Standard producers and some (platform-safe) queue implementations.

src/main/java/rx/internal/operators/OperatorToObservableList.java

@@ -15,12 +15,11 @@
  */
 package rx.internal.operators;
 
-import rx.Observable.Operator;
-import rx.Subscriber;
+import java.util.*;
 
-import java.util.ArrayList;
-import java.util.LinkedList;
-import java.util.List;
+import rx.Observable.Operator;
+import rx.*;
+import rx.internal.producers.SingleDelayedProducer;
 
 /**
  * Returns an {@code Observable} that emits a single item, a list composed of all the items emitted by the
@@ -90,7 +89,7 @@ public void onCompleted() {
                         return;
                     }
                     list = null;
-                    producer.set(result);
+                    producer.setValue(result);
                 }
             }
 

src/main/java/rx/internal/operators/OperatorToObservableSortedList.java

@@ -20,6 +20,7 @@
 import rx.Observable.Operator;
 import rx.*;
 import rx.functions.Func2;
+import rx.internal.producers.SingleDelayedProducer;
 
 /**
  * Return an {@code Observable} that emits the items emitted by the source {@code Observable}, in a sorted order
@@ -77,7 +78,7 @@ public void onCompleted() {
                         onError(e);
                         return;
                     }
-                    producer.set(a);
+                    producer.setValue(a);
                 }
             }
 

src/main/java/rx/internal/producers/ProducerArbiter.java

@@ -0,0 +1,189 @@
+/**
+ * Copyright 2015 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 rx.internal.producers;
+
+import rx.*;
+
+/**
+ * Producer that allows changing an underlying producer atomically and correctly resume with the accumulated
+ * requests.
+ */
+public final class ProducerArbiter implements Producer {
+    long requested;
+    Producer currentProducer;
+
+    boolean emitting;
+    long missedRequested;
+    long missedProduced;
+    Producer missedProducer;
+
+    static final Producer NULL_PRODUCER = new Producer() {
+        @Override
+        public void request(long n) {
+
+        }
+    };
+
+    @Override
+    public void request(long n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("n >= 0 required");
+        }
+        if (n == 0) {
+            return;
+        }
+        synchronized (this) {
+            if (emitting) {
+                missedRequested += n;
+                return;
+            }
+            emitting = true;
+        }
+        boolean skipFinal = false;
+        try {
+            long r = requested;
+            long u = r + n;
+            if (u < 0) {
+                u = Long.MAX_VALUE;
+            }
+            requested = u;
+
+            Producer p = currentProducer;
+            if (p != null) {
+                p.request(n);
+            }
+
+            emitLoop();
+            skipFinal = true;
+        } finally {
+            if (!skipFinal) {
+                synchronized (this) {
+                    emitting = false;
+                }
+            }
+        }
+    }
+
+    public void produced(long n) {
+        if (n <= 0) {
+            throw new IllegalArgumentException("n > 0 required");
+        }
+        synchronized (this) {
+            if (emitting) {
+                missedProduced += n;
+                return;
+            }
+            emitting = true;
+        }
+
+        boolean skipFinal = false;
+        try {
+            long r = requested;
+            long u = r - n;
+            if (u < 0) {
+                throw new IllegalStateException();
+            }
+            requested = u;
+
+            emitLoop();
+            skipFinal = true;
+        } finally {
+            if (!skipFinal) {
+                synchronized (this) {
+                    emitting = false;
+                }
+            }
+        }
+    }
+
+    public void setProducer(Producer newProducer) {
+        synchronized (this) {
+            if (emitting) {
+                missedProducer = newProducer == null ? NULL_PRODUCER : newProducer;
+                return;
+            }
+            emitting = true;
+        }
+        boolean skipFinal = false;
+        try {
+            currentProducer = newProducer;
+            if (newProducer != null) {
+                newProducer.request(requested);
+            }
+
+            emitLoop();
+            skipFinal = true;
+        } finally {
+            if (!skipFinal) {
+                synchronized (this) {
+                    emitting = false;
+                }
+            }
+        }
+    }
+
+    public void emitLoop() {
+        for (;;) {
+            long localRequested;
+            long localProduced;
+            Producer localProducer;
+            synchronized (this) {
+                localRequested = missedRequested;
+                localProduced = missedProduced;
+                localProducer = missedProducer;
+                if (localRequested == 0L 
+                        && localProduced == 0L
+                        && localProducer == null) {
+                    emitting = false;
+                    return;
+                }
+                missedRequested = 0L;
+                missedProduced = 0L;
+                missedProducer = null;
+            }
+
+            long r = requested;
+
+            if (r != Long.MAX_VALUE) {
+                long u = r + localRequested;
+                if (u < 0 || u == Long.MAX_VALUE) {
+                    r = Long.MAX_VALUE;
+                    requested = r;
+                } else {
+                    long v = u - localProduced;
+                    if (v < 0) {
+                        throw new IllegalStateException("more produced than requested");
+                    }
+                    r = v;
+                    requested = v;
+                }
+            }
+            if (localProducer != null) {
+                if (localProducer == NULL_PRODUCER) {
+                    currentProducer = null;
+                } else {
+                    currentProducer = localProducer;
+                    localProducer.request(r);
+                }
+            } else {
+                Producer p = currentProducer;
+                if (p != null && localRequested != 0L) {
+                    p.request(localRequested);
+                }
+            }
+        }
+    }
+}