DnsConfigService and a posix implementation

net/dns/dns_config_service_posix.cc

+// Copyright (c) 2011 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 "net/dns/dns_config_service_posix.h"
+
+#include "base/bind.h"
+#include "base/compiler_specific.h"
+#include "base/file_path.h"
+#include "base/files/file_path_watcher.h"
+#include "base/memory/ref_counted.h"
+#include "base/message_loop.h"
+#include "base/message_loop_proxy.h"
+#include "base/observer_list.h"
+#include "base/scoped_ptr.h"
+#include "base/threading/worker_pool.h"
+#include "net/base/ip_endpoint.h"
+#include "net/base/net_util.h"
+
+#ifndef _PATH_RESCONF  // Normally defined in <resolv.h>
+#define _PATH_RESCONF "/etc/resolv.conf"
+#endif
+
+namespace net {
+
+// FilePathWatcher::Delegate is refcounted, so we separate it from the Service
+// It also hosts callbacks on the WorkerPool.
+class DnsConfigServicePosix::WatcherDelegate :
+    public base::files::FilePathWatcher::Delegate {
+ public:
+  // Takes ownership of |lib|.
+  WatcherDelegate(DnsConfigServicePosix* service,
+                  DnsConfigServicePosix::ResolverLib* lib)
+    : service_(service),
+      resolver_lib_(lib),
+      message_loop_(base::MessageLoopProxy::current()),
+      reading_(false),
+      read_pending_(false) {}
+
+  void Cancel() {
+    DCHECK(message_loop_->BelongsToCurrentThread());
+    service_ = NULL;
+  }
+
+  void RescheduleWatch() {
+    DCHECK(message_loop_->BelongsToCurrentThread());
+    // Retry Watch in 100ms or so.
+    message_loop_->PostDelayedTask(
+        FROM_HERE, base::Bind(&WatcherDelegate::StartWatch, this), 100);
+  }
+
+  // FilePathWatcher::Delegate interface
+  virtual void OnFilePathChanged(const FilePath& path) OVERRIDE {
+    DCHECK(message_loop_->BelongsToCurrentThread());
+    if (!service_)
+      return;
+    ScheduleRead();
+  }
+
+  virtual void OnFilePathError(const FilePath& path) OVERRIDE {
+    DCHECK(message_loop_->BelongsToCurrentThread());
+    StartWatch();
+  }
+
+ private:
+  virtual ~WatcherDelegate() {}
+
+  // Unless already scheduled, post DoRead to WorkerPool.
+  void ScheduleRead() {
+    if (reading_) {
+      // Mark that we need to re-read after DoRead posts results.
+      read_pending_ = true;
+    } else {
+      if (!base::WorkerPool::PostTask(FROM_HERE, base::Bind(
+          &WatcherDelegate::DoRead, this), false)) {
+        // See worker_pool_posix.cc.
+        NOTREACHED() << "WorkerPool::PostTask is not expected to fail on posix";
+      }
+      reading_ = true;
+      read_pending_ = false;
+    }
+  }
+
+  // Reads DnsConfig and posts OnResultAvailable to |message_loop_|.
+  // Must be called on the worker thread.
+  void DoRead() {
+    DnsConfig config;
+    struct __res_state res;
+    bool success = false;
+    if (resolver_lib_->ninit(&res) == 0) {
+      success = ConvertResToConfig(res, &config);
+      resolver_lib_->nclose(&res);
+    }
+    // If this fails, the loop is gone, so there is no point retrying.
+    message_loop_->PostTask(FROM_HERE, base::Bind(
+        &WatcherDelegate::OnResultAvailable, this, config, success));
+  }
+
+  // Communicates result to the service. Must be called on the the same thread
+  // that constructed WatcherDelegate.
+  void OnResultAvailable(const DnsConfig &config, bool success) {
+    DCHECK(message_loop_->BelongsToCurrentThread());
+    if (!service_)
+      return;
+    reading_ = false;
+    if (read_pending_) {
+      // Discard this result and re-schedule.
+      ScheduleRead();
+      return;
+    }
+    if (!success) {
+      VLOG(1) << "Failed to read DnsConfig";
+    } else {
+      service_->OnConfigRead(config);
+    }
+  }
+
+  // To avoid refcounting the service, use this method in tasks/callbacks.
+  void StartWatch() {
+    if (!service_)
+      return;
+    service_->StartWatch();
+  }
+
+  DnsConfigServicePosix* service_;
+  scoped_ptr<DnsConfigServicePosix::ResolverLib> resolver_lib_;
+  // Message loop for the thread on which Watch is called (of TYPE_IO).
+  scoped_refptr<base::MessageLoopProxy> message_loop_;
+  // True after DoRead before OnResultsAvailable.
+  bool reading_;
+  // True after OnFilePathChanged fires while |reading_| is true.
+  bool read_pending_;
+};
+
+DnsConfigServicePosix::DnsConfigServicePosix()
+  : have_config_(false),
+    resolver_lib_(new ResolverLib()),
+    watcher_factory_(new FilePathWatcherFactory()) {
+}
+
+DnsConfigServicePosix::~DnsConfigServicePosix() {
+  DCHECK(CalledOnValidThread());
+  if (watcher_delegate_.get())
+    watcher_delegate_->Cancel();
+}
+
+void DnsConfigServicePosix::AddObserver(Observer* observer) {
+  DCHECK(CalledOnValidThread());
+  observers_.AddObserver(observer);
+  if (have_config_) {
+    observer->OnConfigChanged(dns_config_);
+  }
+}
+
+void DnsConfigServicePosix::RemoveObserver(Observer* observer) {
+  DCHECK(CalledOnValidThread());
+  observers_.RemoveObserver(observer);
+}
+
+void DnsConfigServicePosix::Watch() {
+  DCHECK(CalledOnValidThread());
+  DCHECK(!watcher_delegate_.get());
+  DCHECK(!resolv_file_watcher_.get());
+  DCHECK(resolver_lib_.get());
+  DCHECK(watcher_factory_.get());
+
+  watcher_delegate_ = new WatcherDelegate(this, resolver_lib_.release());
+  StartWatch();
+}
+
+void DnsConfigServicePosix::OnConfigRead(const DnsConfig& config) {

