// Copyright (C) 2010  Internet Systems Consortium, Inc. ("ISC")
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
// AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
// OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
// PERFORMANCE OF THIS SOFTWARE.

#ifndef __ASIOLINK_H
#define __ASIOLINK_H 1

// IMPORTANT NOTE: only very few ASIO headers files can be included in
// this file.  In particular, asio.hpp should never be included here.
// See the description of the namespace below.
#include <unistd.h>             // for some network system calls
#include <asio/ip/address.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/function.hpp>

#include <functional>
#include <string>
#include <vector>
#include <utility>

#include <dns/buffer.h>
#include <dns/message.h>
#include <dns/question.h>

#include <exceptions/exceptions.h>

#include <asiolink/ioaddress.h>
#include <asiolink/ioendpoint.h>
#include <asiolink/iomessage.h>
#include <asiolink/iosocket.h>

namespace asio {
// forward declaration for IOService::get_io_service() below
class io_service;
}

/// \namespace asiolink
/// \brief A wrapper interface for the ASIO library.
///
/// The \c asiolink namespace is used to define a set of wrapper interfaces
/// for the ASIO library.
///
/// BIND 10 uses the non-Boost version of ASIO because it's header-only,
/// i.e., does not require a separate library object to be linked, and thus
/// lowers the bar for introduction.
///
/// But the advantage comes with its own costs: since the header-only version
/// includes more definitions in public header files, it tends to trigger
/// more compiler warnings for our own sources, and, depending on the
/// compiler options, may make the build fail.
///
/// We also found it may be tricky to use ASIO and standard C++ libraries
/// in a single translation unit, i.e., a .cc file: depending on the order
/// of including header files, ASIO may or may not work on some platforms.
///
/// This wrapper interface is intended to centralize these
/// problematic issues in a single sub module.  Other BIND 10 modules should
/// simply include \c asiolink.h and use the wrapper API instead of
/// including ASIO header files and using ASIO-specific classes directly.
///
/// This wrapper may be used for other IO libraries if and when we want to
/// switch, but generality for that purpose is not the primary goal of
/// this module.  The resulting interfaces are thus straightforward mapping
/// to the ASIO counterparts.
///
/// Notes to developers:
/// Currently the wrapper interface is fairly specific to use by a
/// DNS server, i.e., b10-auth or b10-resolver.  But the plan is to
/// generalize it and have other modules use it as well.
///
/// One obvious drawback of this approach is performance overhead
/// due to the additional layer.  We should eventually evaluate the cost
/// of the wrapper abstraction in benchmark tests. Another drawback is
/// that the wrapper interfaces don't provide all features of ASIO
/// (at least for the moment).  We should also re-evaluate the
/// maintenance overhead of providing necessary wrappers as we develop
/// more.
///
/// On the other hand, we may be able to exploit the wrapper approach to
/// simplify the interfaces (by limiting the usage) and unify performance
/// optimization points.
///
/// As for optimization, we may want to provide a custom allocator for
/// the placeholder of callback handlers:
/// http://think-async.com/Asio/asio-1.3.1/doc/asio/reference/asio_handler_allocate.html

namespace asiolink {
class DNSServiceImpl;
struct IOServiceImpl;
struct IntervalTimerImpl;

/// \brief An exception that is thrown if an error occurs within the IO
/// module.  This is mainly intended to be a wrapper exception class for
/// ASIO specific exceptions.
class IOError : public isc::Exception {
public:
    IOError(const char* file, size_t line, const char* what) :
        isc::Exception(file, line, what) {}
};

/// \brief Forward declarations for classes used below
class SimpleCallback;
class DNSLookup;
class DNSAnswer;

/// \brief The \c IOService class is a wrapper for the ASIO \c io_service
/// class.
///
class IOService {
    ///
    /// \name Constructors and Destructor
    ///
    /// Note: The copy constructor and the assignment operator are
    /// intentionally defined as private, making this class non-copyable.
    //@{
private:
    IOService(const IOService& source);
    IOService& operator=(const IOService& source);
public:
    /// \brief The constructor
    IOService();
    /// \brief The destructor.
    ~IOService();
    //@}

