fixes exception for canceled scan task (apache#5093)

An exception was happening when a scan task was canceled before it ever
ran.  This was caused by transitionFromRunning() executing when
transitoinToRunning() had never executed.  When this happened a
precondition check that the scanThread was non-null would fail.  These
changes only execute transitionFromRunning() when transitionToRunning()
has executed.

Author: keith-turner

server/tserver/src/main/java/org/apache/accumulo/tserver/scan/LookupTask.java

@@ -64,15 +64,16 @@ public void run() {
     MultiScanSession session = (MultiScanSession) server.getSession(scanID);
     String oldThreadName = Thread.currentThread().getName();
 
+    if (!transitionToRunning()) {
+      return;
+    }
+    // Do not add any code here. Need to ensure that transitionFromRunning() runs in the finally
+    // block when transitionToRunning() returns true.
     try {
       if (isCancelled() || session == null) {
         return;
       }
 
-      if (!transitionToRunning()) {
-        return;
-      }
-
       TableConfiguration acuTableConf = server.getTableConfiguration(session.threadPoolExtent);
       long maxResultsSize = acuTableConf.getAsBytes(Property.TABLE_SCAN_MAXMEM);
 

server/tserver/src/main/java/org/apache/accumulo/tserver/scan/NextBatchTask.java

@@ -54,15 +54,16 @@ public void run() {
     final SingleScanSession scanSession = (SingleScanSession) server.getSession(scanID);
     String oldThreadName = Thread.currentThread().getName();
 
+    if (!transitionToRunning()) {
+      return;
+    }
+    // Do not add any code here. Need to ensure that transitionFromRunning() runs in the finally
+    // block when transitionToRunning() returns true.
     try {
       if (isCancelled() || scanSession == null) {
         return;
       }
 
-      if (!transitionToRunning()) {
-        return;
-      }
-
       Thread.currentThread()
           .setName("User: " + scanSession.getUser() + " Start: " + scanSession.startTime
               + " Client: " + scanSession.client + " Tablet: " + scanSession.extent);