Add A Shutdown Hook for the Buildserver

Add a Shutdown Hook. In a container swarm, the swarm orchestrator may
choose to shutdown a container (running a buildserver) as part of load
balancing and other maintenance tasks. It will send a SIGTERM signal to
the container which will send it to us. This shutdown hook causes us to
wait until all build tasks are completed before we exit, ensuring that
people's build jobs are not interrupted We combine this code with a
configuration in the swarm service to *not* hard kill a container for a
period of time (say 15 minutes) to give the buildserver a chance to
finish outstanding jobs.

Change-Id: Ie0fed539162e219af3df96a1402f153fc0473f0e

appinventor/buildserver/src/com/google/appinventor/buildserver/BuildServer.java

@@ -683,6 +683,43 @@ public class BuildServer {
       System.exit(1);
     }
 
+    // Add a Shutdown Hook. In a container swarm, the swarm orchestrator
+    // may choose to shutdown a container (running a buildserver) as part
+    // of load balancing and other maintenance tasks. It will send a
+    // SIGTERM signal to the container which will send it to us. This
+    // shutdown hook causes us to wait until all build tasks are completed
+    // before we exit, ensuring that people's build jobs are not interrupted
+    // We combine this code with a configuration in the swarm service to *not*
+    // hard kill a container for a period of time (say 15 minutes) to give
+    // running jobs a chance to finish.
+    Runtime.getRuntime().addShutdownHook(new Thread() {
+        @Override
+        public void run() {
+          shuttingTime = System.currentTimeMillis();
+          if (buildExecutor == null) {
+            /* We haven't really started up yet... */
+            return;
+          }
+          while (true) {
+            int tasks = buildExecutor.getActiveTaskCount();
+            if (tasks <= 0) {
+              try {
+                Thread.sleep(10000); // One final wait so people can get
+                                     // their barcode
+              } catch (InterruptedException e) {
+              }
+              return;
+            }
+            try {
+              Thread.sleep(1000); // Wait one second and try again
+            } catch (InterruptedException e) {
+              // XXX
+            }
+          }
+        }
+      });
+
+
     // Now that the command line options have been processed, we can create the buildExecutor.
     buildExecutor = new NonQueuingExecutor(commandLineOptions.maxSimultaneousBuilds);