Improve build server shutdown procedure

Use ServerCommand.STOP_SERVER as a first approach to make the build-
server shutdown. If that doesn't work send signal SIGTERM and if that
still doesn't work kill the server by sending SIGKILL.

substratevm/src/com.oracle.svm.driver/src/com/oracle/svm/driver/NativeImageServer.java

@@ -22,6 +22,7 @@
  */
 package com.oracle.svm.driver;
 
+import static com.oracle.svm.core.posix.headers.Signal.SignalEnum.SIGKILL;
 import static com.oracle.svm.core.posix.headers.Signal.SignalEnum.SIGTERM;
 
 import java.io.ByteArrayOutputStream;
@@ -223,20 +224,35 @@ void abortTask() {
         }
 
         synchronized void shutdown() {
-            Signal.kill(pid, SIGTERM.getCValue());
+            ByteArrayOutputStream baos = new ByteArrayOutputStream();
+            sendRequest(byteStreamToByteConsumer(baos), byteStreamToByteConsumer(baos), ServerCommand.STOP_SERVER);
+            showVerboseMessage(verboseServer, "Server stop response:" + new String(baos.toByteArray()));
+            long terminationTimeout = System.currentTimeMillis() + 20_000;
+            long killTimeout = terminationTimeout + 40_000;
+            long killedTimeout = killTimeout + 2_000;
+            showVerboseMessage(verboseServer, "Waiting for " + this + " to shutdown");
             /* Release port only after server stops responding to it */
-            long timeout = System.currentTimeMillis() + 60_000;
-            showVerboseMessage(verboseServer, "Waiting for " + this + " to die");
-            while (isAlive()) {
+            boolean sentSIGTERM = false;
+            boolean sentSIGKILL = false;
+            do {
                 try {
                     Thread.sleep(500);
                 } catch (InterruptedException e) {
-                    throw showError("Woke up from waiting for " + this + " to die", e);
+                    throw showError("Woke up from waiting for " + this + " to shutdown", e);
                 }
-                if (timeout < System.currentTimeMillis()) {
-                    throw showError(this + " keeps responding to port " + port + " even after shutdown");
+                long now = System.currentTimeMillis();
+                if (!sentSIGTERM && terminationTimeout < now) {
+                    showWarning(this + " keeps responding to port " + port + " even after sending STOP_SERVER");
+                    Signal.kill(pid, SIGTERM.getCValue());
+                    sentSIGTERM = true;
+                } else if (!sentSIGKILL && killTimeout < now) {
+                    showWarning(this + " keeps responding to port " + port + " even after killing with SIGTERM");
+                    Signal.kill(pid, SIGKILL.getCValue());
+                    sentSIGKILL = true;
+                } else if (killedTimeout < now) {
+                    throw showError(this + " keeps responding to port " + port + " even after killing with SIGKILL");
                 }
-            }
+            } while (isAlive());
             deleteAllFiles(serverDir);
             releasePortNumber(port);
         }