ServiceWorkerWriteToCacheJob: refactor

content/browser/service_worker/service_worker_cache_writer.cc

Index: content/browser/service_worker/service_worker_cache_writer.cc
diff --git a/content/browser/service_worker/service_worker_cache_writer.cc b/content/browser/service_worker/service_worker_cache_writer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..0e517a76dcc4c1797978b46d934357d0c516a629
--- /dev/null
+++ b/content/browser/service_worker/service_worker_cache_writer.cc
@@ -0,0 +1,587 @@
+// Copyright 2015 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 "content/browser/service_worker/service_worker_cache_writer.h"
+
+#include <algorithm>
+#include <string>
+
+#include "content/browser/appcache/appcache_response.h"
+#include "content/browser/service_worker/service_worker_disk_cache.h"
+#include "content/browser/service_worker/service_worker_storage.h"
+
+namespace {
+
+const size_t kCopyBufferSize = 16 * 1024;
+
+// States for the state machine.
+//
+// The state machine flows roughly like this: if there is no existing cache
+// entry, incoming headers and data are written directly back to the cache
+// ("passthrough mode", the PASSTHROUGH states). If there is an existing cache
+// entry, incoming headers and data are compared to the existing cache entry
+// ("compare mode", the COMPARE states); if at any point the incoming
+// headers/data are not equal to the cached headers/data, this class copies the
+// cached data up to the point where the incoming data and the cached data
+// diverged ("copy mode", the COPY states), then switches to "passthrough mode"
+// to write the remainder of the incoming data. The overall effect is to avoid
+// rewriting the cache entry if the incoming data is identical to the cached
+// data.

[Randy Smith (Not in Mondays), 2015/09/13 20:56:36]

This is an awesome explanatory comment, with a goldilocks level of detail (IMO).
 Thank you.

[Elly Fong-Jones, 2015/09/15 15:08:24]

Acknowledged.

+enum {
+  STATE_START,
+  // Control flows linearly through these four states, then loops from
+  // READ_DATA_FOR_COMPARE_DONE to READ_DATA_FOR_COMPARE, or exits to
+  // READ_HEADERS_FOR_COPY.
+  STATE_READ_HEADERS_FOR_COMPARE,
+  STATE_READ_HEADERS_FOR_COMPARE_DONE,
+  STATE_READ_DATA_FOR_COMPARE,
+  STATE_READ_DATA_FOR_COMPARE_DONE,
+
+  // Control flows linearly through these states, with each pass from
+  // READ_DATA_FOR_COPY to WRITE_DATA_FOR_COPY_DONE copying one block of data
+  // at a time. Control loops from WRITE_DATA_FOR_COPY_DONE back to
+  // READ_DATA_FOR_COPY if there is more data to copy, or exits to
+  // WRITE_DATA_FOR_PASSTHROUGH.
+  STATE_READ_HEADERS_FOR_COPY,
+  STATE_READ_HEADERS_FOR_COPY_DONE,
+  STATE_WRITE_HEADERS_FOR_COPY,
+  STATE_WRITE_HEADERS_FOR_COPY_DONE,
+  STATE_READ_DATA_FOR_COPY,
+  STATE_READ_DATA_FOR_COPY_DONE,
+  STATE_WRITE_DATA_FOR_COPY,
+  STATE_WRITE_DATA_FOR_COPY_DONE,
+
+  // Control flows linearly through these states, with a loop between
+  // WRITE_DATA_FOR_PASSTHROUGH and WRITE_DATA_FOR_PASSTHROUGH_DONE.
+  STATE_WRITE_HEADERS_FOR_PASSTHROUGH,
+  STATE_WRITE_HEADERS_FOR_PASSTHROUGH_DONE,
+  STATE_WRITE_DATA_FOR_PASSTHROUGH,
+  STATE_WRITE_DATA_FOR_PASSTHROUGH_DONE,
+  STATE_DONE,
+};
+
+// Shim class used to turn always-async functions into async-or-result
+// functions. See the comments below near ReadInfoHelper.