    /// \brief Start the underlying event loop.
    ///
    /// This method does not return control to the caller until
    /// the \c stop() method is called via some handler.
    void run();

    /// \brief Run the underlying event loop for a single event.
    ///
    /// This method return control to the caller as soon as the
    /// first handler has completed.  (If no handlers are ready when
    /// it is run, it will block until one is.)
    void run_one();

    /// \brief Stop the underlying event loop.
    ///
    /// This will return the control to the caller of the \c run() method.
    void stop();

    /// \brief Return the native \c io_service object used in this wrapper.
    ///
    /// This is a short term work around to support other BIND 10 modules
    /// that share the same \c io_service with the authoritative server.
    /// It will eventually be removed once the wrapper interface is
    /// generalized.
    asio::io_service& get_io_service();

private:
    IOServiceImpl* io_impl_;
};

///
/// DNSService is the service that handles DNS queries and answers with
/// a given IOService. This class is mainly intended to hold all the
/// logic that is shared between the authoritative and the recursive
/// server implementations. As such, it handles asio, including config
/// updates (through the 'Checkinprovider'), and listening sockets.
/// 
class DNSService {
    ///
    /// \name Constructors and Destructor
    ///
    /// Note: The copy constructor and the assignment operator are
    /// intentionally defined as private, making this class non-copyable.
    //@{
private:
    DNSService(const DNSService& source);
    DNSService& operator=(const DNSService& source);

public:
    /// \brief The constructor with a specific IP address and port on which
    /// the services listen on.
    ///
    /// \param io_service The IOService to work with
    /// \param port the port to listen on
    /// \param address the IP address to listen on
    /// \param checkin Provider for cc-channel events (see \c SimpleCallback)
    /// \param lookup The lookup provider (see \c DNSLookup)
    /// \param answer The answer provider (see \c DNSAnswer)
    DNSService(IOService& io_service, const char& port,
               const char& address, SimpleCallback* checkin,
               DNSLookup* lookup, DNSAnswer* answer);
    /// \brief The constructor with a specific port on which the services
    /// listen on.
    ///
    /// It effectively listens on "any" IPv4 and/or IPv6 addresses.
    /// IPv4/IPv6 services will be available if and only if \c use_ipv4
    /// or \c use_ipv6 is \c true, respectively.
    ///
    /// \param io_service The IOService to work with
    /// \param port the port to listen on
    /// \param ipv4 If true, listen on ipv4 'any'
    /// \param ipv6 If true, listen on ipv6 'any'
    /// \param checkin Provider for cc-channel events (see \c SimpleCallback)
    /// \param lookup The lookup provider (see \c DNSLookup)
    /// \param answer The answer provider (see \c DNSAnswer)
    DNSService(IOService& io_service, const char& port,
               const bool use_ipv4, const bool use_ipv6,
               SimpleCallback* checkin, DNSLookup* lookup,
               DNSAnswer* answer);
    /// \brief The constructor without any servers.
    ///
    /// Use addServer() to add some servers.
    DNSService(IOService& io_service, SimpleCallback* checkin,
               DNSLookup* lookup, DNSAnswer* answer);
    /// \brief The destructor.
    ~DNSService();
    //@}

    /// \brief Add another server to the service
    void addServer(uint16_t port, const std::string &address);
    void addServer(const char &port, const std::string &address);
    /// \brief Remove all servers from the service
    void clearServers();

    /// \brief Return the native \c io_service object used in this wrapper.
    ///
    /// This is a short term work around to support other BIND 10 modules
    /// that share the same \c io_service with the authoritative server.
    /// It will eventually be removed once the wrapper interface is
    /// generalized.
    asio::io_service& get_io_service() { return io_service_.get_io_service(); }
private:
    DNSServiceImpl* impl_;
    IOService& io_service_;
};

