[FIX JENKINS-18048] Fix link of SCMTriggerCause from UpstreamCause

- UpstreamCause now tries to initialize upstream causes with the
  correct build in onLoad
- SCMTriggerCause stores build reference and uses its URL to build
  absolute link to changelog

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

@@ -157,6 +157,26 @@ private UpstreamCause(String upstreamProject, int upstreamBuild, String upstream
             this.upstreamCauses = upstreamCauses;
         }
 
+        @Override
+        public void onLoad(@Nonnull AbstractBuild<?, ?> build) {
+            Item i = Jenkins.getInstance().getItemByFullName(this.upstreamProject);
+            if (i == null || !(i instanceof AbstractProject)) {
+                // cannot initialize upstream causes
+                return;
+            }
+
+            AbstractProject j = (AbstractProject)i;
+            AbstractBuild r = j.getBuildByNumber(this.getUpstreamBuild());
+            if (r == null) {
+                // build doesn't exist anymore
+                return;
+            }
+
+            for (Cause c : this.upstreamCauses) {
+                c.onLoad(r);
+            }
+        }
+
         /**
          * @since 1.515
          */

core/src/main/java/hudson/triggers/SCMTrigger.java

@@ -46,6 +46,8 @@
 import hudson.util.SequentialExecutionQueue;
 import org.apache.commons.io.FileUtils;
 import org.apache.commons.jelly.XMLOutput;
+import org.kohsuke.accmod.Restricted;
+import org.kohsuke.accmod.restrictions.DoNotUse;
 import org.kohsuke.stapler.StaplerRequest;
 import org.kohsuke.stapler.DataBoundConstructor;
 
@@ -72,6 +74,8 @@
 import static java.util.logging.Level.*;
 import jenkins.model.RunAction2;
 
+import javax.annotation.Nonnull;
+
 /**
  * {@link Trigger} that checks for SCM updates periodically.
  *
@@ -536,6 +540,8 @@ public static class SCMTriggerCause extends Cause {
          */
         private String pollingLog;
 
+        private transient AbstractBuild<?, ?>  build;
+
         public SCMTriggerCause(File logFile) throws IOException {
             // TODO: charset of this log file?
             this(FileUtils.readFileToString(logFile));
@@ -553,8 +559,14 @@ public SCMTriggerCause() {
             this("");
         }
 
+        @Override
+        public void onLoad(@Nonnull AbstractBuild<?, ?> build) {
+            this.build = build;
+        }
+
         @Override
         public void onAddedTo(AbstractBuild build) {
+            this.build = build;
             try {
                 BuildAction a = new BuildAction(build);
                 FileUtils.writeStringToFile(a.getPollingLogFile(),pollingLog);
@@ -570,6 +582,11 @@ public String getShortDescription() {
             return Messages.SCMTrigger_SCMTriggerCause_ShortDescription();
         }
 
+        @Restricted(DoNotUse.class)
+        public AbstractBuild<?, ?> getBuild() {
+            return this.build;
+        }
+
         @Override
         public boolean equals(Object o) {
             return o instanceof SCMTriggerCause;

core/src/main/resources/hudson/triggers/SCMTrigger/SCMTriggerCause/description.jelly

@@ -23,5 +23,6 @@ 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">
-    <span><a href="pollingLog">${it.shortDescription}</a></span>
+    <!-- this link loses breadcrumb context, but need absolute URL for upstream cause support (JENKINS-18048) -->
+    <span><a href="${rootURL}/${it.build.url}pollingLog">${it.shortDescription}</a></span>
 </j:jelly>