// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef NET_DNS_DNS_CONFIG_SERVICE_H_
#define NET_DNS_DNS_CONFIG_SERVICE_H_

#include <map>
#include <memory>
#include <optional>

#include "base/files/file_path.h"
#include "base/memory/raw_ptr.h"
#include "base/memory/weak_ptr.h"
#include "base/sequence_checker.h"
#include "base/time/time.h"
#include "base/timer/timer.h"
#include "net/base/net_export.h"
#include "net/dns/dns_config.h"
#include "net/dns/dns_hosts.h"
#include "net/dns/serial_worker.h"
#include "url/gurl.h"

namespace net {

// Service for reading system DNS settings, on demand or when signalled by
// internal watchers and NetworkChangeNotifier. This object is not thread-safe
// and methods may perform blocking I/O so methods must be called on a sequence
// that allows blocking (i.e. base::MayBlock).
class NET_EXPORT_PRIVATE DnsConfigService {
 public:
  // Callback interface for the client, called on the same thread as
  // ReadConfig() and WatchConfig().
  typedef base::RepeatingCallback<void(const DnsConfig& config)> CallbackType;

  // DHCP and user-induced changes are on the order of seconds, so 150ms should
  // not add perceivable delay. On the other hand, config readers should finish
  // within 150ms with the rare exception of I/O block or extra large HOSTS.
  static const base::TimeDelta kInvalidationTimeout;

  // Creates the platform-specific DnsConfigService. May return |nullptr| if
  // reading system DNS settings is not supported on the current platform.
  static std::unique_ptr<DnsConfigService> CreateSystemService();

  DnsConfigService(const DnsConfigService&) = delete;
  DnsConfigService& operator=(const DnsConfigService&) = delete;

  virtual ~DnsConfigService();

  // Attempts to read the configuration. Will run |callback| when succeeded.
  // Can be called at most once.
  void ReadConfig(const CallbackType& callback);

  // Registers systems watchers. Will attempt to read config after watch starts,
  // but only if watchers started successfully. Will run |callback| iff config
  // changes from last call or has to be withdrawn. Can be called at most once.
  // Might require MessageLoopForIO.
  void WatchConfig(const CallbackType& callback);

  // Triggers invalidation and re-read of the current configuration (followed by
  // invocation of the callback). For use only on platforms expecting
  // network-stack-external notifications of DNS config changes.
  virtual void RefreshConfig();

  void set_watch_failed_for_testing(bool watch_failed) {
    watch_failed_ = watch_failed;
  }

  // Simulates a watcher trigger by calling OnConfigChanged().
  void TriggerOnConfigChangedForTesting(bool succeeded) {
    // Directly call ...Delayed() version to skip past delay logic.
    OnConfigChangedDelayed(succeeded);
  }

 protected:
  // Watcher to observe for changes to DNS config or HOSTS (via overriding
  // `Watch()` with platform specifics) and trigger necessary refreshes on
  // changes.
  class NET_EXPORT_PRIVATE Watcher {
   public:
    // `service` is expected to own the created Watcher and thus stay valid for
    // the lifetime of the created Watcher.
    explicit Watcher(DnsConfigService& service);
    virtual ~Watcher();

    Watcher(const Watcher&) = delete;
    Watcher& operator=(const Watcher&) = delete;

    virtual bool Watch() = 0;

   protected:
    // Hooks for detected changes. `succeeded` false to indicate that there was
    // an error watching for the change.
    void OnConfigChanged(bool succeeded);
    void OnHostsChanged(bool succeeded);

    void CheckOnCorrectSequence();

   private:
    // Back pointer. `this` is expected to be owned by `service_`, making this
    // raw pointer safe.
    const raw_ptr<DnsConfigService> service_;

    SEQUENCE_CHECKER(sequence_checker_);
  };

  // Reader of HOSTS files. In this base implementation, uses standard logic
  // appropriate to most platforms to read the HOSTS file located at
  // `hosts_file_path`.
  class NET_EXPORT_PRIVATE HostsReader : public SerialWorker {
   public:
    // `service` is expected to own the created reader and thus stay valid for
    // the lifetime of the created reader.
    HostsReader(base::FilePath::StringPieceType hosts_file_path,
                DnsConfigService& service);
    ~HostsReader() override;