/// \brief The \c DNSServer class is a wrapper (and base class) for
/// classes which provide DNS server functionality.
/// 
/// The classes derived from this one, \c TCPServer and \c UDPServer,
/// act as the interface layer between clients sending queries, and
/// functions defined elsewhere that provide answers to those queries.
/// Those functions are described in more detail below under
/// \c SimpleCallback, \c DNSLookup, and \c DNSAnswer.
///
/// Notes to developers:
/// When constructed, this class (and its derived classes) will have its
/// "self_" member set to point to "this".  Objects of this class (as
/// instantiated through a base class) are sometimes passed by
/// reference (as this superclass); calls to methods in the base
/// class are then rerouted via this pointer to methods in the derived
/// class.  This allows code from outside asiolink, with no specific
/// knowledge of \c TCPServer or \c UDPServer, to access their methods.
///
/// This class is both assignable and copy-constructable.  Its subclasses
/// use the "stackless coroutine" pattern, meaning that it will copy itself
/// when "forking", and that instances will be posted as ASIO handler
/// objects, which are always copied.
///
/// Because these objects are frequently copied, it is recommended 
/// that derived classes be kept small to reduce copy overhead.
class DNSServer {
protected: 
    ///
    /// \name Constructors and destructors
    ///
    /// This is intentionally defined as \c protected, as this base class
    /// should never be instantiated except as part of a derived class.
    //@{
    DNSServer() : self_(this) {}
public:
    /// \brief The destructor
    virtual ~DNSServer() {}
    //@}

    ///
    /// \name Class methods
    ///
    /// These methods all make their calls indirectly via the "self_"
    /// pointer, ensuring that the functions ultimately invoked will be
    /// the ones in the derived class.  This makes it possible to pass
    /// instances of derived classes as references to this base class
    /// without losing access to derived class data.
    /// 
    //@{
    /// \brief The funtion operator
    virtual void operator()(asio::error_code ec = asio::error_code(),
                            size_t length = 0)
    {
        (*self_)(ec, length);
    }

    /// \brief Resume processing of the server coroutine after an 
    /// asynchronous call (e.g., to the DNS Lookup provider) has completed.
    ///
    /// \param done If true, this signals the system there is an answer
    ///             to return.
    virtual void resume(const bool done) { self_->resume(done); }

    /// \brief Indicate whether the server is able to send an answer
    /// to a query.
    /// 
    /// This is presently used only for testing purposes.
    virtual bool hasAnswer() { return (self_->hasAnswer()); }

    /// \brief Returns the current value of the 'coroutine' object
    ///
    /// This is a temporary method, intended to be used for debugging
    /// purposes during development and removed later.  It allows
    /// callers from outside the coroutine object to retrieve information
    /// about its current state.
    ///
    /// \return The value of the 'coroutine' object
    virtual int value() { return (self_->value()); }

    /// \brief Returns a pointer to a clone of this DNSServer object.
    ///
    /// When a \c DNSServer object is copied or assigned, the result will
    /// normally be another \c DNSServer object containing a copy
    /// of the original "self_" pointer.  Calling clone() guarantees
    /// that the underlying object is also correctly copied.
    ///
    /// \return A deep copy of this DNSServer object
    virtual DNSServer* clone() { return (self_->clone()); }
    //@}

protected:
    /// \brief Lookup handler object.
    ///
    /// This is a protected class; it can only be instantiated
    /// from within a derived class of \c DNSServer.
    ///
    /// A server object that has received a query creates an instance
    /// of this class and scheudles it on the ASIO service queue
    /// using asio::io_service::post().  When the handler executes, it
    /// calls the asyncLookup() method in the server object to start a
    /// DNS lookup.  When the lookup is complete, the server object is
    /// scheduled to resume, again using io_service::post().
    ///
    /// Note that the calling object is copied into the handler object,
    /// not referenced.  This is because, once the calling object yields
    /// control to the handler, it falls out of scope and may disappear
    template <typename T>
    class AsyncLookup {
    public:
        AsyncLookup(T& caller) : caller_(caller) {}
        void operator()() { caller_.asyncLookup(); }
    private:
        T caller_;
    };

