Critical section support

Fixes #485

Author: dmlloyd

process/src/main/java/io/smallrye/common/process/CriticalSection.java

@@ -0,0 +1,131 @@
+package io.smallrye.common.process;
+
+import java.lang.invoke.ConstantBootstraps;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
+import java.util.concurrent.CopyOnWriteArraySet;
+import java.util.concurrent.locks.LockSupport;
+
+/**
+ * Methods for managing "critical sections" of code.
+ * A "critical section" is defined as any program subunit within a thread which could have
+ * a deleterious impact on the state of the running system if
+ * interrupted by process exit partway through execution.
+ * <p>
+ * This is intended to be used with a {@code try}-with-resources construct
+ * in order to ensure that the critical section is safely concluded.
+ * If the returned handle is not properly used to {@code close()} the critical section,
+ * the process may never exit.
+ */
+public final class CriticalSection {
+    private static final CriticalSection INSTANCE = new CriticalSection();
+    private static final Thread shutdownThread = new Thread(CriticalSection::run, "Critical section exit thread");
+    private static final VarHandle stateHandle = ConstantBootstraps.fieldVarHandle(MethodHandles.lookup(), "state", VarHandle.class, CriticalSection.class, long.class);
+    private static final CopyOnWriteArraySet<Thread> interruptibleThreads = new CopyOnWriteArraySet<>();
+
+    private static final long ST_EXITING = 1L << 63;
+    private static final long CNT_MASK = -1L & ~ST_EXITING;
+
+    static {
+        Runtime.getRuntime().addShutdownHook(shutdownThread);
+    }
+
+    private CriticalSection() {}
+
+    @SuppressWarnings("unused")
+    private long state;
+
+    static void run() {
+        long oldVal = (long) stateHandle.getAndBitwiseOr(ST_EXITING) & CNT_MASK;
+        while (oldVal > 0) {
+            interruptibleThreads.forEach(Thread::interrupt);
+            // clear interrupted flag
+            Thread.interrupted();
+            LockSupport.park(INSTANCE);
+            oldVal = (long) stateHandle.getVolatile() & CNT_MASK;
+        }
+        // done
+    }
+
+    /**
+     * Attempt to enter a critical section.
+     * If the process is shutting down, and no other critical sections are entered,
+     * this method does not return.
+     *
+     * @return a handle to use to exit the critical section (not {@code null})
+     */
+    public static Closer enter() {
+        tryEnter();
+        return new Closer(false);
+    }
+
+    /**
+     * Attempt to enter a critical section.
+     * If the process is shutting down, and no other critical sections are entered,
+     * this method does not return.
+     * Otherwise, this thread will be interrupted no less than one time when the
+     * process is exiting.
+     *
+     * @return a handle to use to exit the critical section (not {@code null})
+     */
+    public static Closer enterInterruptibly() {
+        long oldVal = tryEnter();
+        if ((oldVal & ST_EXITING) != 0) {
+            Thread.currentThread().interrupt();
+        }
+        // if this thread is already in the set, then an outer block carries the interruptible status
+        return new Closer(interruptibleThreads.add(Thread.currentThread()));
+    }
+
+    private static long tryEnter() {
+        long oldVal = (long) stateHandle.getVolatile();
+        for (;;) {
+            if (oldVal == ST_EXITING) {
+                // no critical sections and exit was requested, so shutdown will commence soon
+                for (;;) {
+                    Thread.interrupted();
+                    LockSupport.park(INSTANCE);
+                }
+            } else {
+                long witness = (long) stateHandle.compareAndExchange(oldVal, oldVal + 1);
+                if (witness == oldVal) {
+                    break;
+                }
+                oldVal = witness;
+            }
+        }
+        return oldVal;
+    }
+
+    /**
+     * The close handle for the critical section.
+     */
+    public static final class Closer implements AutoCloseable {
+        private static final VarHandle closedHandle = ConstantBootstraps.fieldVarHandle(MethodHandles.lookup(), "closed", VarHandle.class, Closer.class, boolean.class);
+
+        private final boolean interruptible;
+
+        @SuppressWarnings("unused")
+        private boolean closed;
+
+        private Closer(final boolean interruptible) {
+            this.interruptible = interruptible;
+        }
+
+        /**
+         * Finish the critical section (idempotent).
+         */
+        public void close() {
+            if (closedHandle.compareAndSet(this, false, true)) {
+                long oldVal = (long) stateHandle.getAndAdd(-1L);
+                if (oldVal == ST_EXITING + 1) {
+                    // we were the last one, so wake up the runner
+                    LockSupport.unpark(shutdownThread);
+                }
+                if (interruptible) {
+                    interruptibleThreads.remove(Thread.currentThread());
+                }
+            }
+        }
+    }
+}