keith-turner commented on code in PR #4840:
URL: https://github.com/apache/accumulo/pull/4840#discussion_r1733383593


##########
server/tserver/src/main/java/org/apache/accumulo/tserver/session/ScanSession.java:
##########
@@ -184,17 +191,71 @@ public TabletResolver getTabletResolver() {
   }
 
   public ScanTask<T> getScanTask() {
-    return scanTask;
+    return scanTaskRef.get();
   }
 
   public void setScanTask(ScanTask<T> scanTask) {
-    this.scanTask = scanTask;
+    Objects.requireNonNull(scanTask);
+    scanTaskRef.getAndUpdate(currScanTask -> {
+      Preconditions.checkState(currScanTask == null,
+          "Unable to set a scan task when one is already set");
+      return scanTask;
+    });
+  }
+
+  public void clearScanTask() {
+    scanTaskRef.getAndUpdate(currScanTask -> {
+      // For tracking zombie scan threads, do not want to clear the scan task 
if it has an active
+      // thread. When the thread is not null and the task has produced a 
result, the thread should
+      // be in
+      // the process of clearing itself from the scan task.
+      Preconditions.checkState(
+          currScanTask == null || currScanTask.getScanThread() == null
+              || currScanTask.producedResult(),
+          "Can not clear scan task that is still running and has not produced 
a result");
+      return null;
+    });
+  }
+
+  private boolean loggedZombieStackTrace = false;
+
+  public void logZombieStackTrace() {
+    Preconditions.checkState(getState() == State.REMOVED);
+    var scanTask = scanTaskRef.get();
+    if (scanTask != null) {
+      ScanTask.ScanThreadStackTrace scanStackTrace = scanTask.getStackTrace();
+      if (scanStackTrace != null && !loggedZombieStackTrace) {
+        var changeTimeMillis = elaspedSinceStateChange(TimeUnit.MILLISECONDS);
+        var exception =

Review Comment:
   > @keith-turner do you think we should also include the specific tablet info 
the zombie scan was attempting? Or is the tableID enough?
   
   It may be good to include the scan session id in order to correlate w/ 
output of list scans, however that is a bit of mess at the moment.   I am going 
to open a follow on issue about that.
   
   Including the thead name seems useful, I can add that.



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