    /// \brief Carries out a DNS lookup.
    ///
    /// This function calls the \c DNSLookup object specified by the
    /// DNS server when the \c IOService was created, passing along
    /// the details of the query and a pointer back to the current
    /// server object.  It is called asynchronously via the AsyncLookup
    /// handler class.
    virtual void asyncLookup() { self_->asyncLookup(); }

private:
    DNSServer* self_;
};

/// \brief The \c DNSLookup class is an abstract base class for a DNS
/// Lookup provider function.
///
/// Specific derived class implementations are hidden within the
/// implementation.  Instances of the derived classes can be called
/// as functions via the operator() interface.  Pointers to these
/// instances can then be provided to the \c IOService class
/// via its constructor.
///
/// A DNS Lookup provider function obtains the data needed to answer
/// a DNS query (e.g., from authoritative data source, cache, or upstream
/// query).  After it has run, the OutputBuffer object passed to it
/// should contain the answer to the query, in an internal representation.
class DNSLookup {
    ///
    /// \name Constructors and Destructor
    ///
    /// Note: The copy constructor and the assignment operator are
    /// intentionally defined as private, making this class non-copyable.
    //@{
private:
    DNSLookup(const DNSLookup& source);
    DNSLookup& operator=(const DNSLookup& source);
protected:
    /// \brief The default constructor.
    ///
    /// This is intentionally defined as \c protected as this base class
    /// should never be instantiated (except as part of a derived class).
    DNSLookup() : self_(this) {}
public:
    /// \brief The destructor
    virtual ~DNSLookup() {}
    //@}
    /// \brief The function operator
    ///
    /// This makes its call indirectly via the "self" pointer, ensuring
    /// that the function ultimately invoked will be the one in the derived
    /// class.
    ///
    /// \param io_message The event message to handle
    /// \param message The DNS MessagePtr that needs handling
    /// \param buffer The final answer is put here
    /// \param DNSServer DNSServer object to use
    virtual void operator()(const IOMessage& io_message,
                            isc::dns::MessagePtr message,
                            isc::dns::MessagePtr answer_message,
                            isc::dns::OutputBufferPtr buffer,
                            DNSServer* server) const
    {
        (*self_)(io_message, message, answer_message, buffer, server);
    }
private:
    DNSLookup* self_;
};

/// \brief The \c DNSAnswer class is an abstract base class for a DNS
/// Answer provider function.
///
/// Specific derived class implementations are hidden within the
/// implementation.  Instances of the derived classes can be called
/// as functions via the operator() interface.  Pointers to these
/// instances can then be provided to the \c IOService class
/// via its constructor.
///
/// A DNS Answer provider function takes answer data that has been obtained
/// from a DNS Lookup provider functon and readies it to be sent to the
/// client.  After it has run, the OutputBuffer object passed to it should
/// contain the answer to the query rendered into wire format.
class DNSAnswer {
    ///
    /// \name Constructors and Destructor
    ///
    /// Note: The copy constructor and the assignment operator are
    /// intentionally defined as private, making this class non-copyable.
    //@{
private:
    DNSAnswer(const DNSAnswer& source);
    DNSAnswer& operator=(const DNSAnswer& source);
protected:
    /// \brief The default constructor.
    ///
    /// This is intentionally defined as \c protected as this base class
    /// should never be instantiated (except as part of a derived class).
    DNSAnswer() {}
public:
    /// \brief The destructor
    virtual ~DNSAnswer() {}
    //@}
    /// \brief The function operator
    ///
    /// This makes its call indirectly via the "self" pointer, ensuring
    /// that the function ultimately invoked will be the one in the derived
    /// class.
    ///
    /// \param io_message The event message to handle
    /// \param message The DNS MessagePtr that needs handling
    /// \param buffer The result is put here
    virtual void operator()(const IOMessage& io_message,
                            isc::dns::MessagePtr message,
                            isc::dns::MessagePtr answer_message,
                            isc::dns::OutputBufferPtr buffer) const = 0;
};