+class AsyncOnlyCompletionCallbackAdaptor
+    : public base::RefCounted<AsyncOnlyCompletionCallbackAdaptor> {
+ public:
+  explicit AsyncOnlyCompletionCallbackAdaptor(
+      const net::CompletionCallback& callback)
+      : async_(false), result_(net::ERR_IO_PENDING), callback_(callback) {}
+
+  void set_async(bool async) { async_ = async; }
+  bool async() { return async_; }
+  int result() { return result_; }
+
+  void WrappedCallback(int result) {
+    result_ = result;
+    if (async_)
+      callback_.Run(result);
+  }
+
+ private:
+  friend class base::RefCounted<AsyncOnlyCompletionCallbackAdaptor>;
+  virtual ~AsyncOnlyCompletionCallbackAdaptor() {}
+
+  bool async_;
+  int result_;
+  net::CompletionCallback callback_;
+};
+
+}  // namespace
+
+namespace content {
+
+int ServiceWorkerCacheWriter::DoLoop(int status) {
+  bool pause = false;
+  do {
+    int next_state = -1;
+    switch (state_) {
+      case STATE_START:
+        status = DoStart(&next_state, &pause, status);
+        break;
+      case STATE_READ_HEADERS_FOR_COMPARE:
+        status = DoReadHeadersForCompare(&next_state, &pause, status);
+        break;
+      case STATE_READ_HEADERS_FOR_COMPARE_DONE:
+        status = DoReadHeadersForCompareDone(&next_state, &pause, status);
+        break;
+      case STATE_READ_DATA_FOR_COMPARE:
+        status = DoReadDataForCompare(&next_state, &pause, status);
+        break;
+      case STATE_READ_DATA_FOR_COMPARE_DONE:
+        status = DoReadDataForCompareDone(&next_state, &pause, status);
+        break;
+      case STATE_READ_HEADERS_FOR_COPY:
+        status = DoReadHeadersForCopy(&next_state, &pause, status);
+        break;
+      case STATE_READ_HEADERS_FOR_COPY_DONE:
+        status = DoReadHeadersForCopyDone(&next_state, &pause, status);
+        break;
+      case STATE_READ_DATA_FOR_COPY:
+        status = DoReadDataForCopy(&next_state, &pause, status);
+        break;
+      case STATE_READ_DATA_FOR_COPY_DONE:
+        status = DoReadDataForCopyDone(&next_state, &pause, status);
+        break;
+      case STATE_WRITE_HEADERS_FOR_PASSTHROUGH:
+        status = DoWriteHeadersForPassthrough(&next_state, &pause, status);
+        break;
+      case STATE_WRITE_HEADERS_FOR_PASSTHROUGH_DONE:
+        status = DoWriteHeadersForPassthroughDone(&next_state, &pause, status);
+        break;
+      case STATE_WRITE_DATA_FOR_PASSTHROUGH:
+        status = DoWriteDataForPassthrough(&next_state, &pause, status);
+        break;
+      case STATE_WRITE_DATA_FOR_PASSTHROUGH_DONE:
+        status = DoWriteDataForPassthroughDone(&next_state, &pause, status);
+        break;
+      case STATE_WRITE_HEADERS_FOR_COPY:
+        status = DoWriteHeadersForCopy(&next_state, &pause, status);
+        break;
+      case STATE_WRITE_HEADERS_FOR_COPY_DONE:
+        status = DoWriteHeadersForCopyDone(&next_state, &pause, status);
+        break;
+      case STATE_WRITE_DATA_FOR_COPY:
+        status = DoWriteDataForCopy(&next_state, &pause, status);
+        break;
+      case STATE_WRITE_DATA_FOR_COPY_DONE:
+        status = DoWriteDataForCopyDone(&next_state, &pause, status);
+        break;
+      case STATE_DONE:
+        status = DoDone(&next_state, &pause, status);
+        break;
+      default:
+        next_state = STATE_DONE;
+        break;
+    }
+    state_ = next_state;
+  } while (status >= net::OK && state_ != STATE_DONE && !pause);
+  return status;
+}
+
+ServiceWorkerCacheWriter::ServiceWorkerCacheWriter(
+    const ResponseReaderCreator& reader_creator,
+    const ResponseWriterCreator& writer_creator)
+    : state_(STATE_START),
+      did_replace_(false),
+      reader_creator_(reader_creator),
+      writer_creator_(writer_creator),
+      weak_factory_(this) {}
+
+ServiceWorkerCacheWriter::~ServiceWorkerCacheWriter() {}
+
+net::Error ServiceWorkerCacheWriter::MaybeWriteHeaders(
+    HttpResponseInfoIOBuffer* headers,
+    const OnWriteCompleteCallback& callback) {
+  headers_to_write_ = headers;
+  pending_callback_ = callback;
+  DCHECK_EQ(STATE_START, state_);
+  int result = DoLoop(net::OK);
+
+  // Ensure that the state machine is in one of the expected pause states.
