Removes the purge thread in favor of standard ScheduledThreadPoolExecutor APIs (#7293)

The RXSchedulerPurge thread is currently enabled by default and runs every second
to call purge() on each executor in the pool. This is causing significant issues
for low powered devices (e.g. mobile phones), because it needs to periodically
wake up the CPU to perform purging. The RXSchedulerPurge thread could be completely
removed in favor of using the standard setRemoveOnCancelPolicy() API on the
ScheduledThreadPoolExecutor which became available in Java 7 and offers removal
of cancelled tasks at the moment they are cancelled in O(1).

Author: chggr

src/main/java/io/reactivex/rxjava3/internal/schedulers/SchedulerPoolFactory.java

@@ -31,86 +31,11 @@ private SchedulerPoolFactory() {
 
     static final String PURGE_ENABLED_KEY = "rx3.purge-enabled";
 
-    /**
-     * Indicates the periodic purging of the ScheduledExecutorService is enabled.
-     */
     public static final boolean PURGE_ENABLED;
 
-    static final String PURGE_PERIOD_SECONDS_KEY = "rx3.purge-period-seconds";
-
-    /**
-     * Indicates the purge period of the ScheduledExecutorServices created by create().
-     */
-    public static final int PURGE_PERIOD_SECONDS;
-
-    static final AtomicReference<ScheduledExecutorService> PURGE_THREAD =
-            new AtomicReference<>();
-
-    // Upcast to the Map interface here to avoid 8.x compatibility issues.
-    // See http://stackoverflow.com/a/32955708/61158
-    static final Map<ScheduledThreadPoolExecutor, Object> POOLS =
-            new ConcurrentHashMap<>();
-
-    /**
-     * Starts the purge thread if not already started.
-     */
-    public static void start() {
-        tryStart(PURGE_ENABLED);
-    }
-
-    static void tryStart(boolean purgeEnabled) {
-        if (purgeEnabled) {
-            for (;;) {
-                ScheduledExecutorService curr = PURGE_THREAD.get();
-                if (curr != null) {
-                    return;
-                }
-                ScheduledExecutorService next = Executors.newScheduledThreadPool(1, new RxThreadFactory("RxSchedulerPurge"));
-                if (PURGE_THREAD.compareAndSet(curr, next)) {
-
-                    next.scheduleAtFixedRate(new ScheduledTask(), PURGE_PERIOD_SECONDS, PURGE_PERIOD_SECONDS, TimeUnit.SECONDS);
-
-                    return;
-                } else {
-                    next.shutdownNow();
-                }
-            }
-        }
-    }
-
-    /**
-     * Stops the purge thread.
-     */
-    public static void shutdown() {
-        ScheduledExecutorService exec = PURGE_THREAD.getAndSet(null);
-        if (exec != null) {
-            exec.shutdownNow();
-        }
-        POOLS.clear();
-    }
-
     static {
         SystemPropertyAccessor propertyAccessor = new SystemPropertyAccessor();
         PURGE_ENABLED = getBooleanProperty(true, PURGE_ENABLED_KEY, true, true, propertyAccessor);
-        PURGE_PERIOD_SECONDS = getIntProperty(PURGE_ENABLED, PURGE_PERIOD_SECONDS_KEY, 1, 1, propertyAccessor);
-
-        start();
-    }
-
-    static int getIntProperty(boolean enabled, String key, int defaultNotFound, int defaultNotEnabled, Function<String, String> propertyAccessor) {
-        if (enabled) {
-            try {
-                String value = propertyAccessor.apply(key);
-                if (value == null) {
-                    return defaultNotFound;
-                }
-                return Integer.parseInt(value);
-            } catch (Throwable ex) {
-                Exceptions.throwIfFatal(ex);
-                return defaultNotFound;
-            }
-        }
-        return defaultNotEnabled;
     }
 
