[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 slice_offset,
+                     uint64_t slice_size) {
+  const auto& source_items = source.items();
+  const auto& offsets = source.offsets();
+  DCHECK_LE(slice_offset + slice_size, source.total_size());
+  size_t item_index =
+      std::upper_bound(offsets.begin(), offsets.end(), slice_offset) -
+      offsets.begin();
+  uint64_t item_offset =
+      item_index == 0 ? slice_offset : slice_offset - offsets[item_index - 1];
+  size_t num_items = source_items.size();
+
+  LOG(ERROR) << "doing a slice at " << slice_offset << " with size "
+             << slice_size;
+  size_t first_item_index = item_index;
+  copying_memory_size = 0;
+
+  bool first_item = true;

[michaeln, 2016/07/14 01:44:43]

probably don't need this local, could item_index == first_item_index instead,
the value is only needed at one callsite

[dmurph, 2016/07/15 20:18:15]

Done.

+  bool last_item_memory_sliced = false;
+  // Read starting from 'mid' and 'offset_from_mid'.

[michaeln, 2016/07/14 01:44:43]

comment is stale

[dmurph, 2016/07/15 20:18:15]

Done.

+  for (uint64_t total_sliced = 0;
+       item_index < num_items && total_sliced < slice_size; item_index++) {
+    const scoped_refptr<BlobDataItem>& item = source_items[item_index]->item();

[michaeln, 2016/07/14 01:44:43]

might call this source_item for clarity

[dmurph, 2016/07/15 20:18:15]

Done.

+    uint64_t length = item->length();

[michaeln, 2016/07/14 01:44:43]

probably dont need this local
(the value is not used at many callsites and half of them refer to the value
thru the item instance pointer)

[dmurph, 2016/07/15 20:18:15]

Replaced callsites, I like the local.

+    DCHECK_NE(length, std::numeric_limits<uint64_t>::max());
+    DCHECK_NE(length, 0ull);
+
+    last_item_memory_sliced = false;
+
+    if (item_offset == 0 && slice_size - total_sliced >= item->length()) {
+      // We can share the entire item.
+      LOG(ERROR) << "we can share";
+      dest_items.push_back(source_items[item_index]);
+      total_sliced += length;
+      first_item = false;
+      continue;
+    }
+
+    uint64_t read_size =
+        std::min(item->length() - item_offset, slice_size - total_sliced);
+    total_sliced += read_size;

[michaeln, 2016/07/14 01:44:43]

might help readability to move the read_size computation and total_sliced
incremntation up above the test for whether the whole thing can be taken, gets
rid of one dup'd line of code and might be easier to grok

  if (item_offset == 0 && read_size == item->length()) {
    // Take the whole thing
    dest_items.push_back(source_items[item_index]);
    continue;
  }

[dmurph, 2016/07/15 20:18:15]

Done.

+
+    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 = item_offset;
+          first_source_item = source_items[first_item_index];
+        }

[michaeln, 2016/07/14 01:44:43]

i think the handling of the last partial item maybe could be done more directly
right here?

if (first_item) {
} else {
  // we must be taking a partial last item
}

[dmurph, 2016/07/15 20:18:15]

Ah, much better.

+        LOG(ERROR) << "we're sliceing a bytes item!";
+        last_item_memory_sliced = true;
+        copying_memory_size += read_size;
+        // Since we don't have quota yet for memory, we create temporary items
+        // for this data. When our blob is finished constructing, all dependent
+        // blobs are done, and we have enough memory quota, we'll copy the data
+        // over.
+        std::unique_ptr<DataElement> element(new DataElement());
+        element->SetToBytesDescription(base::checked_cast<size_t>(read_size));
+        data_item = new BlobDataItem(std::move(element), nullptr);
+      } break;
+      case DataElement::TYPE_FILE: {
+        std::unique_ptr<DataElement> element(new DataElement());
+        element->SetToFilePathRange(item->path(), item->offset() + item_offset,
+                                    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() + item_offset, 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() + item_offset,
+                                          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(std::move(data_item)));
+    first_item = false;
+    item_offset = 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[item_index - 1];
+  }
+}
+
+BlobSlice::~BlobSlice() {}
+
+}  // namespace storage