+  // Synchronous errors always go to ERR_IO_PENDING.
+  if (result < 0 && result != net::ERR_IO_PENDING)
+    DCHECK_EQ(STATE_DONE, state_);
+
+  // Synchronous successes either go into passthrough mode or compare/copy mode.
+  if (result >= 0) {
+    DCHECK(state_ == STATE_READ_DATA_FOR_COMPARE ||
+           state_ == STATE_WRITE_DATA_FOR_PASSTHROUGH)
+        << "Unexpected state: " << state_;
+  }
+
+  // ERR_IO_PENDING has to have one of the "done" states as the next state.
+  if (result == net::ERR_IO_PENDING) {
+    DCHECK(state_ == STATE_READ_HEADERS_FOR_COMPARE_DONE ||
+           state_ == STATE_WRITE_HEADERS_FOR_COPY_DONE ||
+           state_ == STATE_WRITE_HEADERS_FOR_PASSTHROUGH_DONE)
+        << "Unexpected state: " << state_;
+  }
+
+  return result >= 0 ? net::OK : static_cast<net::Error>(result);
+}
+
+net::Error ServiceWorkerCacheWriter::MaybeWriteData(
+    net::IOBuffer* buf,
+    size_t buf_size,
+    const OnWriteCompleteCallback& callback) {
+  data_to_write_ = buf;
+  len_to_write_ = buf_size;
+  pending_callback_ = callback;
+  int result = DoLoop(net::OK);
+
+  // Check that the state machine is in one of the expected pause states.
+
+  // Synchronous errors are always STATE_DONE.
+  if (result < 0 && result != net::ERR_IO_PENDING)
+    DCHECK_EQ(STATE_DONE, state_);
+
+  // Synchronous successes mean that this object is immediately ready for a new
+  // call to MaybeWriteData, so it must be waiting in one of the ReadData
+  // states, or writing data directly in passthrough mode:
+  if (result >= 0) {
+    DCHECK(state_ == STATE_READ_DATA_FOR_COMPARE ||
+           state_ == STATE_READ_DATA_FOR_COPY ||
+           state_ == STATE_WRITE_DATA_FOR_PASSTHROUGH)
+        << "Unexpected state: " << state_;
+  }
+
+  // Asynchronous completion means the state machine must be waiting in one of
+  // the Done states for an IO operation to complete:
+  if (result == net::ERR_IO_PENDING) {
+    // Note that STATE_READ_HEADERS_FOR_COMPARE_DONE is excluded because the
+    // headers are compared in MaybeWriteHeaders, not here, and
+    // STATE_WRITE_HEADERS_FOR_PASSTHROUGH_DONE is excluded because that write
+    // is done by MaybeWriteHeaders.
+    DCHECK(state_ == STATE_READ_DATA_FOR_COMPARE_DONE ||
+           state_ == STATE_READ_HEADERS_FOR_COPY_DONE ||
+           state_ == STATE_READ_DATA_FOR_COPY_DONE ||
+           state_ == STATE_WRITE_HEADERS_FOR_COPY_DONE ||
+           state_ == STATE_WRITE_DATA_FOR_COPY_DONE ||
+           state_ == STATE_WRITE_DATA_FOR_PASSTHROUGH_DONE)
+        << "Unexpected state: " << state_;
+  }
+
+  return result >= 0 ? net::OK : static_cast<net::Error>(result);
+}
+
+int ServiceWorkerCacheWriter::DoStart(int* next_state,
+                                      bool* pause,
+                                      int result) {
+  bytes_written_ = 0;
+  compare_reader_ = reader_creator_.Run();
+  if (compare_reader_.get()) {
+    *next_state = STATE_READ_HEADERS_FOR_COMPARE;
+  } else {
+    // No existing reader, just write the headers back directly.
