[FIXED JENKINS-31518] Translate Job.logRotator to BuildDiscarderProperty more seamlessly.

core/src/main/java/hudson/model/Job.java

@@ -430,7 +430,7 @@ public abstract class Job<JobT extends Job<JobT, RunT>, RunT extends Run<JobT, R
      * Returns the configured build discarder for this job, via {@link BuildDiscarderProperty}, or null if none.
      */
     public synchronized BuildDiscarder getBuildDiscarder() {
-        BuildDiscarderProperty prop = getProperty(BuildDiscarderProperty.class);
+        BuildDiscarderProperty prop = _getProperty(BuildDiscarderProperty.class);
         return prop != null ? prop.getStrategy() : /* settings compatibility */ logRotator;
     }
 
@@ -553,9 +553,13 @@ public abstract class Job<JobT extends Job<JobT, RunT>, RunT extends Run<JobT, R
     /**
      * Gets all the job properties configured for this job.
      */
-    @SuppressWarnings("unchecked")
+    @SuppressWarnings({"unchecked", "rawtypes"})
     public Map<JobPropertyDescriptor, JobProperty<? super JobT>> getProperties() {
-        return Descriptor.toMap((Iterable) properties);
+        Map result = Descriptor.toMap((Iterable) properties);
+        if (logRotator != null) {
+            result.put(Jenkins.getActiveInstance().getDescriptorByType(BuildDiscarderProperty.DescriptorImpl.class), new BuildDiscarderProperty(logRotator));
+        }
+        return result;
     }
 
     /**
@@ -572,6 +576,13 @@ public abstract class Job<JobT extends Job<JobT, RunT>, RunT extends Run<JobT, R
      * this job.
      */
     public <T extends JobProperty> T getProperty(Class<T> clazz) {
+        if (clazz == BuildDiscarderProperty.class && logRotator != null) {
+            return clazz.cast(new BuildDiscarderProperty(logRotator));
+        }
+        return _getProperty(clazz);
+    }
+
+    private <T extends JobProperty> T _getProperty(Class<T> clazz) {
         for (JobProperty p : properties) {
             if (clazz.isInstance(p))
                 return clazz.cast(p);
@@ -1196,6 +1207,8 @@ public abstract class Job<JobT extends Job<JobT, RunT>, RunT extends Run<JobT, R
         try {
             setDisplayName(json.optString("displayNameOrNull"));
 
+            logRotator = null;
+
             DescribableList<JobProperty<?>, JobPropertyDescriptor> t = new DescribableList<JobProperty<?>, JobPropertyDescriptor>(NOOP,getAllProperties());
             JSONObject jsonProperties = json.optJSONObject("properties");
             if (jsonProperties != null) {

test/src/test/java/jenkins/model/BuildDiscarderPropertyTest.java

@@ -25,6 +25,7 @@
 package jenkins.model;
 
 import hudson.model.AbstractProject;
+import hudson.model.FreeStyleProject;
 import hudson.tasks.LogRotator;
 import java.io.StringReader;
 import javax.xml.transform.Source;
@@ -41,6 +42,29 @@ public class BuildDiscarderPropertyTest {
     @Rule
     public JenkinsRule r = new JenkinsRule();
 
+    @Issue("JENKINS-31518")
+    @LocalData
+    @Test
+    public void buildDiscarderField() throws Exception {
+        FreeStyleProject p = r.jenkins.getItemByFullName("p", FreeStyleProject.class);
+        verifyBuildDiscarder(p);
+        r.configRoundtrip(p);
+        verifyBuildDiscarder(p);
+        String xml = p.getConfigFile().asString();
+        assertFalse(xml, xml.contains("<logRotator class="));
+        assertTrue(xml, xml.contains("<" + BuildDiscarderProperty.class.getName() + ">"));
+    }
+
+    private void verifyBuildDiscarder(FreeStyleProject p) {
+        BuildDiscarder bd = p.getBuildDiscarder();
+        assertNotNull(bd);
+        LogRotator lr = (LogRotator) bd;
+        assertEquals(7, lr.getDaysToKeep());
+        assertEquals(10, lr.getNumToKeep());
+        assertNotNull(p.getProperty(BuildDiscarderProperty.class));
+        assertEquals(1, p.getProperties().size());
+    }
+
     @Issue("JENKINS-16979")
     @LocalData
     @Test