[fsp] Improve performance for reading small chunks of data.

webkit/browser/fileapi/file_system_backend.h

Index: webkit/browser/fileapi/file_system_backend.h
diff --git a/webkit/browser/fileapi/file_system_backend.h b/webkit/browser/fileapi/file_system_backend.h
index b71c8cab06d7fce114b06a3ade098dff8084f165..b6c83095f922d7d9727bc27b92f7551acf6bf9d8 100644
--- a/webkit/browser/fileapi/file_system_backend.h
+++ b/webkit/browser/fileapi/file_system_backend.h
@@ -35,6 +35,10 @@ class FileSystemOperation;
 class FileSystemQuotaUtil;
 class WatcherManager;
 
+// Maximum value of length arguments used for
+// FileSystemBackend::CreateFileStreamReader().
+const int64 kMaximumLength = std::numeric_limits<int64>::max();

[hashimoto, 2014/09/04 11:21:25]

Why is this constant needed?

[mtomasz, 2014/09/05 01:04:40]

Having a call:

new FileStreamReader(0, kMaximumLength) is saying more than
new FileStreamReader(0, std::numeric_limits<int64>::max()).

It's also shorter, since we don't need to include <limits> everywhere we want to
use a maximum length for a file stream reader.

Finally, if we change the type from int64 to uint64, then we can just do it in
one place.

+
 // An interface for defining a file system backend.
 //
 // NOTE: when you implement a new FileSystemBackend for your own
@@ -108,12 +112,12 @@ class STORAGE_EXPORT FileSystemBackend {
   // if it does any succeeding read operations should fail with
   // ERR_UPLOAD_FILE_CHANGED error.
   // This method itself does *not* check if the given path exists and is a
-  // regular file.
-  // The |length| argument says how many bytes are going to be read using the
-  // instance of the file stream reader. If unknown, then equal to -1.
+  // regular file. At most |limit| bytes can be fetched from the file stream
+  // reader.
   virtual scoped_ptr<storage::FileStreamReader> CreateFileStreamReader(
       const FileSystemURL& url,
       int64 offset,
+      int64 limit,

[hashimoto, 2014/09/04 11:21:25]

It's not unclear from the word |limit| if this value is measured from the
beginning of the file, or from the offset.
How about using something like "max_length", "max_bytes_to_read", or anything
which clearly says this value is the number of bytes measured from |offset|?

[mtomasz, 2014/09/05 01:04:40]

This naming OFFSET and LIMIT is used widely in databases, and it means a limit
starting from the offset. max_length is not much better here since it may mean a
length of the file. max_bytes_to_read makes sense. Let's go with it.

       const base::Time& expected_modification_time,
       FileSystemContext* context) const = 0;