Strawman proposal for uncaught exception configuration

This is a strawman example/proposal for configurable uncaught exception handling in RxJavaPlugins. This is in ref to ReactiveX#5234.

The two options are to defer to current thread's handler (the current implementation and remains the default) and an optional `throw` option that wraps the throwable into an `UncaughtRxJavaException` for developers to watch for separately (and rethrowing, which requires some sort of `RuntimeException` wrapping).

src/main/java/io/reactivex/plugins/RxJavaPlugins.java

@@ -30,8 +30,27 @@
 import io.reactivex.schedulers.Schedulers;
 /**
  * Utility class to inject handlers to certain standard RxJava operations.
+ * <p>
+ * <strong>Supported system properties ({@code System.getProperty()}):</strong>
+ * <ul>
+ * <li>{@code rx2.uncaught-behavior} (String): sets the behavior of the uncaught exception handling, default is {@link #UNCAUGHT_HANDLER}. This is {@link Experimental}.</li>
+ * </ul>
  */
 public final class RxJavaPlugins {
+
+    /**
+     * System property value for uncaught exceptions that passes the exception directly to the current thread's
+     * {@link UncaughtExceptionHandler}. This is the default behavior.
+     */
+    @Experimental
+    public static final String UNCAUGHT_HANDLER = "handler";
+
+    /**
+     * System property value for uncaught exceptions that just immediately throws the uncaught exception.
+     */
+    @Experimental
+    public static final String UNCAUGHT_THROW = "throw";
+
     @Nullable
     static volatile Consumer<? super Throwable> errorHandler;
 
@@ -123,6 +142,9 @@ public final class RxJavaPlugins {
      */
     static volatile boolean failNonBlockingScheduler;
 
+    /** The name of the system property for setting the uncaught exception behavior. */
+    private static final String KEY_UNCAUGHT_BEHAVIOR = "rx2.uncaught-behavior";
+
     /**
      * Prevents changing the plugins from then on.
      * <p>This allows container-like environments to prevent clients
@@ -406,9 +428,15 @@ static boolean isBug(Throwable error) {
     }
 
     static void uncaught(@NonNull Throwable error) {
-        Thread currentThread = Thread.currentThread();
-        UncaughtExceptionHandler handler = currentThread.getUncaughtExceptionHandler();
-        handler.uncaughtException(currentThread, error);
+        String uncaughtBehavior = System.getProperty(KEY_UNCAUGHT_BEHAVIOR, UNCAUGHT_HANDLER);
+        if (UNCAUGHT_THROW.equals(uncaughtBehavior)) {
+            throw new UncaughtRxJavaException(error);
+        } else {
+            Thread currentThread = Thread.currentThread();
+            UncaughtExceptionHandler handler = currentThread.getUncaughtExceptionHandler();
+            handler.uncaughtException(currentThread, error);
+        }
+        // TODO Error on invalid inputs somehow?
     }
 
     /**

src/main/java/io/reactivex/plugins/UncaughtRxJavaException.java

@@ -0,0 +1,16 @@
+package io.reactivex.plugins;
+
+import io.reactivex.annotations.Experimental;
+
+/**
+ * An exception thrown when an uncaught throwable was received and the {@link RxJavaPlugins#UNCAUGHT_THROW} property was
+ * set. This guarantees the existence of an underlying cause retrievable via {@link #getCause()}.
+ */
+@Experimental
+public final class UncaughtRxJavaException extends RuntimeException {
+
+    public UncaughtRxJavaException(Throwable cause) {
+        super(cause);
+    }
+
+}