/// \brief The \c SimpleCallback class is an abstract base class for a
/// simple callback function with the signature:
///
/// void simpleCallback(const IOMessage& io_message) const;
///
/// Specific derived class implementations are hidden within the
/// implementation.  Instances of the derived classes can be called
/// as functions via the operator() interface.  Pointers to these
/// instances can then be provided to the \c IOService class
/// via its constructor.
///
/// The \c SimpleCallback is expected to be used for basic, generic
/// tasks such as checking for configuration changes.  It may also be
/// used for testing purposes.
class SimpleCallback {
    ///
    /// \name Constructors and Destructor
    ///
    /// Note: The copy constructor and the assignment operator are
    /// intentionally defined as private, making this class non-copyable.
    //@{
private:
    SimpleCallback(const SimpleCallback& source);
    SimpleCallback& operator=(const SimpleCallback& source);
protected:
    /// \brief The default constructor.
    ///
    /// This is intentionally defined as \c protected as this base class
    /// should never be instantiated (except as part of a derived class).
    SimpleCallback() : self_(this) {}
public:
    /// \brief The destructor
    virtual ~SimpleCallback() {}
    /// \brief The function operator
    //@}
    ///
    /// This makes its call indirectly via the "self" pointer, ensuring
    /// that the function ultimately invoked will be the one in the derived
    /// class.
    ///
    /// \param io_message The event message to handle
    virtual void operator()(const IOMessage& io_message) const {
        (*self_)(io_message);
    }
private:
    SimpleCallback* self_;
};

/// \brief The \c RecursiveQuery class provides a layer of abstraction around
/// the ASIO code that carries out an upstream query.
///
/// This design is very preliminary; currently it is only capable of
/// handling simple forward requests to a single resolver.
class RecursiveQuery {
    ///
    /// \name Constructors
    ///
    //@{
public:
    /// \brief Constructor for use when acting as a forwarder
    ///
    /// This is currently the only way to construct \c RecursiveQuery
    /// object.  The addresses of the forward nameservers is specified,
    /// and every upstream query will be sent to one random address.
    /// \param dns_service The DNS Service to perform the recursive
    ///        query on.
    /// \param upstream Addresses and ports of the upstream servers
    ///        to forward queries to.
    /// \param upstream_root Addresses and ports of the root servers
    ///        to use when resolving.
    /// \param timeout How long to timeout the query, in ms
    ///     -1 means never timeout (but do not use that).
    ///     TODO: This should be computed somehow dynamically in future
    /// \param retries how many times we try again (0 means just send and
    ///     and return if it returs).
    RecursiveQuery(DNSService& dns_service,
                   const std::vector<std::pair<std::string, uint16_t> >&
                   upstream, 
                   const std::vector<std::pair<std::string, uint16_t> >&
                   upstream_root, 
                   int timeout = -1, unsigned retries = 0);
    //@}

    /// \brief Initiates an upstream query in the \c RecursiveQuery object.
    ///
    /// When sendQuery() is called, a message is sent asynchronously to
    /// the upstream name server.  When a reply arrives, 'server'
    /// is placed on the ASIO service queue via io_service::post(), so
    /// that the original \c DNSServer objct can resume processing.
    ///
    /// \param question The question being answered <qname/qclass/qtype>
    /// \param buffer An output buffer into which the response can be copied
    /// \param server A pointer to the \c DNSServer object handling the client
    void sendQuery(const isc::dns::Question& question,
                   isc::dns::MessagePtr answer_message,
                   isc::dns::OutputBufferPtr buffer,
                   DNSServer* server);
private:
    DNSService& dns_service_;
    boost::shared_ptr<std::vector<std::pair<std::string, uint16_t> > >
        upstream_;
    boost::shared_ptr<std::vector<std::pair<std::string, uint16_t> > >
        upstream_root_;
    int timeout_;
    unsigned retries_;
};

