[BlobAsync] Disk support for blob storage

storage/browser/blob/blob_slice.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "storage/browser/blob/blob_slice.h"
+
+#include <algorithm>
+
+#include "storage/browser/blob/blob_data_item.h"
+#include "storage/browser/blob/blob_storage_context.h"
+#include "storage/browser/blob/shareable_blob_data_item.h"
+
+namespace storage {
+
+BlobSlice::BlobSlice(const InternalBlobData& source,
+                     uint64_t blob_offset,
+                     uint64_t size) {

[michaeln, 2016/07/07 20:05:23]

some naming suggestions: source, slice_offset, slice_size

[dmurph, 2016/07/11 23:33:27]

Done.

+  const auto& source_items = source.items();

[michaeln, 2016/07/07 20:05:23]

what if source_items is empty? can that happen?

[dmurph, 2016/07/11 23:33:27]

That shouldn't happen, as we check to see if we need to slice the blob before we
create the slice. Even so, it should correctly create the blob w/
copying_memory_size = 0, and no items. I'll start adding tests soon, as this
architecture seems pretty good.

+  const auto& offsets = source.offsets();
+  size_t mid = std::upper_bound(offsets.begin(), offsets.end(), blob_offset) -
+               offsets.begin();
+  uint64_t offset_from_mid =

[michaeln, 2016/07/07 20:05:23]

for these two locals, names like item_index and item_offset would've helped me

are there any assumptions being made about the inputs that could be DCHECK'd,
like...
DCHECK(!items.empty())
DCHECK_LE(slice_offset + slice_size, source.size())

[dmurph, 2016/07/11 23:33:27]

Done.

+      mid == 0 ? blob_offset : blob_offset - offsets[mid - 1];
+  size_t num_items = source_items.size();
+
+  LOG(ERROR) << "doing a slice at " << blob_offset << " with size " << size;
+  size_t first_item_index = mid;
+  copying_memory_size = 0;
+
+  bool first_item = true;
+  bool last_item_memory_sliced = false;
+  // Read starting from 'mid' and 'offset_from_mid'.
+  for (uint64_t memory_read = 0; mid < num_items && memory_read < size; mid++) {

[michaeln, 2016/07/07 20:05:23]

it looks like |memory_read| also accumulates the sizes of file backed items? is
|memory_read| misnamed? maybe total_sliced?

[dmurph, 2016/07/11 23:33:27]

Done.

+    const scoped_refptr<BlobDataItem>& item = source_items[mid]->item();
+    uint64_t length = item->length();
+    if (length == 0) {

[michaeln, 2016/07/07 20:05:23]

do we have zero length items? i think at some point we didn't

[dmurph, 2016/07/11 23:33:27]

We shouldn't. I'll dcheck this.

+      continue;
+    }
+    DCHECK_NE(length, std::numeric_limits<uint64_t>::max());
+    DCHECK_NE(length, 0ull);
+
+    last_item_memory_sliced = false;
+    LOG(ERROR) << item->length();
+    bool can_share_item =

[michaeln, 2016/07/07 20:05:23]

since this local is only used in the if below, getting rid of the local might be
a small improvement. a code comment that says // We can share the entire item
would satisfy the purpose it looks like your want the local to serve

[dmurph, 2016/07/11 23:33:27]

Done.

+        offset_from_mid == 0 && size - memory_read >= item->length();
+
+    if (can_share_item) {
+      LOG(ERROR) << "we can share";
+      dest_items.push_back(source_items[mid]);
+      memory_read += length;
+      first_item = false;
+      continue;
+    }
+
+    uint64_t read_size =
+        std::min(item->length() - offset_from_mid, size - memory_read);
+    memory_read += read_size;
+
+    scoped_refptr<BlobDataItem> data_item;
+    switch (item->type()) {
+      case DataElement::TYPE_BYTES_DESCRIPTION:
+      case DataElement::TYPE_BYTES: {
+        if (first_item) {
+          has_sliced_first_memory_item = true;
+          first_item_slice_offset = offset_from_mid;
+          first_source_item = source_items[first_item_index];
+        }
+        LOG(ERROR) << "we're sliceing a bytes item!";
+        last_item_memory_sliced = true;
+        copying_memory_size += read_size;
+        std::unique_ptr<DataElement> element(new DataElement());
+        element->SetToBytesDescription(base::checked_cast<size_t>(read_size));
+        data_item = new BlobDataItem(std::move(element), nullptr);

[michaeln, 2016/07/07 20:05:23]

when do we really copy the data?

[dmurph, 2016/07/11 23:33:27]

When the blob is finished constructing. The flattener records all of these
'copies', and then we store them. When we have all other blob data (& we've
asked the memory manager if we can store this data) we do these copies.

+      } break;
+      case DataElement::TYPE_FILE: {
+        std::unique_ptr<DataElement> element(new DataElement());
+        element->SetToFilePathRange(item->path(),
+                                    item->offset() + offset_from_mid, read_size,
+                                    item->expected_modification_time());
+        data_item = new BlobDataItem(std::move(element), item->data_handle_);
+      } break;
+      case DataElement::TYPE_FILE_FILESYSTEM: {
+        std::unique_ptr<DataElement> element(new DataElement());
+        element->SetToFileSystemUrlRange(
+            item->filesystem_url(), item->offset() + offset_from_mid, read_size,
+            item->expected_modification_time());
+        data_item = new BlobDataItem(std::move(element), nullptr);
+      } break;
+      case DataElement::TYPE_DISK_CACHE_ENTRY: {
+        std::unique_ptr<DataElement> element(new DataElement());
+        element->SetToDiskCacheEntryRange(item->offset() + offset_from_mid,
+                                          read_size);
+        data_item = new BlobDataItem(std::move(element), item->data_handle_,
+                                     item->disk_cache_entry(),
+                                     item->disk_cache_stream_index(),
+                                     item->disk_cache_side_stream_index());
+      } break;
+      case DataElement::TYPE_BLOB:
+      case DataElement::TYPE_UNKNOWN:
+        CHECK(false) << "Illegal blob item type: " << item->type();
+    }
+    dest_items.push_back(new ShareableBlobDataItem(
+        BlobStorageContext::GetAndIncrementItemId(), std::move(data_item)));
+    first_item = false;
+    offset_from_mid = 0;
+  }
+  if (last_item_memory_sliced &&
+      !(has_sliced_first_memory_item && dest_items.size() == 1)) {
+    has_sliced_last_memory_item = true;
+    last_source_item = source_items[mid - 1];
+  }
+}
+
+BlobSlice::~BlobSlice() {}
+
+}  // namespace storage