Componentize renderer side of DNS prefetching.

chrome/renderer/net/predictor_queue.cc

-// Copyright (c) 2006-2008 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.
-
-// See header file for description of DnsQueue class
-
-#include "chrome/renderer/net/predictor_queue.h"
-
-#include "base/logging.h"
-#include "base/metrics/stats_counters.h"
-
-DnsQueue::DnsQueue(BufferSize size)
-    : buffer_(new char[size + 2]),
-      buffer_size_(size + 1),
-      buffer_sentinel_(size + 1),
-      size_(0) {
-  CHECK(0 < static_cast<BufferSize>(size + 3));  // Avoid overflow worries.
-  buffer_[buffer_sentinel_] = '\0';  // Guard byte to help reading data.
-  readable_ = writeable_ = 0;  // Buffer starts empty.
-}
-
-DnsQueue::~DnsQueue(void) {
-}
-
-void DnsQueue::Clear() {
-  size_ = 0;
-  readable_ = writeable_;
-  DCHECK(Validate());
-}
-
-// Push takes an unterminated string plus its length.
-// The string must not contain a null terminator.
-// Exactly length chars are written, or nothing is written.
-// Returns true for success, false there was no room to push.
-DnsQueue::PushResult DnsQueue::Push(const char* source,
-                                    const size_t unsigned_length) {
-  BufferSize length = static_cast<BufferSize>(unsigned_length);
-  if (0 > length+1)  // Avoid overflows in conversion to signed.
-    return OVERFLOW_PUSH;
-
-  // To save on sites with a LOT of links to the SAME domain, we have a
-  // a compaction hack that removes duplicates when we try to push() a
-  // match with the last push.
-  if (0 < size_ && readable_ + length < buffer_sentinel_ &&
-    0 == strncmp(source, &buffer_[readable_], unsigned_length) &&
-    '\0' == buffer_[readable_ + unsigned_length]) {
-    SIMPLE_STATS_COUNTER("DNS.PrefetchDnsRedundantPush");
-
-    // We already wrote this name to the queue, so we'll skip this repeat.
-    return REDUNDANT_PUSH;
-  }
-
-  // Calling convention precludes nulls.
-  DCHECK(!length || '\0' != source[length - 1]);
-
-  DCHECK(Validate());
-
-  BufferSize available_space = readable_ - writeable_;
-
-  if (0 >= available_space) {
-    available_space += buffer_size_;
-  }
-
-  if (length + 1 >= available_space) {
-    SIMPLE_STATS_COUNTER("DNS.PrefetchDnsQueueFull");
-    return OVERFLOW_PUSH;  // Not enough space to push.
-  }
-
-  BufferSize dest = writeable_;
-  BufferSize space_till_wrap = buffer_sentinel_ - dest;
-  if (space_till_wrap < length + 1) {
-    // Copy until we run out of room at end of buffer.
-    std::memcpy(&buffer_[dest], source, space_till_wrap);
-    // Ensure caller didn't have embedded '\0' and also
-    // ensure trailing sentinel was in place.
-    // Relies on sentinel.
-    DCHECK(static_cast<size_t>(space_till_wrap) == strlen(&buffer_[dest]));
-
-    length -= space_till_wrap;
-    source += space_till_wrap;
-    dest = 0;  // Continue writing at start of buffer.
-  }
-
-  // Copy any remaining portion of source.
-  std::memcpy(&buffer_[dest], source, length);
-  DCHECK(dest + length < buffer_sentinel_);
-  buffer_[dest + length] = '\0';  // We need termination in our buffer.
-  // Preclude embedded '\0'.
-  DCHECK(static_cast<size_t>(length) == strlen(&buffer_[dest]));
-
-  dest += length + 1;
-  if (dest == buffer_sentinel_)
-    dest = 0;
-
-  writeable_ = dest;
-  size_++;
-  DCHECK(Validate());
-  return SUCCESSFUL_PUSH;
-}
-
-// Extracts the next available string from the buffer.
-// The returned string is null terminated, and hence has length
-// that is exactly one greater than the written string.
-// If the buffer is empty, then the Pop and returns false.
-bool DnsQueue::Pop(std::string* out_string) {
-  DCHECK(Validate());
-  // Sentinel will preclude memory reads beyond buffer's end.
-  DCHECK('\0' == buffer_[buffer_sentinel_]);
-
-  if (readable_ == writeable_) {
-    return false;  // buffer was empty
-  }
-
-  // Constructor *may* rely on sentinel for null termination.
-  (*out_string) = &buffer_[readable_];
-  // Our sentinel_ at end of buffer precludes an overflow in cast.
-  BufferSize first_fragment_size = static_cast<BufferSize> (out_string->size());
-
-  BufferSize terminal_null;
-  if (readable_ + first_fragment_size >= buffer_sentinel_) {
-    // Sentinel was used, so we need the portion after the wrap.
-    out_string->append(&buffer_[0]);  // Fragment at start of buffer.
-    // Sentinel precludes overflow in cast to signed type.
-    terminal_null = static_cast<BufferSize>(out_string->size())
-                    - first_fragment_size;
-  } else {
-    terminal_null = readable_ + first_fragment_size;
-  }
-  DCHECK('\0' == buffer_[terminal_null]);
-
-  BufferSize new_readable = terminal_null + 1;
-  if (buffer_sentinel_ == new_readable)
-    new_readable = 0;
-
-  readable_ = new_readable;
-  size_--;
-  if (readable_ == writeable_ || 0 == size_) {
-    // Queue is empty, so reset to start of buffer to help with peeking.
-    readable_ = writeable_ = 0;
-  }
-  DCHECK(Validate());
-  return true;
-}
-
-bool DnsQueue::Validate() {
-  return (readable_ >= 0) &&
-          readable_ < buffer_sentinel_ &&
-          writeable_ >= 0 &&
-          writeable_ < buffer_sentinel_ &&
-          '\0' == buffer_[buffer_sentinel_] &&
-          ((0 == size_) == (readable_ == writeable_));
-}