+    *next_state = STATE_WRITE_HEADERS_FOR_PASSTHROUGH;
+  }
+  return net::OK;
+}
+
+int ServiceWorkerCacheWriter::DoReadHeadersForCompare(int* next_state,
+                                                      bool* pause,
+                                                      int result) {
+  DCHECK(headers_to_write_);
+
+  headers_to_read_ = new HttpResponseInfoIOBuffer;
+  *next_state = STATE_READ_HEADERS_FOR_COMPARE_DONE;
+  return ReadInfoHelper(compare_reader_, headers_to_read_.get());
+}
+
+int ServiceWorkerCacheWriter::DoReadHeadersForCompareDone(int* next_state,
+                                                          bool* pause,
+                                                          int result) {
+  if (result < 0) {
+    *next_state = STATE_DONE;
+    return result;
+  }
+  cached_length_ = headers_to_read_->response_data_size;
+  bytes_compared_ = 0;
+  *pause = true;
+  *next_state = STATE_READ_DATA_FOR_COMPARE;
+  return net::OK;
+}
+
+int ServiceWorkerCacheWriter::DoReadDataForCompare(int* next_state,
+                                                   bool* pause,
+                                                   int result) {
+  DCHECK(data_to_write_);
+
+  data_to_read_ = new net::IOBuffer(len_to_write_);
+  len_to_read_ = len_to_write_;
+  *next_state = STATE_READ_DATA_FOR_COMPARE_DONE;
+  compare_offset_ = 0;
+  // If this was an EOF, don't issue a read.
+  if (len_to_write_ > 0)
+    result = ReadDataHelper(compare_reader_, data_to_read_.get(), len_to_read_);
+  return result;
+}
+
+int ServiceWorkerCacheWriter::DoReadDataForCompareDone(int* next_state,
+                                                       bool* pause,
+                                                       int result) {
+  DCHECK(data_to_read_);
+  DCHECK(data_to_write_);
+  DCHECK_EQ(len_to_read_, len_to_write_);
+  DCHECK_LE(result + compare_offset_, static_cast<size_t>(len_to_write_));
+
+  if (result < 0) {
+    *next_state = STATE_DONE;
+    return result;
+  }
+
+  // Premature EOF while reading the service worker script cache data to
+  // compare. Fail the comparison.
+  if (result == 0 && len_to_write_ != 0) {
+    *next_state = STATE_READ_HEADERS_FOR_COPY;
+    return net::OK;
+  }
+
+  // Compare the data from the ServiceWorker script cache to the data from the
+  // network.
+  if (memcmp(data_to_read_->data(), data_to_write_->data() + compare_offset_,
+             result)) {
+    // Data mismatched. This method already validated that all the bytes through
+    // |bytes_compared_| were identical, so copy the first |bytes_compared_|
+    // over, then start writing network data back after the changed point.
+    //
+    // Note that the state machine does NOT get paused here, since there is
+    // still further work to do before this object is ready to handle another
+    // WriteData call.
+    *next_state = STATE_READ_HEADERS_FOR_COPY;
+    return net::OK;
+  }
+
+  compare_offset_ += result;
+
+  // This is a little bit tricky. It is possible that not enough data was read
+  // to finish comparing the entire block of data from the network (which is
+  // kept in len_to_write_), so this method may need to issue another read and
+  // return to this state.
+  //
+  // Compare isn't complete yet. Issue another read for the remaining data. Note
+  // that this reuses the same IOBuffer.
+  if (compare_offset_ < static_cast<size_t>(len_to_read_)) {
+    *next_state = STATE_READ_DATA_FOR_COMPARE_DONE;
+    return ReadDataHelper(compare_reader_, data_to_read_.get(),
+                          len_to_read_ - compare_offset_);
+  }
+
+  // Cached entry is longer than the network entry but the prefix matches. Copy
+  // just the prefix.
+  if (len_to_read_ == 0 && bytes_compared_ + compare_offset_ < cached_length_) {
+    *next_state = STATE_READ_HEADERS_FOR_COPY;
+    return net::OK;
+  }
+
+  // bytes_compared_ only gets incremented when a full block is compared, to
+  // avoid having to use only parts of the buffered network data.
+  bytes_compared_ += result;
+  *next_state = STATE_READ_DATA_FOR_COMPARE;
+  *pause = true;
+  return net::OK;
+}
+
+int ServiceWorkerCacheWriter::DoReadHeadersForCopy(int* next_state,
+                                                   bool* pause,
+                                                   int result) {
+  bytes_copied_ = 0;
+  copy_reader_ = reader_creator_.Run();
+  headers_to_read_ = new HttpResponseInfoIOBuffer;
+  data_to_copy_ = new net::IOBuffer(kCopyBufferSize);
+  *next_state = STATE_READ_HEADERS_FOR_COPY_DONE;
+  return ReadInfoHelper(copy_reader_, headers_to_read_.get());
+}
+
+int ServiceWorkerCacheWriter::DoReadHeadersForCopyDone(int* next_state,
+                                                       bool* pause,
+                                                       int result) {
+  if (result < 0) {
+    *next_state = STATE_DONE;
+    return result;
+  }
+  *next_state = STATE_WRITE_HEADERS_FOR_COPY;
+  return net::OK;
+}
+
+// Write the just-read headers back to the cache.
