2.x: Add Completable.takeUntil(Completable) operator

src/main/java/io/reactivex/Completable.java

@@ -2031,6 +2031,32 @@ public final Completable subscribeOn(final Scheduler scheduler) {
         return RxJavaPlugins.onAssembly(new CompletableSubscribeOn(this, scheduler));
     }
 
+    /**
+     * Terminates the downstream if this or the other {@code Completable}
+     * terminates (wins the termination race) while disposing the connection to the losing source.
+     * <p>
+     * <img width="640" height="468" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.takeuntil.c.png" alt="">
+     * <dl>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code takeUntil} does not operate by default on a particular {@link Scheduler}.</dd>
+     *  <dt><b>Error handling:</b></dt>
+     *  <dd>If both this and the other sources signal an error, only one of the errors
+     *  is signaled to the downstream and the other error is signaled to the global
+     *  error handler via {@link RxJavaPlugins#onError(Throwable)}.</dd>
+     * </dl>
+     * @param other the other completable source to observe for the terminal signals
+     * @return the new Completable instance
+     * @since 2.1.17 - experimental
+     */
+    @CheckReturnValue
+    @Experimental
+    @SchedulerSupport(SchedulerSupport.NONE)
+    public final Completable takeUntil(CompletableSource other) {
+        ObjectHelper.requireNonNull(other, "other is null");
+
+        return RxJavaPlugins.onAssembly(new CompletableTakeUntilCompletable(this, other));
+    }
+
     /**
      * Returns a Completable that runs this Completable and emits a TimeoutException in case
      * this Completable doesn't complete within the given time.

src/main/java/io/reactivex/internal/operators/completable/CompletableTakeUntilCompletable.java

@@ -0,0 +1,145 @@
+/**
+ * Copyright (c) 2016-present, RxJava Contributors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is
+ * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
+ * the License for the specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivex.internal.operators.completable;
+
+import java.util.concurrent.atomic.*;
+
+import io.reactivex.*;
+import io.reactivex.annotations.Experimental;
+import io.reactivex.disposables.Disposable;
+import io.reactivex.internal.disposables.DisposableHelper;
+import io.reactivex.plugins.RxJavaPlugins;
+
+/**
+ * Terminates the sequence if either the main or the other Completable terminate.
+ * @since 2.1.17 - experimental
+ */
+@Experimental
+public final class CompletableTakeUntilCompletable extends Completable {
+
+    final Completable source;
+
+    final CompletableSource other;
+
+    public CompletableTakeUntilCompletable(Completable source,
+            CompletableSource other) {
+        this.source = source;
+        this.other = other;
+    }
+
+    @Override
+    protected void subscribeActual(CompletableObserver s) {
+        TakeUntilMainObserver parent = new TakeUntilMainObserver(s);
+        s.onSubscribe(parent);
+
+        other.subscribe(parent.other);
+        source.subscribe(parent);
+    }
+
+    static final class TakeUntilMainObserver extends AtomicReference<Disposable>
+    implements CompletableObserver, Disposable {
+
+        private static final long serialVersionUID = 3533011714830024923L;
+
+        final CompletableObserver downstream;
+
+        final OtherObserver other;
+
+        final AtomicBoolean once;
+
+        TakeUntilMainObserver(CompletableObserver downstream) {
+            this.downstream = downstream;
+            this.other = new OtherObserver(this);
+            this.once = new AtomicBoolean();
+        }
+
+        @Override
+        public void dispose() {
+            if (once.compareAndSet(false, true)) {
+                DisposableHelper.dispose(this);
+                DisposableHelper.dispose(other);
+            }
+        }
+
+        @Override
+        public boolean isDisposed() {
+            return once.get();
+        }
+
+        @Override
+        public void onSubscribe(Disposable d) {
+            DisposableHelper.setOnce(this, d);
+        }
+
+        @Override
+        public void onComplete() {
+            if (once.compareAndSet(false, true)) {
+                DisposableHelper.dispose(other);
+                downstream.onComplete();
+            }
+        }
+
+        @Override
+        public void onError(Throwable e) {
+            if (once.compareAndSet(false, true)) {
+                DisposableHelper.dispose(other);
+                downstream.onError(e);
+            } else {
+                RxJavaPlugins.onError(e);
+            }
+        }
+
+        void innerComplete() {
+            if (once.compareAndSet(false, true)) {
+                DisposableHelper.dispose(this);
+                downstream.onComplete();
+            }
+        }
+
+        void innerError(Throwable e) {
+            if (once.compareAndSet(false, true)) {
+                DisposableHelper.dispose(this);
+                downstream.onError(e);
+            } else {
+                RxJavaPlugins.onError(e);
+            }
+        }
+
+        static final class OtherObserver extends AtomicReference<Disposable>
+        implements CompletableObserver {
+
+            private static final long serialVersionUID = 5176264485428790318L;
+            final TakeUntilMainObserver parent;
+
+            OtherObserver(TakeUntilMainObserver parent) {
+                this.parent = parent;
+            }
+
+            @Override
+            public void onSubscribe(Disposable d) {
+                DisposableHelper.setOnce(this, d);
+            }
+
+            @Override
+            public void onComplete() {
+                parent.innerComplete();
+            }
+
+            @Override
+            public void onError(Throwable e) {
+                parent.innerError(e);
+            }
+        }
+    }
+}