[cbentzel, 2011/08/16 17:44:24]

Maybe DCHECK(CalledOnValidThread()). It's not a public method, but I sometimes
find these very useful as both documentation and sanity checking. 

+  if (!config.Equals(dns_config_)) {
+    dns_config_ = config;
+    have_config_ = true;
+    FOR_EACH_OBSERVER(Observer, observers_, OnConfigChanged(config));
+  }
+}
+
+void DnsConfigServicePosix::StartWatch() {
+  DCHECK(watcher_delegate_.get());
+
+  FilePath path(FILE_PATH_LITERAL(_PATH_RESCONF));
+
+  // FilePathWatcher allows only one Watch per lifetime, so we need a new one.
+  resolv_file_watcher_.reset(watcher_factory_->CreateFilePathWatcher());
+  if (resolv_file_watcher_->Watch(path, watcher_delegate_.get())) {
+    // Make the initial read after watch is installed.
+    watcher_delegate_->OnFilePathChanged(path);
+  } else {
+    VLOG(1) << "Watch failed, scheduling restart";
+    watcher_delegate_->RescheduleWatch();
+  }
+}
+
+// static
+DnsConfigService* DnsConfigService::CreateSystemService() {
+  return new DnsConfigServicePosix();
+}
+
+bool ConvertResToConfig(const struct __res_state& res, DnsConfig* dns_config) {
+  CHECK(dns_config != NULL);
+  DCHECK(res.options & RES_INIT);
+
+  dns_config->nameservers.clear();
+
+#if OS_LINUX
+  // Initially, glibc stores IPv6 in _ext.nsaddrs and IPv4 in nsaddr_list.
+  // Next (res_send.c::__libc_res_nsend), it copies nsaddr_list after nsaddrs.
+  // If RES_ROTATE is enabled, the list is shifted left after each res_send.
+  // However, if nsaddr_list changes, it will refill nsaddr_list (IPv4) but
+  // leave the IPv6 entries in nsaddr in the same (shifted) order.
+
+  // Put IPv6 addresses ahead of IPv4.
+  for (int i = 0; i < res._u._ext.nscount6; ++i) {
+    IPEndPoint ipe;
+    if (ipe.FromSockAddr(
+        reinterpret_cast<const struct sockaddr*>(res._u._ext.nsaddrs[i]),
+        sizeof *res._u._ext.nsaddrs[i])) {
+      dns_config->nameservers.push_back(ipe);
+    } else {
+      return false;
+    }
+  }
+#endif
+
+  for (int i = 0; i < res.nscount; ++i) {
+    IPEndPoint ipe;
+    if (ipe.FromSockAddr(
+        reinterpret_cast<const struct sockaddr*>(&res.nsaddr_list[i]),
+        sizeof res.nsaddr_list[i])) {
+      dns_config->nameservers.push_back(ipe);
+    } else {
+      return false;
+    }
+  }
+
+  dns_config->search.clear();
+  for (int i = 0; (i < MAXDNSRCH) && res.dnsrch[i]; ++i) {
+    dns_config->search.push_back(std::string(res.dnsrch[i]));
+  }
+
+  dns_config->ndots = res.ndots;
+  dns_config->timeout = base::TimeDelta::FromSeconds(res.retrans);
+  dns_config->attempts = res.retry;
+  dns_config->rotate = res.options & RES_ROTATE;
+  dns_config->edns0 = res.options & RES_USE_EDNS0;
+
+  return true;
+}
+
+}  // namespace net
+