potential race condition in result aggregation

In attempting to diagnose an ATH failure I noticed what looks like a
potential race condition that could lead to results not being correctly
updated.

For example in rare cases (but maybe not so rare in our ATH) if 2 builds finish at
T1, a web request comes in a T2 and then another build fishes at T3
followed by a web request at T4.

When T1 arrives the timestamp is set to T1, and then in T2 the current
value is checked against the timestamp and set to T1+1

When T3 arrives the timestamp is set to T3.

THe issue occurs if you have multiple CPUs and updating the code is fast
and T1 = T2 = T3.

When the request at T4 arrives the code thinks everything is up-to-date when it is not.

Author: jtnord

src/main/java/hudson/tasks/test/AggregatedTestResultPublisher.java

@@ -53,6 +53,7 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
 import jenkins.model.Jenkins;
 import net.sf.json.JSONObject;
 import org.kohsuke.accmod.Restricted;
@@ -126,13 +127,9 @@ public static final class TestResultAction extends AbstractTestResultAction {
         private final boolean includeFailedBuilds;
 
         /**
-         * The last time the fields of this object is computed from the rest.
+         * flag set when we need to recompute the aggregation.
          */
-        private transient long lastUpdated = 0;
-        /**
-         * When was the last time any build completed?
-         */
-        private static long lastChanged = 0;
+        private static final AtomicBoolean RECOMPUTE_NEEDED = new AtomicBoolean(true);
 
         private transient int failCount;
         private transient int totalCount;
@@ -271,11 +268,9 @@ public List<AbstractProject> getNoFingerprints() {
                 justification = "False positive. Short-circuited")
         private synchronized void upToDateCheck() {
             // up to date check
-            if (lastUpdated > lastChanged) {
+            if (!RECOMPUTE_NEEDED.compareAndSet(true, false)) {
                 return;
             }
-            lastUpdated = lastChanged + 1;
-
             int failCount = 0;
             int totalCount = 0;
             List<AbstractTestResultAction> individuals = new ArrayList<>();
@@ -352,7 +347,7 @@ public String getUrlName() {
         public static class RunListenerImpl extends RunListener<Run> {
             @Override
             public void onCompleted(Run run, TaskListener listener) {
-                lastChanged = System.currentTimeMillis();
+                RECOMPUTE_NEEDED.set(true);
             }
         }
     }