Fix race condition with disposal in MemberSingle.tryEmit issue #58, pr #59

* fix race condition with disposal in MemberSingle.tryEmit #58
* organize imports

Author: davidmoten

rxjava2-jdbc/src/test/java/org/davidmoten/rx/jdbc/SelectTest.java

@@ -1,6 +1,5 @@
 package org.davidmoten.rx.jdbc;
 
-import org.davidmoten.rx.jdbc.Select;
 import org.junit.Test;
 
 import com.github.davidmoten.junit.Asserts;

rxjava2-pool/src/main/java/org/davidmoten/rx/pool/MemberSingle.java

@@ -259,21 +259,25 @@ private void scheduleReleasesNoDelay() {
     }
 
     private boolean tryEmit(Observers<T> obs, DecoratingMember<T> m) {
-        // get a fresh worker each time so we jump threads to
-        // break the stack-trace (a long-enough chain of
-        // checkout-checkins could otherwise provoke stack
-        // overflow)
-
+        // note that tryEmit is protected by the drain method so will 
+        // not be run concurrently. We do have to be careful with 
+        // concurrent disposal of observers though.
+        
+        
         // advance counter to the next and choose an Observer to emit to (round robin)
 
         int index = obs.index;
+        // a precondition of this method is that obs.activeCount > 0 (enforced by drain method)
         MemberSingleObserver<T> o = obs.observers[index];
         MemberSingleObserver<T> oNext = o;
-        // atomically bump up the index (if that entry has not been deleted in
-        // the meantime by disposal)
+        
+        // atomically bump up the index to select the next Observer by round-robin
+        // (if that entry has not been deleted in the meantime by disposal). Need 
+        // to be careful too that ALL observers have not been deleted via a race 
+        // with disposal.
         while (true) {
             Observers<T> x = observers.get();
-            if (x.index == index && x.observers[index] == o) {
+            if (x.index == index && x.activeCount > 0 && x.observers[index] == o) {
                 boolean[] active = new boolean[x.active.length];
                 System.arraycopy(x.active, 0, active, 0, active.length);
                 int nextIndex = (index + 1) % active.length;
@@ -292,6 +296,10 @@ private boolean tryEmit(Observers<T> obs, DecoratingMember<T> m) {
                 return false;
             }
         }
+        // get a fresh worker each time so we jump threads to
+        // break the stack-trace (a long-enough chain of
+        // checkout-checkins could otherwise provoke stack
+        // overflow)
         Worker worker = scheduler.createWorker();
         worker.schedule(new Emitter<T>(worker, oNext, m));
         return true;
@@ -479,7 +487,7 @@ private static final class Observers<T> {
         final int requested;
 
         Observers(MemberSingleObserver<T>[] observers, boolean[] active, int activeCount, int index, int requested) {
-            Preconditions.checkArgument(observers.length > 0 || index == 0, "index must be -1 for zero length array");
+            Preconditions.checkArgument(observers.length > 0 || index == 0, "index must be 0 for zero length array");
             Preconditions.checkArgument(observers.length == active.length);
             this.observers = observers;
             this.index = index;