    HostsReader(const HostsReader&) = delete;
    HostsReader& operator=(const HostsReader&) = delete;

   protected:
    class NET_EXPORT_PRIVATE WorkItem : public SerialWorker::WorkItem {
     public:
      explicit WorkItem(std::unique_ptr<DnsHostsParser> dns_hosts_parser);
      ~WorkItem() override;

      // Override if needed to implement platform-specific behavior, e.g. for a
      // platform-specific HOSTS format.
      virtual std::optional<DnsHosts> ReadHosts();

      // Adds any necessary additional entries to the given `DnsHosts`. Returns
      // false on failure.
      //
      // Override if needed to implement platform-specific behavior.
      virtual bool AddAdditionalHostsTo(DnsHosts& in_out_dns_hosts);

      // SerialWorker::WorkItem:
      void DoWork() final;

     private:
      friend HostsReader;

      std::optional<DnsHosts> hosts_;
      std::unique_ptr<DnsHostsParser> dns_hosts_parser_;
    };

    // SerialWorker:
    std::unique_ptr<SerialWorker::WorkItem> CreateWorkItem() override;
    bool OnWorkFinished(
        std::unique_ptr<SerialWorker::WorkItem> work_item) final;

   private:
    // Raw pointer to owning DnsConfigService. This must never be accessed
    // inside DoWork(), since service may be destroyed while SerialWorker is
    // running on worker thread.
    const raw_ptr<DnsConfigService> service_;

    const base::FilePath hosts_file_path_;
  };

  // On detecting config change, will post and wait `config_change_delay` before
  // triggering refreshes. Will trigger refreshes synchronously on nullopt.
  // Useful for platforms where multiple changes may be made and detected before
  // the config is stabilized and ready to be read.
  explicit DnsConfigService(base::FilePath::StringPieceType hosts_file_path,
                            std::optional<base::TimeDelta> config_change_delay =
                                base::Milliseconds(50));

  // Immediately attempts to read the current configuration.
  virtual void ReadConfigNow() = 0;
  virtual void ReadHostsNow();
  // Registers system watchers. Returns true iff succeeds.
  virtual bool StartWatching() = 0;

  // Called when the current config (except hosts) has changed.
  void InvalidateConfig();
  // Called when the current hosts have changed.
  void InvalidateHosts();

  // Called with new config. |config|.hosts is ignored.
  void OnConfigRead(DnsConfig config);
  // Called with new hosts. Rest of the config is assumed unchanged.
  void OnHostsRead(DnsHosts hosts);

  // Called when config refresh is required. `succeeded` false to indicate that
  // there was an error while watching for the change.
  void OnConfigChanged(bool succeeded);

  SEQUENCE_CHECKER(sequence_checker_);

 private:
  // The timer counts from the last Invalidate* until complete config is read.
  void StartTimer();
  void OnTimeout();
  // Called when the config becomes complete. Stops the timer.
  void OnCompleteConfig();

  // Hooks for Watcher change notifications. `succeeded` false to indicate that
  // there was an error watching for the change.
  void OnHostsChanged(bool succeeded);
  void OnConfigChangedDelayed(bool succeeded);

  CallbackType callback_;

  DnsConfig dns_config_;

  // True if any of the necessary watchers failed. In that case, the service
  // will communicate changes via OnTimeout, but will only send empty DnsConfig.
  bool watch_failed_ = false;
  // True after On*Read, before Invalidate*. Tells if the config is complete.
  bool have_config_ = false;
  bool have_hosts_ = false;
  // True if receiver needs to be updated when the config becomes complete.
  bool need_update_ = false;
  // True if the last config sent was empty (instead of |dns_config_|).
  // Set when |timer_| expires.
  bool last_sent_empty_ = true;

  const std::optional<base::TimeDelta> config_change_delay_;
  const base::FilePath hosts_file_path_;

  // Created only if needed in ReadHostsNow() to avoid creating unnecessarily if
  // overridden for a platform-specific implementation.
  std::unique_ptr<HostsReader> hosts_reader_;

  // Started in Invalidate*, cleared in On*Read.
  base::OneShotTimer timer_;

  base::WeakPtrFactory<DnsConfigService> weak_factory_{this};
};

}  // namespace net

#endif  // NET_DNS_DNS_CONFIG_SERVICE_H_