slightly refactor signal delivery to make spotbugs happy

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SignalModuleBuiltins.java

@@ -45,6 +45,7 @@
 import java.util.List;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentLinkedDeque;
+import java.util.concurrent.Semaphore;
 
 import com.oracle.graal.python.builtins.Builtin;
 import com.oracle.graal.python.builtins.CoreFunctions;
@@ -77,6 +78,8 @@ public class SignalModuleBuiltins extends PythonBuiltins {
 
     private static final HiddenKey signalQueueKey = new HiddenKey("signalQueue");
     private final ConcurrentLinkedDeque<SignalTriggerAction> signalQueue = new ConcurrentLinkedDeque<>();
+    private static final HiddenKey signalSemaKey = new HiddenKey("signalQueue");
+    private final Semaphore signalSema = new Semaphore(0);
 
     @Override
     protected List<? extends NodeFactory<? extends PythonBuiltinBaseNode>> getNodeFactories() {
@@ -102,15 +105,18 @@ public void postInitialize(PythonCore core) {
 
         PythonModule signalModule = core.lookupBuiltinModule("_signal");
         signalModule.setAttribute(signalQueueKey, signalQueue);
+        signalModule.setAttribute(signalSemaKey, signalSema);
 
         core.getContext().registerAsyncAction(() -> {
-            synchronized (signalQueue) {
-                try {
-                    signalQueue.wait();
-                } catch (InterruptedException e) {
+            SignalTriggerAction poll = signalQueue.poll();
+            try {
+                while (poll == null) {
+                    signalSema.acquire();
+                    poll = signalQueue.poll();
                 }
+            } catch (InterruptedException e) {
             }
-            return signalQueue.poll();
+            return poll;
         });
     }
 
@@ -230,17 +236,17 @@ Object signal(@SuppressWarnings("unused") PythonModule self, long signalNumber,
         @Specialization
         @TruffleBoundary
         Object signal(PythonModule self, long signalNumber, Object handler,
-                        @Cached("create()") ReadAttributeFromObjectNode readNode) {
+                        @Cached("create()") ReadAttributeFromObjectNode readQueueNode,
+                        @Cached("create()") ReadAttributeFromObjectNode readSemaNode) {
             int signum = getSignum(signalNumber);
-            ConcurrentLinkedDeque<SignalTriggerAction> queue = getQueue(self, readNode);
+            ConcurrentLinkedDeque<SignalTriggerAction> queue = getQueue(self, readQueueNode);
+            Semaphore semaphore = getSemaphore(self, readSemaNode);
             Object retval;
             SignalTriggerAction signalTrigger = new SignalTriggerAction(handler, signum);
             try {
                 retval = Signals.setSignalHandler(signum, () -> {
                     queue.add(signalTrigger);
-                    synchronized (queue) {
-                        queue.notify();
-                    }
+                    semaphore.release();
                 });
             } catch (IllegalArgumentException e) {
                 throw raise(PythonErrorType.ValueError, e);
@@ -266,6 +272,15 @@ private static ConcurrentLinkedDeque<SignalTriggerAction> getQueue(PythonModule
                 throw new IllegalStateException("the signal trigger queue was modified!");
             }
         }
+
+        private static Semaphore getSemaphore(PythonModule self, ReadAttributeFromObjectNode readNode) {
+            Object semaphore = readNode.execute(self, signalSemaKey);
+            if (semaphore instanceof Semaphore) {
+                return (Semaphore) semaphore;
+            } else {
+                throw new IllegalStateException("the signal trigger semaphore was modified!");
+            }
+        }
     }
 }