3 files changed, 27 insertions, 10 deletions

diff --git a/qpid/cpp/src/qpid/ha/QueueGuard.cpp b/qpid/cpp/src/qpid/ha/QueueGuard.cpp
index 28ec855c2d..85feadd2ab 100644
--- a/qpid/cpp/src/qpid/ha/QueueGuard.cpp
+++ b/qpid/cpp/src/qpid/ha/QueueGuard.cpp
@@ -50,15 +50,15 @@ class QueueGuard::QueueObserver : public broker::QueueObserver
 
 
 QueueGuard::QueueGuard(broker::Queue& q, const BrokerInfo& info)
-    : queue(q), subscription(0), range(q)
+    : queue(q), subscription(0)
 {
-    // NOTE: The QueueGuard is created before the queue becomes active: either
-    // when a backup is promoted, or when a new queue is created on the primary.
     std::ostringstream os;
     os << "Primary guard " << queue.getName() << "@" << info.getLogId() << ": ";
     logPrefix = os.str();
     observer.reset(new QueueObserver(*this));
     queue.addObserver(observer);
+    // Set range after addObserver so we know that range.back+1 is a guarded position.
+    range = QueueRange(q);
 }
 
 QueueGuard::~QueueGuard() { cancel(); }
@@ -112,6 +112,7 @@ void completeBefore(QueueGuard* guard, SequenceNumber position, const QueuedMess
 
 bool QueueGuard::subscriptionStart(SequenceNumber position) {
    // Complete any messages before or at the ReplicatingSubscription start position.
+   // Those messages are already on the backup.
     if (!delayed.empty() && delayed.front() <= position) {
         // FIXME aconway 2012-06-15: queue iteration, only messages in delayed
         queue.eachMessage(boost::bind(&completeBefore, this, position, _1));
diff --git a/qpid/cpp/src/qpid/ha/QueueGuard.h b/qpid/cpp/src/qpid/ha/QueueGuard.h
index 9c6fb55015..8cc2055381 100644
--- a/qpid/cpp/src/qpid/ha/QueueGuard.h
+++ b/qpid/cpp/src/qpid/ha/QueueGuard.h
@@ -83,12 +83,14 @@ class QueueGuard {
      * QueueGuard is created before the queue becomes active: either when a
      * backup is promoted, or when a new queue is created on the primary.
      *
-     * NOTE: The first position protected by the guard is getRange().getBack()+1
+     * NOTE: The first position guaranteed to be protected by the guard is
+     * getRange().getBack()+1. It is possible that the guard has protected
+     * some messages before that point.
      */
     const QueueRange& getRange() const { return range; } // range is immutable, no lock needed.
 
     /** Inform the guard of the stating position for the attached subscription.
-     * Complete messages that will not be seen by the subscriptino.
+     * Complete messages that will not be seen by the subscription.
      *@return true if the subscription has already advanced to a guarded position.
      */
     bool subscriptionStart(framing::SequenceNumber position);
@@ -102,7 +104,7 @@ class QueueGuard {
     framing::SequenceSet delayed;
     ReplicatingSubscription* subscription;
     boost::shared_ptr<QueueObserver> observer;
-    const QueueRange range;
+    QueueRange range;
 };
 }} // namespace qpid::ha
 
diff --git a/qpid/cpp/src/qpid/ha/QueueRange.h b/qpid/cpp/src/qpid/ha/QueueRange.h
index 3e83ff795e..d734326910 100644
--- a/qpid/cpp/src/qpid/ha/QueueRange.h
+++ b/qpid/cpp/src/qpid/ha/QueueRange.h
@@ -33,17 +33,31 @@ namespace ha {
 
 /**
  * Get the front/back range of a queue or from a ReplicatingSubscription arguments table.
+ *
+ * The *back* of the queue is the position of the latest (most recently pushed)
+ * message on the queue or, if the queue is empty, the back is n-1 where n is
+ * the position that will be assigned to the next message pushed onto the queue.
+ *
+ * The *front* of the queue is the position of the oldest (next to be consumed) message
+ * on the queue or, if the queue is empty, it is the position that will be occupied
+ * by the next message pushed onto the queue.
+ *
+ * This leads to the slightly surprising conclusion that for an empty queue
+ * front = back+1
  */
 struct QueueRange {
   public:
     framing::SequenceNumber front, back;
 
-    QueueRange() { }
+    QueueRange() : front(1), back(0) { } // Empty range.
 
     QueueRange(broker::Queue& q) {
-        back = q.getPosition();
-        front = back+1;         // assume empty
-        ReplicatingSubscription::getFront(q, front);
+        if (ReplicatingSubscription::getFront(q, front))
+            back = q.getPosition();
+        else {
+            back = q.getPosition();
+            front = back+1;     // empty
+        }
         assert(front <= back + 1);
     }