3.x: Introduce property rx3.scheduler.use-nanotime (#7169)

Issue-Id: #7154

Co-authored-by: Sergej Isbrecht <sergej.isbrecht@gmail.com>

src/main/java/io/reactivex/rxjava3/core/Scheduler.java

@@ -60,8 +60,9 @@
  * interface which can grant access to the original or hooked {@code Runnable}, thus, a repeated {@code RxJavaPlugins.onSchedule}
  * can detect the earlier hook and not apply a new one over again.
  * <p>
- * The default implementation of {@link #now(TimeUnit)} and {@link Worker#now(TimeUnit)} methods to return current
- * {@link System#currentTimeMillis()} value in the desired time unit. Custom {@code Scheduler} implementations can override this
+ * The default implementation of {@link #now(TimeUnit)} and {@link Worker#now(TimeUnit)} methods to return current {@link System#currentTimeMillis()}
+ * value in the desired time unit, unless {@code rx3.scheduler.use-nanotime} (boolean) is set. When the property is set to
+ * {@code true}, the method uses {@link System#nanoTime()} as its basis instead. Custom {@code Scheduler} implementations can override this
  * to provide specialized time accounting (such as virtual time to be advanced programmatically).
  * Note that operators requiring a {@code Scheduler} may rely on either of the {@code now()} calls provided by
  * {@code Scheduler} or {@code Worker} respectively, therefore, it is recommended they represent a logically
@@ -88,6 +89,34 @@
  * All methods on the {@code Scheduler} and {@code Worker} classes should be thread safe.
  */
 public abstract class Scheduler {
+    /**
+     * Value representing whether to use {@link System#nanoTime()}, or default as clock for {@link #now(TimeUnit)}
+     * and {@link Scheduler.Worker#now(TimeUnit)}
+     * <p>
+     * Associated system parameter:
+     * <ul>
+     *   <li>{@code rx3.scheduler.use-nanotime}, boolean, default {@code false}
+     * </ul>
+     */
+    static boolean IS_DRIFT_USE_NANOTIME = Boolean.getBoolean("rx3.scheduler.use-nanotime");
+
+    /**
+     * Returns the current clock time depending on state of {@link Scheduler#IS_DRIFT_USE_NANOTIME} in given {@code unit}
+     * <p>
+     * By default {@link System#currentTimeMillis()} will be used as the clock. When the property is set
+     * {@link System#nanoTime()} will be used.
+     * <p>
+     * @param unit the time unit
+     * @return the 'current time' in given unit
+     * @throws NullPointerException if {@code unit} is {@code null}
+     */
+    static long computeNow(TimeUnit unit) {
+        if(!IS_DRIFT_USE_NANOTIME) {
+            return unit.convert(System.currentTimeMillis(), TimeUnit.MILLISECONDS);
+        }
+        return unit.convert(System.nanoTime(), TimeUnit.NANOSECONDS);
+    }
+
     /**
      * The tolerance for a clock drift in nanoseconds where the periodic scheduler will rebase.
      * <p>
@@ -156,7 +185,7 @@ public static long clockDriftTolerance() {
      * @since 2.0
      */
     public long now(@NonNull TimeUnit unit) {
-        return unit.convert(System.currentTimeMillis(), TimeUnit.MILLISECONDS);
+        return computeNow(unit);
     }
 
     /**
@@ -362,8 +391,9 @@ public <S extends Scheduler & Disposable> S when(@NonNull Function<Flowable<Flow
      * track the individual {@code Runnable} tasks while they are waiting to be executed (with or without delay) so that
      * {@link #dispose()} can prevent their execution or potentially interrupt them if they are currently running.
      * <p>
-     * The default implementation of the {@link #now(TimeUnit)} method returns current
-     * {@link System#currentTimeMillis()} value in the desired time unit. Custom {@code Worker} implementations can override this
+     * The default implementation of the {@link #now(TimeUnit)} method returns current {@link System#currentTimeMillis()}
+     * value in the desired time unit, unless {@code rx3.scheduler.use-nanotime} (boolean) is set. When the property is set to
+     * {@code true}, the method uses {@link System#nanoTime()} as its basis instead. Custom {@code Worker} implementations can override this
      * to provide specialized time accounting (such as virtual time to be advanced programmatically).
      * Note that operators requiring a scheduler may rely on either of the {@code now()} calls provided by
      * {@code Scheduler} or {@code Worker} respectively, therefore, it is recommended they represent a logically
@@ -482,7 +512,7 @@ public Disposable schedulePeriodically(@NonNull Runnable run, final long initial
          * @since 2.0
          */
         public long now(@NonNull TimeUnit unit) {
-            return unit.convert(System.currentTimeMillis(), TimeUnit.MILLISECONDS);
+            return computeNow(unit);
         }
 
         /**

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

@@ -40,6 +40,8 @@
  * <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.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>
  */
 public final class Schedulers {

src/test/java/io/reactivex/rxjava3/core/SchedulerTest.java

@@ -16,10 +16,48 @@
 package io.reactivex.rxjava3.core;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
 
+import org.junit.After;
 import org.junit.Test;
 
+import java.util.concurrent.TimeUnit;
+
 public class SchedulerTest {
+    private static final String DRIFT_USE_NANOTIME = "rx3.scheduler.use-nanotime";
+
+    @After
+    public void cleanup() {
+      // reset value to default in order to not influence other tests
+      Scheduler.IS_DRIFT_USE_NANOTIME = false;
+    }
+
+    @Test
+    public void driftUseNanoTimeNotSetByDefault() {
+      assertFalse(Scheduler.IS_DRIFT_USE_NANOTIME);
+      assertFalse(Boolean.getBoolean(DRIFT_USE_NANOTIME));
+    }
+
+    @Test
+    public void computeNow_currentTimeMillis() {
+      TimeUnit unit = TimeUnit.MILLISECONDS;
+      assertTrue(isInRange(System.currentTimeMillis(), Scheduler.computeNow(unit), unit, 250, TimeUnit.MILLISECONDS));
+    }
+
+    @Test
+    public void computeNow_nanoTime() {
+      TimeUnit unit = TimeUnit.NANOSECONDS;
+      Scheduler.IS_DRIFT_USE_NANOTIME = true;
+
+      assertFalse(isInRange(System.currentTimeMillis(), Scheduler.computeNow(unit), unit, 250, TimeUnit.MILLISECONDS));
+      assertTrue(isInRange(System.nanoTime(), Scheduler.computeNow(unit), TimeUnit.NANOSECONDS, 250, TimeUnit.MILLISECONDS));
+    }
+
+    private boolean isInRange(long start, long stop, TimeUnit source, long maxDiff, TimeUnit diffUnit) {
+      long diff = Math.abs(stop - start);
+      return diffUnit.convert(diff, source) <= maxDiff;
+    }
 
     @Test
     public void clockDriftCalculation() {