2.x: test sync, fix retryWhen/redoWhen/publish(Function) (#4361)

* 2.x: test sync, fix retryWhen/redoWhen/publish(Function)

* Fix spelling error, use common parameter validation at more places

Author: akarnokd

src/main/java/io/reactivex/BackpressureOverflow.java

@@ -0,0 +1,28 @@
+/**
+ * Copyright 2016 Netflix, Inc.
+ *
+ * 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;
+
+/**
+ * Options to deal with buffer overflow when using onBackpressureBuffer.
+ */
+public enum BackpressureOverflowStrategy {
+    /** Signal a MissingBackpressureException and terminate the sequence. */
+    ERROR,
+    /** Drop the oldest value from the buffer. */
+    DROP_OLDEST,
+    /** Drop the latest value from the buffer. */
+    DROP_LATEST
+}