+// Note that this method must create |writer_|, since the only paths to this
+// state never create a writer.
+// Also note that this *discards* the read headers and replaces them with the
+// net headers.
+int ServiceWorkerCacheWriter::DoWriteHeadersForCopy(int* next_state,
+                                                    bool* pause,
+                                                    int result) {
+  DCHECK(!writer_);
+  writer_ = writer_creator_.Run();
+  *next_state = STATE_WRITE_HEADERS_FOR_COPY_DONE;
+  return WriteInfoHelper(writer_, headers_to_write_.get());
+}
+
+int ServiceWorkerCacheWriter::DoWriteHeadersForCopyDone(int* next_state,
+                                                        bool* pause,
+                                                        int result) {
+  if (result < 0) {
+    *next_state = STATE_DONE;
+    return result;
+  }
+  *next_state = STATE_READ_DATA_FOR_COPY;
+  return net::OK;
+}
+
+int ServiceWorkerCacheWriter::DoReadDataForCopy(int* next_state,
+                                                bool* pause,
+                                                int result) {
+  size_t to_read = std::min(kCopyBufferSize, bytes_compared_ - bytes_copied_);
+  // At this point, all compared bytes have been read. Currently
+  // |data_to_write_| and |len_to_write_| hold the chunk of network input that
+  // caused the comparison failure, so those need to be written back and this
+  // object needs to go into passthrough mode.
+  if (to_read == 0) {
+    *next_state = STATE_WRITE_DATA_FOR_PASSTHROUGH;
+    return net::OK;
+  }
+  *next_state = STATE_READ_DATA_FOR_COPY_DONE;
+  return ReadDataHelper(copy_reader_, data_to_copy_.get(), to_read);
+}
+
+int ServiceWorkerCacheWriter::DoReadDataForCopyDone(int* next_state,
+                                                    bool* pause,
+                                                    int result) {
+  if (result < 0) {
+    *next_state = STATE_DONE;
+    return result;
+  }
+  *next_state = STATE_WRITE_DATA_FOR_COPY;
+  return result;
+}
+
+int ServiceWorkerCacheWriter::DoWriteDataForCopy(int* next_state,
+                                                 bool* pause,
+                                                 int result) {
+  *next_state = STATE_WRITE_DATA_FOR_COPY_DONE;
+  DCHECK_GT(result, 0);
+  return WriteDataHelper(writer_, data_to_copy_.get(), result);
+}
+
+int ServiceWorkerCacheWriter::DoWriteDataForCopyDone(int* next_state,
+                                                     bool* pause,
+                                                     int result) {
+  if (result < 0) {
+    *next_state = STATE_DONE;
+    return result;
+  }
+  bytes_written_ += result;
+  bytes_copied_ += result;
+  *next_state = STATE_READ_DATA_FOR_COPY;
+  return result;
+}
+
+int ServiceWorkerCacheWriter::DoWriteHeadersForPassthrough(int* next_state,
+                                                           bool* pause,
+                                                           int result) {
+  writer_ = writer_creator_.Run();
+  *next_state = STATE_WRITE_HEADERS_FOR_PASSTHROUGH_DONE;
+  return WriteInfoHelper(writer_, headers_to_write_.get());
+}
+
+int ServiceWorkerCacheWriter::DoWriteHeadersForPassthroughDone(int* next_state,
+                                                               bool* pause,
+                                                               int result) {
+  *next_state = STATE_WRITE_DATA_FOR_PASSTHROUGH;
+  *pause = true;
+  return net::OK;
+}
+
+int ServiceWorkerCacheWriter::DoWriteDataForPassthrough(int* next_state,
+                                                        bool* pause,
+                                                        int result) {
+  *next_state = STATE_WRITE_DATA_FOR_PASSTHROUGH_DONE;
+  if (len_to_write_ > 0)
+    result = WriteDataHelper(writer_, data_to_write_.get(), len_to_write_);
+  return result;
+}
+
+int ServiceWorkerCacheWriter::DoWriteDataForPassthroughDone(int* next_state,
+                                                            bool* pause,
+                                                            int result) {
+  if (result < 0) {
+    *next_state = STATE_DONE;
+    return result;
+  }
+  bytes_written_ += result;
+  *next_state = STATE_WRITE_DATA_FOR_PASSTHROUGH;
+  *pause = true;
+  return net::OK;
+}
+
+int ServiceWorkerCacheWriter::DoDone(int* next_state, bool* pause, int result) {
+  *next_state = STATE_DONE;
+  return net::OK;
+}
+
+// These helpers adapt the AppCache "always use the callback" pattern to the
+// //net "only use the callback for async" pattern using
+// AsyncCompletionCallbackAdaptor.
