Fix potential buffer vs. IO size mismatch in StorageBlock.

I don't think this happens in practice, or we would likely have
seen crashes or corruption. When calling CopyFrom(), the other
storage block must have the same num_buffers() value, otherwise
we will allocate buffers when calling the Data() method based
on the old num_buffers() value, and then update it to the other's
num_buffer() value. Later on, Store() may calculate a size based
upon the new value, but the buffer is still sized according to the
old value.

-- Add test to show this situation.

Change-Id: Ib72215dd6c51d90be0483158543daf6a1d814b69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5039450
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1226956}

net/disk_cache/blockfile/storage_block-inl.h

@@ -36,10 +36,10 @@ void StorageBlock<T>::CopyFrom(StorageBlock<T>* other) {
   DCHECK(!modified_);
   DCHECK(!other->modified_);
   Discard();
-  *Data() = *other->Data();
-  file_ = other->file_;
   address_ = other->address_;
   extended_ = other->extended_;
+  file_ = other->file_;
+  *Data() = *other->Data();
 }
 
 template<typename T> void* StorageBlock<T>::buffer() const {

net/disk_cache/blockfile/storage_block_unittest.cc

@@ -70,3 +70,22 @@ TEST_F(DiskCacheTest, StorageBlock_SetModified) {
   EXPECT_TRUE(entry2.Load());
   EXPECT_TRUE(0x45687912 == entry2.Data()->hash);
 }
+
+TEST_F(DiskCacheTest, StorageBlock_DifferentNumBuffers) {
+  base::FilePath filename = cache_path_.AppendASCII("a_test");
+  auto file = base::MakeRefCounted<disk_cache::MappedFile>();
+  ASSERT_TRUE(CreateCacheTestFile(filename));
+  ASSERT_TRUE(file->Init(filename, 8192));
+
+  // 2 buffers at index 1.
+  CacheEntryBlock entry1(file.get(), disk_cache::Addr(0xa1010001));
+  EXPECT_TRUE(entry1.Load());
+
+  // 1 buffer at index 3.
+  CacheEntryBlock entry2(file.get(), disk_cache::Addr(0xa0010003));
+  EXPECT_TRUE(entry2.Load());
+
+  // Now specify 2 buffers at index 1.
+  entry2.CopyFrom(&entry1);
+  EXPECT_TRUE(entry2.Load());
+}