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 <resolv.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"
+#include "net/dns/dns_config_service.h"
+
+#ifndef _PATH_RESCONF
+#define _PATH_RESCONF "/etc/resolv.conf"
+#endif
+
+namespace net {
+
+namespace {
+
+using base::files::FilePathWatcher;
+
+// Fills in |dns_config| from |res|.
+bool ConfigFromResolver(const struct __res_state& res, DnsConfig* dns_config) {
+  CHECK(dns_config != NULL);
+  DCHECK(res.options & RES_INIT);
+
+  dns_config->nameservers.clear();
+  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;
+    }
+  }
+
+  // TODO(szym): this might not compile on mac.

[cbentzel, 2011/08/05 19:11:39]

Can you run try jobs to confirm? 

May also want to comment that this is adding IPV6 server addresses.

Do you know if there is a way via res_init to interleave ordering of IPv6 and
IPv4 nameservers? In this case, it seems like the IPv6 ones will always be
processed last.

[szym, 2011/08/15 14:27:42]

glibc uses funny code to determine the ordering of IPv6 vs. IPv4. I elaborated
in a comment.

+  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;
+      }
+    }
+
+  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 = res.retrans;
+  dns_config->attempts = res.retry;
+  dns_config->rotate = res.options & RES_ROTATE;
+  dns_config->edns0 = res.options & RES_USE_EDNS0;
+
+  return true;
+}
+
+// Fills in |dns_config| using res_ninit, returns true on success.
+bool ReadConfig(DnsConfig* dns_config) {
+  // res_ninit is available on Linux (glibc 2+), Mac OS X, and Android.
+  // It might be thread-safe, but to ensure ordering we run only one at a time.
+  struct __res_state res;
+  if (res_ninit(&res) != 0)
+    return false;
+
+  return ConfigFromResolver(res, dns_config);
+}
+
+class DnsConfigServicePosix : public DnsConfigService {
+ public:
+  DnsConfigServicePosix();
+  virtual ~DnsConfigServicePosix() {
+    // The watcher must be destroyed on the same thread that called Watch.
+    if (watcher_delegate_.get())
+      watcher_delegate_->Cancel();
+  }
+
+  virtual void AddObserver(Observer* observer) {
+    observers_.AddObserver(observer);
+  }
+
+  virtual void RemoveObserver(Observer* observer) {
+    observers_.RemoveObserver(observer);
+  }
+
+ private:
+  // FilePathWatcher::Delegate is refcounted, so we separate it from the Service
+  // We'll also use it on the WorkerPool.
+  class WatcherDelegate : public FilePathWatcher::Delegate {

[cbentzel, 2011/08/05 19:11:39]

Might be clearer to make this a non-nested class - especially since this is only
in this .cc file. I had a hard time when reading understanding which parts were
on the WatcherDelegate and which parts on the DnsConfigServicePosix.

[szym, 2011/08/15 14:27:42]

Done.

+   public:
+    explicit WatcherDelegate(DnsConfigServicePosix* service)
+      : message_loop_(base::MessageLoopProxy::CreateForCurrentThread()),
+        service_(service) {}

[cbentzel, 2011/08/05 19:11:39]

You need to initialize reading_(false) and read_pending_(false)

Could be arbitrary data at this point. 

[szym, 2011/08/15 14:27:42]

Done. Good catch. I wrongly assumed bool() == false.

+
+    // 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();
+    }
+
+    void Cancel() {
+      DCHECK(message_loop_->BelongsToCurrentThread());
+      service_ = NULL;
+    }
+
+    void ScheduleWatch() {
+      // Retry Watch in 100ms or more.
+      message_loop_->PostDelayedTask(
+          FROM_HERE, base::Bind(&WatcherDelegate::StartWatch, this), 100);
+    }
+
+   private:
+    virtual ~WatcherDelegate() {}
+
+    void ScheduleRead() {
+      if (reading_) {
+        read_pending_ = true;
+      } else {
+        // This can't fail on Linux.
+        base::WorkerPool::PostTask(FROM_HERE, base::Bind(
+            &WatcherDelegate::DoRead, this), false);
+        reading_ = true;
+        read_pending_ = false;
+      }
+    }
+
+    // Reads DnsConfig and posts OnResultAvailable to |message_loop_|.
+    // Must be called on the worker thread.
+    void DoRead() {
+      DnsConfig config;
+      bool success = ReadConfig(&config);
+      // 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);
+      }
+    }
+
+    void StartWatch() {
+      if (!service_)
+        return;
+      service_->StartWatch();
+    }
+
+    // Message loop for the thread on which Watch is called (of TYPE_IO).
+    scoped_refptr<base::MessageLoopProxy> message_loop_;
+    DnsConfigServicePosix* service_;
+    // True after DoRead before OnResultsAvailable
+    bool reading_;
+    // True after OnFilePathChanged fires while |reading_| is true.
+    bool read_pending_;
+  };
+
+  void OnConfigRead(const DnsConfig& config) {
+    if (!config.Equals(dns_config_)) {
+      dns_config_ = config;
+      FOR_EACH_OBSERVER(Observer, observers_, OnConfigChanged(config));
+    }
+  }
+
+  // Configure a FilePathWatcher and install watcher_delegate_.
+  void StartWatch();
+
+  scoped_ptr<FilePathWatcher> resolv_file_watcher_;
+  scoped_refptr<WatcherDelegate> watcher_delegate_;
+  ObserverList<Observer> observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(DnsConfigServicePosix);
+};
+
+DnsConfigServicePosix::DnsConfigServicePosix() {
+  DCHECK(!resolv_file_watcher_.get());
+  DCHECK(!watcher_delegate_.get());
+  DCHECK(MessageLoopForIO::current());
+
+  watcher_delegate_ = new WatcherDelegate(this);
+  StartWatch();
+}
+
+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(new FilePathWatcher());
+  if (!resolv_file_watcher_->Watch(path, watcher_delegate_)) {
+    watcher_delegate_->ScheduleWatch();
+  } else {
+    // Make the initial read after watch is installed.
+    watcher_delegate_->OnFilePathChanged(path);
+  }
+}
+
+}  // namespace
+
+DnsConfigService* DnsConfigService::CreateSystemService() {
+  return new DnsConfigServicePosix();
+}
+
+}  // namespace net
+