[FIX JENKINS-20520] Save Jenkins after adding global user permission

src/main/java/hudson/security/GlobalMatrixAuthorizationStrategy.java

@@ -533,6 +533,11 @@ private String makeImg(String img, String tooltip, boolean inPlugin) {
         @Override public boolean add(AuthorizationStrategy strategy, User user, Permission perm) {
             if (strategy instanceof GlobalMatrixAuthorizationStrategy) {
                 ((GlobalMatrixAuthorizationStrategy) strategy).add(perm, user.getId());
+                try {
+                    Jenkins.getInstance().save();
+                } catch (IOException ioe) {
+                    LOGGER.log(Level.WARNING, "Failed to save Jenkins after adding permission for user: " + user.getId(), ioe);
+                }
                 return true;
             } else {
                 return false;

src/test/java/org/jenkinsci/plugins/matrixauth/PermissionAdderTest.java

@@ -0,0 +1,51 @@
+package org.jenkinsci.plugins.matrixauth;
+
+import com.gargoylesoftware.htmlunit.html.HtmlPage;
+import hudson.model.User;
+import hudson.security.ACL;
+import hudson.security.ACLContext;
+import hudson.security.GlobalMatrixAuthorizationStrategy;
+import hudson.security.HudsonPrivateSecurityRealm;
+import hudson.security.pages.SignupPage;
+import jenkins.model.Jenkins;
+import org.junit.Assert;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runners.model.Statement;
+import org.jvnet.hudson.test.Issue;
+import org.jvnet.hudson.test.JenkinsRule;
+import org.jvnet.hudson.test.RestartableJenkinsRule;
+
+public class PermissionAdderTest {
+
+    @Rule
+    public RestartableJenkinsRule r = new RestartableJenkinsRule();
+
+    @Test
+    @Issue("JENKINS-20520")
+    public void ensureSavingAfterInitialUser() {
+        r.addStep(new Statement() {
+            @Override
+            public void evaluate() throws Throwable {
+                r.j.jenkins.setSecurityRealm(new HudsonPrivateSecurityRealm(true));
+                r.j.jenkins.setAuthorizationStrategy(new GlobalMatrixAuthorizationStrategy());
+                r.j.jenkins.save();
+
+                JenkinsRule.WebClient wc = r.j.createWebClient();
+                SignupPage signup = new SignupPage(wc.goTo("signup"));
+                signup.enterUsername("alice");
+                signup.enterPassword("alice");
+                signup.enterFullName("Alice User");
+                HtmlPage success = signup.submit(r.j);
+
+                Assert.assertTrue(r.j.jenkins.getACL().hasPermission(User.get("alice").impersonate(), Jenkins.ADMINISTER));
+            }
+        });
+        r.addStep(new Statement() {
+            @Override
+            public void evaluate() throws Throwable {
+                Assert.assertTrue(r.j.jenkins.getACL().hasPermission(User.get("alice").impersonate(), Jenkins.ADMINISTER));
+            }
+        });
+    }
+}