if any, to allow Pipelines to check and test for file changes.
+  By default, no changelog is generated for the first build because the first build has no predecessor build for comparison.
+  When the first build changelog option is enabled, the most recent commit on the branch will be used as the changelog of the first build.
+</div>

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

@@ -1188,6 +1188,25 @@ public void testCleanBeforeCheckout() throws Exception {
         assertThat("Cleaning should happen before fetch", cleaningLogLine, is(lessThan(fetchingLogLine)));
     }
 
+    @Test
+    public void testFirstBuiltChangelog() throws Exception {
+        assumeTrue("Test class max time " + MAX_SECONDS_FOR_THESE_TESTS + " exceeded", isTimeAvailable());
+        FreeStyleProject p = setupProject("master", false, null, null, "Jane Doe", null);
+        FirstBuildChangelog fbc = new FirstBuildChangelog();
+        ((GitSCM) p.getScm()).getExtensions().add(fbc);
+
+        /* First build should should generate a changelog */
+        final String commitFile1 = "commitFile1";
+        commit(commitFile1, johnDoe, janeDoe, "Commit number 1");
+        final FreeStyleBuild firstBuild = build(p, Result.SUCCESS, commitFile1);
+        assertThat(firstBuild.getLog(50), hasItem("First time build. Latest changes added to changelog."));
+        /* Second build should have normal behavior */
+        final String commitFile2 = "commitFile2";
+        commit(commitFile2, johnDoe, janeDoe, "Commit number 2");
+        final FreeStyleBuild secondBuild = build(p, Result.SUCCESS, commitFile2);
+        assertThat(secondBuild.getLog(50), not(hasItem("First time build. Latest changes added to changelog.")));
+    }
+
     @Issue("JENKINS-8342")
     @Test
     public void testExcludedRegionMultiCommit() throws Exception {