From c7d56010815b0e7104dfc1d4bba6d16cdcbffec4 Mon Sep 17 00:00:00 2001
From: Mathias Agopian <mathias@google.com>
Date: Mon, 14 Sep 2009 15:48:42 -0700
Subject: [PATCH] make sure to update the tail pointer when undoing a dequeue

---
 include/private/ui/SharedBufferStack.h |  2 ++
 libs/ui/SharedBufferStack.cpp          | 20 +++++++++++++++-----
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/include/private/ui/SharedBufferStack.h b/include/private/ui/SharedBufferStack.h
index 6181f5576..8b0f154f5 100644
--- a/include/private/ui/SharedBufferStack.h
+++ b/include/private/ui/SharedBufferStack.h
@@ -228,6 +228,8 @@ private:
     friend struct Condition;
     friend struct DequeueCondition;
     friend struct LockCondition;
+    
+    int32_t computeTail() const;
 
     struct QueueUpdate : public UpdateBase {
         inline QueueUpdate(SharedBufferBase* sbb);
diff --git a/libs/ui/SharedBufferStack.cpp b/libs/ui/SharedBufferStack.cpp
index 436d79320..7789a3f86 100644
--- a/libs/ui/SharedBufferStack.cpp
+++ b/libs/ui/SharedBufferStack.cpp
@@ -245,20 +245,27 @@ ssize_t SharedBufferServer::StatusUpdate::operator()() {
 SharedBufferClient::SharedBufferClient(SharedClient* sharedClient,
         int surface, int num)
     : SharedBufferBase(sharedClient, surface, num), tail(0)
+{
+    tail = computeTail();
+}
+
+int32_t SharedBufferClient::computeTail() const
 {
     SharedBufferStack& stack( *mSharedStack );
-    int32_t avail;
-    int32_t head;
     // we need to make sure we read available and head coherently,
     // w.r.t RetireUpdate.
+    int32_t newTail;
+    int32_t avail;
+    int32_t head;
     do {
         avail = stack.available;
         head = stack.head;
     } while (stack.available != avail);
-    tail = head - avail + 1;
-    if (tail < 0) {
-        tail += num;
+    newTail = head - avail + 1;
+    if (newTail < 0) {
+        newTail += mNumBuffers;
     }
+    return newTail;
 }
 
 ssize_t SharedBufferClient::dequeue()
@@ -296,6 +303,9 @@ status_t SharedBufferClient::undoDequeue(int buf)
 {
     UndoDequeueUpdate update(this);
     status_t err = updateCondition( update );
+    if (err == NO_ERROR) {
+        tail = computeTail();
+    }
     return err;
 }