[JENKINS-24380]

- Recorded the fix in changelog.
- Added a separate screen to show the unmigration instruction, which
  makes it easier for people to find it than search logs.

changelog.html

@@ -55,6 +55,11 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class='major rfe'>
+    <b><tt>JENKINS_HOME</tt> layout change</b>: builds are now keyed by build numbers and not timestamps.
+    See <a href="https://wiki.jenkins-ci.org/display/JENKINS/JENKINS-24380+Migration">Wiki</a> for details
+    and downgrade.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-24380">issue 24380</a>)
   <li class=bug>
     Fingerprint compaction aggravated lazy-loading performance issues.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-19392">issue 19392</a>)

core/src/main/java/jenkins/model/RunIdMigrator.java

@@ -24,8 +24,10 @@
 
 package jenkins.model;
 
+import hudson.Extension;
 import hudson.Util;
 import hudson.model.Job;
+import hudson.model.RootAction;
 import hudson.model.Run;
 import hudson.util.AtomicFileWriter;
 import hudson.util.StreamTaskListener;
@@ -58,6 +60,8 @@ import org.apache.commons.lang.time.FastDateFormat;
 import org.apache.tools.ant.BuildException;
 import org.kohsuke.accmod.Restricted;
 import org.kohsuke.accmod.restrictions.NoExternalUse;
+import org.kohsuke.stapler.HttpResponse;
+import org.kohsuke.stapler.StaplerProxy;
 import org.kohsuke.stapler.framework.io.WriterOutputStream;
 
 import static java.util.logging.Level.*;
@@ -158,7 +162,12 @@ public final class RunIdMigrator {
         LOGGER.log(INFO, "Migrating build records in {0}", dir);
         doMigrate(dir);
         save(dir);
-        if (jenkinsHome != null && offeredToUnmigrate.add(jenkinsHome)) {
+        if (jenkinsHome != null && offeredToUnmigrate.add(jenkinsHome))
+            LOGGER.log(WARNING, "Build record migration is one-way. If you need to downgrade Jenkins, run: {0}", getUnmigrationCommandLine(jenkinsHome));
+        return true;
+    }
+
+    private static String getUnmigrationCommandLine(File jenkinsHome) {
         StringBuilder cp = new StringBuilder();
         for (Class<?> c : new Class<?>[] {RunIdMigrator.class, /* TODO how to calculate transitive dependencies automatically? */Charsets.class, WriterOutputStream.class, BuildException.class, FastDateFormat.class}) {
             URL location = c.getProtectionDomain().getCodeSource().getLocation();
@@ -175,9 +184,7 @@ public final class RunIdMigrator {
             }
             cp.append(locationS);
         }
-            LOGGER.log(WARNING, "Build record migration is one-way. If you need to downgrade Jenkins, run: java -classpath {0} {1} \"{2}\"", new Object[] {cp, RunIdMigrator.class.getName(), jenkinsHome});
-        }
-        return true;
+        return String.format("java -classpath \"%s\" %s \"%s\"", cp, RunIdMigrator.class.getName(), jenkinsHome);
     }
 
     private static final Pattern NUMBER_ELT = Pattern.compile("(?m)^  <number>(\\d+)</number>(\r?\n)");
@@ -367,4 +374,34 @@ public final class RunIdMigrator {
         System.err.println(builds + " has been restored to its original format");
     }
 
+    /**
+     * Expose unmigration instruction to the user.
+     */
+    @Extension
+    public static class UnmigrationInstruction implements RootAction, StaplerProxy {
+        @Override
+        public String getIconFileName() {
+            return null;
+        }
+
+        @Override
+        public String getDisplayName() {
+            return null;
+        }
+
+        @Override
+        public String getUrlName() {
+            return "JENKINS-24380";
+        }
+
+        @Override
+        public Object getTarget() {
+            Jenkins.getInstance().checkPermission(Jenkins.ADMINISTER);
+            return this;
+        }
+
+        public String getCommand() {
+            return RunIdMigrator.getUnmigrationCommandLine(Jenkins.getInstance().getRootDir());
+        }
+    }
 }

core/src/main/resources/jenkins/model/RunIdMigrator/UnmigrationInstruction/index.jelly

+<!--
+The MIT License
+
+Copyright (c) 2014, CloudBees, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
+-->
+
+<?jelly escape-by-default='true'?>
+<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form" xmlns:i="jelly:fmt">
+  <l:layout title="Jenkins">
+    <l:header />
+    <l:main-panel>
+      <p>
+        To reverse the effect of <a href="http://jenkins-ci.org/issue/24380">JENKINS-24380</a> fix, run the following command
+        on the server. See <a href="https://wiki.jenkins-ci.org/display/JENKINS/JENKINS-24380+Migration">Wiki page</a>
+        for more details:
+      </p>
+      <table style="width:100%">
+        <tr>
+          <td style="line-height:2em; width:80%">
+            <input type="text" value="${it.command}" style="width:100%"/>
+          </td>
+          <td style="line-height:2em">
+            <l:copyButton message="${%Copied}" text="${it.command}"/>
+          </td>
+        </tr>
+      </table>
+    </l:main-panel>
+  </l:layout>
+</j:jelly>