/// \brief The \c IntervalTimer class is a wrapper for the ASIO
/// \c asio::deadline_timer class.
///
/// This class is implemented to use \c asio::deadline_timer as
/// interval timer.
///
/// \c setupTimer() sets a timer to expire on (now + interval) and
/// a call back function.
///
/// \c IntervalTimerImpl::callback() is called by the timer when
/// it expires.
///
/// The function calls the call back function set by \c setupTimer()
/// and updates the timer to expire in (now + interval) seconds.
/// The type of call back function is \c void(void).
/// 
/// The call back function will not be called if the instance of this
/// class is destructed before the timer is expired.
///
/// Note: Destruction of an instance of this class while call back
/// is pending causes throwing an exception from \c IOService.
///
/// Sample code:
/// \code
///  void function_to_call_back() {
///      // this function will be called periodically
///  }
///  int interval_in_seconds = 1;
///  IOService io_service;
///
///  IntervalTimer intervalTimer(io_service);
///  intervalTimer.setupTimer(function_to_call_back, interval_in_seconds);
///  io_service.run();
/// \endcode
///
class IntervalTimer {
public:
    /// \name The type of timer callback function
    typedef boost::function<void()> Callback;

    ///
    /// \name Constructors and Destructor
    ///
    /// Note: The copy constructor and the assignment operator are
    /// intentionally defined as private, making this class non-copyable.
    //@{
private:
    IntervalTimer(const IntervalTimer& source);
    IntervalTimer& operator=(const IntervalTimer& source);
public:
    /// \brief The constructor with \c IOService.
    ///
    /// This constructor may throw a standard exception if
    /// memory allocation fails inside the method.
    /// This constructor may also throw \c asio::system_error.
    ///
    /// \param io_service A reference to an instance of IOService
    ///
    IntervalTimer(IOService& io_service);

    /// \brief The destructor.
    ///
    /// This destructor never throws an exception.
    ///
    /// On the destruction of this class the timer will be canceled
    /// inside \c asio::deadline_timer.
    ///
    ~IntervalTimer();
    //@}

    /// \brief Register timer callback function and interval.
    ///
    /// This function sets callback function and interval in seconds.
    /// Timer will actually start after calling \c IOService::run().
    ///
    /// \param cbfunc A reference to a function \c void(void) to call back
    /// when the timer is expired (should not be an empty functor)
    /// \param interval Interval in seconds (greater than 0)
    ///
    /// Note: IntervalTimer will not pass \c asio::error_code to
    /// call back function. In case the timer is cancelled, the function
    /// will not be called.
    ///
    /// \throw isc::InvalidParameter cbfunc is empty
    /// \throw isc::BadValue interval is 0
    /// \throw isc::Unexpected ASIO library error
    ///
    void setupTimer(const Callback& cbfunc, const uint32_t interval);

    /// Cancel the timer.
    ///
    /// If the timer has been set up, this method cancels any asynchronous
    /// events waiting on the timer and stops the timer itself.
    /// If the timer has already been canceled, this method effectively does
    /// nothing.
    ///
    /// This method never throws an exception.
    void cancel();

    /// Return the timer interval.
    ///
    /// This method returns the timer interval in seconds if it's running;
    /// if the timer has been canceled it returns 0.
    ///
    /// This method never throws an exception.
    ///
    /// Note: We may want to change the granularity of the timer to
    /// milliseconds or even finer.  If and when this happens the semantics
    /// of the return value of this method will be changed accordingly.
    uint32_t getInterval() const;

private:
    IntervalTimerImpl* impl_;
};

}      // asiolink
#endif // __ASIOLINK_H

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