     static boolean getBooleanProperty(boolean enabled, String key, boolean defaultNotFound, boolean defaultNotEnabled, Function<String, String> propertyAccessor) {
@@ -142,28 +67,8 @@ public String apply(String t) {
      * @return the ScheduledExecutorService
      */
     public static ScheduledExecutorService create(ThreadFactory factory) {
-        final ScheduledExecutorService exec = Executors.newScheduledThreadPool(1, factory);
-        tryPutIntoPool(PURGE_ENABLED, exec);
+        final ScheduledThreadPoolExecutor exec = new ScheduledThreadPoolExecutor(1, factory);
+        exec.setRemoveOnCancelPolicy(PURGE_ENABLED);
         return exec;
     }
-
-    static void tryPutIntoPool(boolean purgeEnabled, ScheduledExecutorService exec) {
-        if (purgeEnabled && exec instanceof ScheduledThreadPoolExecutor) {
-            ScheduledThreadPoolExecutor e = (ScheduledThreadPoolExecutor) exec;
-            POOLS.put(e, exec);
-        }
-    }
-
-    static final class ScheduledTask implements Runnable {
-        @Override
-        public void run() {
-            for (ScheduledThreadPoolExecutor e : new ArrayList<>(POOLS.keySet())) {
-                if (e.isShutdown()) {
-                    POOLS.remove(e);
-                } else {
-                    e.purge();
-                }
-            }
-        }
-    }
 }

src/main/java/io/reactivex/rxjava3/schedulers/Schedulers.java

@@ -38,8 +38,7 @@
  * <li>{@code rx3.computation-priority} (int): sets the thread priority of the {@link #computation()} {@code Scheduler}, default is {@link Thread#NORM_PRIORITY}</li>
  * <li>{@code rx3.newthread-priority} (int): sets the thread priority of the {@link #newThread()} {@code Scheduler}, default is {@link Thread#NORM_PRIORITY}</li>
  * <li>{@code rx3.single-priority} (int): sets the thread priority of the {@link #single()} {@code Scheduler}, default is {@link Thread#NORM_PRIORITY}</li>
- * <li>{@code rx3.purge-enabled} (boolean): enables periodic purging of all {@code Scheduler}'s backing thread pools, default is {@code false}</li>
- * <li>{@code rx3.purge-period-seconds} (int): specifies the periodic purge interval of all {@code Scheduler}'s backing thread pools, default is 1 second</li>
+ * <li>{@code rx3.purge-enabled} (boolean): enables purging of all {@code Scheduler}'s backing thread pools, default is {@code true}</li>
  * <li>{@code rx3.scheduler.use-nanotime} (boolean): {@code true} instructs {@code Scheduler} to use {@link System#nanoTime()} for {@link Scheduler#now(TimeUnit)},
  * instead of default {@link System#currentTimeMillis()} ({@code false})</li>
  * </ul>
@@ -556,7 +555,6 @@ public static void shutdown() {
         newThread().shutdown();
         single().shutdown();
         trampoline().shutdown();
-        SchedulerPoolFactory.shutdown();
     }
 
     /**
@@ -569,7 +567,6 @@ public static void start() {
         newThread().start();
         single().start();
         trampoline().start();
-        SchedulerPoolFactory.start();
     }
 
     static final class IOTask implements Supplier<Scheduler> {

src/test/java/io/reactivex/rxjava3/internal/schedulers/SchedulerPoolFactoryTest.java

@@ -30,50 +30,6 @@ public void utilityClass() {
         TestHelper.checkUtilityClass(SchedulerPoolFactory.class);
     }
 
