Prevent dormant multi branch project continiously building.

JENKINS-63494 and JENKINS-64193

When scanning a multi branch project if the history of changes is outside of the `Head change query limit` then no changes are found.  The old behaviour used the latest change and triggered a build; continuously triggering dormant projects with each new submit.

The new behaviour uses the last build change if no changes are found (preventing a triggered build) or if there was no previous builds (in the case of new projects) then the latest changes is used to triggering a build.

src/main/java/org/jenkinsci/plugins/p4/client/ClientHelper.java

@@ -193,7 +193,7 @@ private void updateClient() throws Exception {
 				sb.append(view.getRight());
 				sb.append("\n");
 			}
-			if ( iclient.getStream() != null ) {
+			if (iclient.getStream() != null) {
 				sb.append("...   Stream: " + iclient.getStream());
 				sb.append("\n");
 			}
@@ -717,8 +717,8 @@ private List<String> buildPaths(Publish publish, String clientBase) {
 		}
 
 		String[] array = rawPaths.split("\n\\s*");
-		for(String a : array) {
-			if(a.startsWith("//")) {
+		for (String a : array) {
+			if (a.startsWith("//")) {
 				list.add(a);
 			} else {
 				list.add(clientBase + a);
@@ -1089,16 +1089,11 @@ public Changelist getChange(long id) throws Exception {
 	 * workspace view up to the specified revision
 	 *
 	 * @param from From revision (change or label)
-	 * @param to To revision (change or label)
+	 * @param to   To revision (change or label)
 	 * @return Perforce change
 	 * @throws Exception push up stack
 	 */
 	public long getClientHead(P4Ref from, P4Ref to) throws Exception {
-		// get last change in server
-		// This will also return shelved CLs
-		String latestChange = getConnection().getCounter("change");
-		long change = Long.parseLong(latestChange);
-
 		// build file revision spec
 		String path = "//" + iclient.getName() + "/...";
 		String revisionPath = buildRevisionLimit(path, from, to);
@@ -1111,25 +1106,29 @@ public long getClientHead(P4Ref from, P4Ref to) throws Exception {
 		List<IChangelistSummary> list = getConnection().getChangelists(files, opts);
 
 		if (!list.isEmpty() && list.get(0) != null) {
-			change = list.get(0).getId();
-		} else if ( to != null) {
-			change = to.getChange();
-			log("P4: no revisions under " + revisionPath + " using change: " + change);
+			long change = list.get(0).getId();
+			log("P4: found " + change + " revision in " + revisionPath);
+			return change;
 		} else {
-			log("P4: no revisions under " + revisionPath + " using latest change: " + change);
+			log("P4: no revisions under " + revisionPath);
+			return 0L;
 		}
-		return change;
 	}
 
 	/**
 	 * Get the change number for the last change within the scope of the
-	 * workspace view.
+	 * workspace view. If there are no recent changes use the latest change.
 	 *
 	 * @return Perforce change
 	 * @throws Exception push up stack
 	 */
 	public long getClientHead() throws Exception {
-		return getClientHead(null, null);
+		// get last change in server, may return shelved CLs
+		String latestChange = getConnection().getCounter("change");
+		long latest = Long.parseLong(latestChange);
+		P4Ref to = new P4ChangeRef(latest);
+		long head = getClientHead(null, to);
+		return (head == 0L) ? latest : head;
 	}
 
 	public List<IChangelistSummary> getPendingChangelists(boolean includeLongDescription, String clientName) throws Exception {

src/main/java/org/jenkinsci/plugins/p4/scm/AbstractP4ScmSource.java

@@ -320,7 +320,7 @@ protected List<String> toLines(String value) {
 	/**
 	 * Get the Latest change for the path specified in P4SCMHead.
 	 *
-	 * @param p4 TempClient instance
+	 * @param p4   TempClient instance
 	 * @param head SCMHead
 	 * @return The latest change as a P4SCMRevision object
 	 * @throws Exception pushed up stack
@@ -338,12 +338,28 @@ public P4SCMRevision getRevision(TempClientHelper p4, P4SCMHead head) throws Exc
 		// Fetch last scan
 		P4SCMRevision last = getLastScan(head);
 
-		/* If 'Polling per Change' is disabled then get the latest change; otherwise use the latest change for the
-		 * first scan then the oldest un-built change.
+		/* Calculate changes to use for SCMRevision:
+		 *   If 'Polling per Change' use the next change on the code-line, else the latest change for the code-line.
+		 *   If there are no changes within the the scan limits (set by `Head change query limit`) the change will be 0;
+		 *   in these cases return the last built change, or for now projects the latest change.
 		 */
-		if (last == null || !perChange) {
+		if (last == null) {
+			// possibly a new project or broken configuration...
 			change = findLatestChange(p4, path);
+			if (change == 0) {
+				// no changes? - use the latest and trigger a build
+				change = p4.getClientHead();
+			}
+		} else if (!perChange) {
+			// Typical case...
+			change = findLatestChange(p4, path);
+			if (change == 0) {
+				// JENKINS-63494 and JENKINS-64193
+				// dormant project outside the change limits; use last built change
+				change = last.getRef().getChange();
+			}
 		} else {
+			// Polling per Change - get next change.
 			change = findIncrementalChange(p4, path, last.getRef());
 		}
 

src/main/java/org/jenkinsci/plugins/p4/tasks/CheckoutTask.java

@@ -122,7 +122,8 @@ public void initialise() throws AbortException {
 				long rangeLimit = buildChange.getChange() - p4.getHeadLimit();
 				rangeLimit = (rangeLimit < 0) ? 1 : rangeLimit;
 				P4ChangeRef limitRef = (p4.getHeadLimit() == 0) ? null : new P4ChangeRef(rangeLimit);
-				buildChange = new P4ChangeRef(p4.getClientHead(limitRef, buildChange));
+				long headChange = p4.getClientHead(limitRef, buildChange);
+				buildChange = (headChange == 0) ? buildChange : new P4ChangeRef(headChange);
 			}
 
 			// add buildChange to list of changes to builds