Add debug sentinel to GrMemoryPool to check for memory stomping

src/gpu/GrMemoryPool.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrMemoryPool.h"
 
 #ifdef SK_DEBUG
@@ skipping 22 matching lines @@
 GrMemoryPool::~GrMemoryPool() {
     VALIDATE;
     SkASSERT(0 == fAllocationCnt);
     SkASSERT(fHead == fTail);
     SkASSERT(0 == fHead->fLiveCount);
     DeleteBlock(fHead);
 };
 
 void* GrMemoryPool::allocate(size_t size) {
     VALIDATE;
+    size += kPerAllocPad;
     size = GrSizeAlignUp(size, kAlignment);
-    size += kPerAllocPad;
     if (fTail->fFreeSize < size) {
         size_t blockSize = size;
         blockSize = SkTMax<size_t>(blockSize, fMinAllocSize);
         BlockHeader* block = CreateBlock(blockSize);
 
         block->fPrev = fTail;
         block->fNext = nullptr;
         SkASSERT(nullptr == fTail->fNext);
         fTail->fNext = block;
         fTail = block;
         fSize += block->fSize;
         SkDEBUGCODE(++fAllocBlockCnt);
     }
     SkASSERT(fTail->fFreeSize >= size);
     intptr_t ptr = fTail->fCurrPtr;
     // We stash a pointer to the block header, just before the allocated space,
     // so that we can decrement the live count on delete in constant time.
-    *reinterpret_cast<BlockHeader**>(ptr) = fTail;
+    AllocHeader* allocData = reinterpret_cast<AllocHeader*>(ptr);
+    SkDEBUGCODE(allocData->fSentinal = kAssignedMarker);
+    allocData->fHeader = fTail;
     ptr += kPerAllocPad;
     fTail->fPrevPtr = fTail->fCurrPtr;
     fTail->fCurrPtr += size;
     fTail->fFreeSize -= size;
     fTail->fLiveCount += 1;
 
     SkDEBUGCODE(++fAllocationCnt);
     VALIDATE;
     return reinterpret_cast<void*>(ptr);
 }
 
 void GrMemoryPool::release(void* p) {
     VALIDATE;
     intptr_t ptr = reinterpret_cast<intptr_t>(p) - kPerAllocPad;
-    BlockHeader* block = *reinterpret_cast<BlockHeader**>(ptr);
+    AllocHeader* allocData = reinterpret_cast<AllocHeader*>(ptr);
+    SkASSERT(kAssignedMarker == allocData->fSentinal);
+    SkDEBUGCODE(allocData->fSentinal = kFreedMarker);
+    BlockHeader* block = allocData->fHeader;
     if (1 == block->fLiveCount) {
         // the head block is special, it is reset rather than deleted
         if (fHead == block) {
             fHead->fCurrPtr = reinterpret_cast<intptr_t>(fHead) + kHeaderSize;
             fHead->fLiveCount = 0;
             fHead->fFreeSize = fPreallocSize;
         } else {
             BlockHeader* prev = block->fPrev;
             BlockHeader* next = block->fNext;
             SkASSERT(prev);
@@ skipping 64 matching lines @@
         if (fHead != block) {
             SkASSERT(block->fLiveCount);
             SkASSERT(userSize >= fMinAllocSize);
         } else {
             SkASSERT(userSize == fPreallocSize);
         }
         if (!block->fLiveCount) {
             SkASSERT(ptrOffset ==  kHeaderSize);
             SkASSERT(userStart == block->fCurrPtr);
         } else {
-            SkASSERT(block == *reinterpret_cast<BlockHeader**>(userStart));
+            AllocHeader* allocData = reinterpret_cast<AllocHeader*>(userStart);
+            SkASSERT(allocData->fSentinal == kAssignedMarker || 
+                     allocData->fSentinal == kFreedMarker);
+            SkASSERT(block == allocData->fHeader);
         }
+
         prev = block;
     } while ((block = block->fNext));
     SkASSERT(allocCount == fAllocationCnt);
     SkASSERT(prev == fTail);
     SkASSERT(fAllocBlockCnt != 0 || fSize == 0);
 #endif
 }