-    @Test
-    public void multiStartStop() {
-        SchedulerPoolFactory.shutdown();
-
-        SchedulerPoolFactory.shutdown();
-
-        SchedulerPoolFactory.tryStart(false);
-
-        assertNull(SchedulerPoolFactory.PURGE_THREAD.get());
-
-        SchedulerPoolFactory.start();
-
-        // restart schedulers
-        Schedulers.shutdown();
-
-        Schedulers.start();
-    }
-
-    @Test
-    public void startRace() throws InterruptedException {
-        try {
-            for (int i = 0; i < TestHelper.RACE_DEFAULT_LOOPS; i++) {
-                SchedulerPoolFactory.shutdown();
-
-                Runnable r1 = new Runnable() {
-                    @Override
-                    public void run() {
-                        SchedulerPoolFactory.start();
-                    }
-                };
-
-                TestHelper.race(r1, r1);
-            }
-
-        } finally {
-            // restart schedulers
-            Schedulers.shutdown();
-
-            Thread.sleep(200);
-
-            Schedulers.start();
-        }
-    }
-
     @Test
     public void boolPropertiesDisabledReturnsDefaultDisabled() throws Throwable {
         assertTrue(SchedulerPoolFactory.getBooleanProperty(false, "key", false, true, failingPropertiesAccessor));
@@ -98,30 +54,6 @@ public void boolPropertiesReturnsValue() throws Throwable {
         assertFalse(SchedulerPoolFactory.getBooleanProperty(true, "false", false, true, Functions.<String>identity()));
     }
 
-    @Test
-    public void intPropertiesDisabledReturnsDefaultDisabled() throws Throwable {
-        assertEquals(-1, SchedulerPoolFactory.getIntProperty(false, "key", 0, -1, failingPropertiesAccessor));
-        assertEquals(-1, SchedulerPoolFactory.getIntProperty(false, "key", 1, -1, failingPropertiesAccessor));
-    }
-
-    @Test
-    public void intPropertiesEnabledMissingReturnsDefaultMissing() throws Throwable {
-        assertEquals(-1, SchedulerPoolFactory.getIntProperty(true, "key", -1, 0, missingPropertiesAccessor));
-        assertEquals(-1, SchedulerPoolFactory.getIntProperty(true, "key", -1, 1, missingPropertiesAccessor));
-    }
-
-    @Test
-    public void intPropertiesFailureReturnsDefaultMissing() throws Throwable {
-        assertEquals(-1, SchedulerPoolFactory.getIntProperty(true, "key", -1, 0, failingPropertiesAccessor));
-        assertEquals(-1, SchedulerPoolFactory.getIntProperty(true, "key", -1, 1, failingPropertiesAccessor));
-    }
-
-    @Test
-    public void intPropertiesReturnsValue() throws Throwable {
-        assertEquals(1, SchedulerPoolFactory.getIntProperty(true, "1", 0, 4, Functions.<String>identity()));
-        assertEquals(2, SchedulerPoolFactory.getIntProperty(true, "2", 3, 5, Functions.<String>identity()));
-    }
-
     static final Function<String, String> failingPropertiesAccessor = new Function<String, String>() {
         @Override
         public String apply(String v) throws Throwable {
@@ -135,22 +67,4 @@ public String apply(String v) throws Throwable {
             return null;
         }
     };
-
-    @Test
-    public void putIntoPoolNoPurge() {
-        int s = SchedulerPoolFactory.POOLS.size();
-
-        SchedulerPoolFactory.tryPutIntoPool(false, null);
-
-        assertEquals(s, SchedulerPoolFactory.POOLS.size());
-    }
-
-    @Test
-    public void putIntoPoolNonThreadPool() {
-        int s = SchedulerPoolFactory.POOLS.size();
-
-        SchedulerPoolFactory.tryPutIntoPool(true, null);
-
-        assertEquals(s, SchedulerPoolFactory.POOLS.size());
-    }
 }

src/test/java/io/reactivex/rxjava3/schedulers/ExecutorSchedulerTest.java

@@ -93,7 +93,7 @@ public void run() {
 
         System.out.println("Wait before second GC");
         System.out.println("JDK 6 purge is N log N because it removes and shifts one by one");
-        int t = (int)(n * Math.log(n) / 100) + SchedulerPoolFactory.PURGE_PERIOD_SECONDS * 1000;
+        int t = (int)(n * Math.log(n) / 100) + 1000;
         int sleepStep = 100;
         while (t > 0) {
             System.out.printf("  >> Waiting for purge: %.2f s remaining%n", t / 1000d);