+//
+// Specifically, these methods return result codes directly for synchronous
+// completions, and only run their callback (which is AsyncDoLoop) for
+// asynchronous completions.
+
+int ServiceWorkerCacheWriter::ReadInfoHelper(
+    const scoped_ptr<ServiceWorkerResponseReader>& reader,
+    HttpResponseInfoIOBuffer* buf) {
+  net::CompletionCallback run_callback = base::Bind(
+      &ServiceWorkerCacheWriter::AsyncDoLoop, weak_factory_.GetWeakPtr());
+  scoped_refptr<AsyncOnlyCompletionCallbackAdaptor> adaptor(
+      new AsyncOnlyCompletionCallbackAdaptor(run_callback));
+  reader->ReadInfo(
+      buf, base::Bind(&AsyncOnlyCompletionCallbackAdaptor::WrappedCallback,
+                      adaptor));
+  adaptor->set_async(true);
+  return adaptor->result();
+}
+
+int ServiceWorkerCacheWriter::ReadDataHelper(
+    const scoped_ptr<ServiceWorkerResponseReader>& reader,
+    net::IOBuffer* buf,
+    int buf_len) {
+  net::CompletionCallback run_callback = base::Bind(
+      &ServiceWorkerCacheWriter::AsyncDoLoop, weak_factory_.GetWeakPtr());
+  scoped_refptr<AsyncOnlyCompletionCallbackAdaptor> adaptor(
+      new AsyncOnlyCompletionCallbackAdaptor(run_callback));
+  reader->ReadData(
+      buf, buf_len,
+      base::Bind(&AsyncOnlyCompletionCallbackAdaptor::WrappedCallback,
+                 adaptor));
+  adaptor->set_async(true);
+  return adaptor->result();
+}
+
+int ServiceWorkerCacheWriter::WriteInfoHelper(
+    const scoped_ptr<ServiceWorkerResponseWriter>& writer,
+    HttpResponseInfoIOBuffer* buf) {
+  did_replace_ = true;
+  net::CompletionCallback run_callback = base::Bind(
+      &ServiceWorkerCacheWriter::AsyncDoLoop, weak_factory_.GetWeakPtr());
+  scoped_refptr<AsyncOnlyCompletionCallbackAdaptor> adaptor(
+      new AsyncOnlyCompletionCallbackAdaptor(run_callback));
+  writer->WriteInfo(
+      buf, base::Bind(&AsyncOnlyCompletionCallbackAdaptor::WrappedCallback,
+                      adaptor));
+  adaptor->set_async(true);
+  return adaptor->result();
+}
+
+int ServiceWorkerCacheWriter::WriteDataHelper(
+    const scoped_ptr<ServiceWorkerResponseWriter>& writer,
+    net::IOBuffer* buf,
+    int buf_len) {
+  net::CompletionCallback run_callback = base::Bind(
+      &ServiceWorkerCacheWriter::AsyncDoLoop, weak_factory_.GetWeakPtr());
+  scoped_refptr<AsyncOnlyCompletionCallbackAdaptor> adaptor(
+      new AsyncOnlyCompletionCallbackAdaptor(run_callback));
+  writer->WriteData(
+      buf, buf_len,
+      base::Bind(&AsyncOnlyCompletionCallbackAdaptor::WrappedCallback,
+                 adaptor));
+  adaptor->set_async(true);
+  return adaptor->result();
+}
+
+void ServiceWorkerCacheWriter::AsyncDoLoop(int result) {
+  result = DoLoop(result);
+  // If the result is ERR_IO_PENDING, the pending callback will be run by a
+  // later invocation of AsyncDoLoop.
+  if (result != net::ERR_IO_PENDING) {
+    OnWriteCompleteCallback callback = pending_callback_;
+    pending_callback_.Reset();
+    net::Error error = result >= 0 ? net::OK : static_cast<net::Error>(result);
+    callback.Run(error);
+  }
+}
+
+}  // namespace content