AU: Speed up updates by using buffered writes.

buffered_file_writer.cc

Index: buffered_file_writer.cc
diff --git a/buffered_file_writer.cc b/buffered_file_writer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..03b8c77bc9616298d4a64e80752fa9d0c1f4265a
--- /dev/null
+++ b/buffered_file_writer.cc
@@ -0,0 +1,60 @@
+// Copyright (c) 2010 The Chromium OS 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 "update_engine/buffered_file_writer.h"
+
+namespace chromeos_update_engine {
+
+BufferedFileWriter::BufferedFileWriter(FileWriter* next, size_t buffer_size)
+    : next_(next),
+      buffer_(new char[buffer_size]),
+      buffer_size_(buffer_size),
+      buffered_bytes_(0) {}
+
+BufferedFileWriter::~BufferedFileWriter() {}
+
+int BufferedFileWriter::Open(const char* path, int flags, mode_t mode) {
+  return next_->Open(path, flags, mode);
+}
+
+ssize_t BufferedFileWriter::Write(const void* bytes, size_t count) {
+  size_t copied_bytes = 0;
+  while (copied_bytes < count) {
+    // Buffers as many bytes as possible.
+    size_t remaining_free = buffer_size_ - buffered_bytes_;
+    size_t copy_bytes = count - copied_bytes;
+    if (copy_bytes > remaining_free) {
+      copy_bytes = remaining_free;
+    }
+    const char* char_bytes = reinterpret_cast<const char*>(bytes);
+    memcpy(&buffer_[buffered_bytes_], char_bytes + copied_bytes, copy_bytes);
+    buffered_bytes_ += copy_bytes;
+    copied_bytes += copy_bytes;
+
+    // If full, sends the buffer to the next FileWriter.
+    if (buffered_bytes_ == buffer_size_) {
+      ssize_t rc = WriteBuffer();
+      if (rc < 0)
+        return rc;
+    }
+  }
+  return count;
+}
+
+int BufferedFileWriter::Close() {
+  // Flushes the buffer first, then closes the next FileWriter.
+  ssize_t rc_write = WriteBuffer();
+  int rc_close = next_->Close();
+  return (rc_write < 0) ? rc_write : rc_close;
+}
+
+ssize_t BufferedFileWriter::WriteBuffer() {
+  if (buffered_bytes_ == 0)
+    return 0;
+  ssize_t rc = next_->Write(&buffer_[0], buffered_bytes_);
+  buffered_bytes_ = 0;
+  return rc;
+}
+
+}  // namespace chromeos_update_engine