# dlib C++ Library - subprocess_stream.h

    // Copyright (C) 2016  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_SUBPROCeSS_STREAM_H_
    #define DLIB_SUBPROCeSS_STREAM_H_

    #include <utility>
    #include <unistd.h>
    #include <iostream>
    #include <memory>
    #include <dlib/matrix.h>
    #include <sys/types.h>
    #include <sys/socket.h>

    namespace dlib
    {

    // --------------------------------------------------------------------------------------

        // Call dlib's serialize and deserialize by default.   The point of this version of
        // serialize is to do something fast that normally we wouldn't do, like directly copy
        // memory.  This is safe since this is an interprocess communication happening the same
        // machine.
        template <typename T> void interprocess_serialize ( const T& item, std::ostream& out) { serialize(item, out); }
        template <typename T> void interprocess_deserialize (T& item, std::istream& in) { deserialize(item, in); }

        // But have overloads for direct memory copies for some types since this is faster than
        // their default serialization.
        template <typename T, long NR, long NC, typename MM, typename L>
        void interprocess_serialize(const dlib::matrix<T,NR,NC,MM,L>& item, std::ostream& out)
        {
            dlib::serialize(item.nr(), out);
            dlib::serialize(item.nc(), out);
            if (item.size() != 0)
                out.write((const char*)&item(0,0), sizeof(T)*item.size());
            if (!out)
                throw dlib::serialization_error("Error writing matrix to interprocess iostream.");
        }

        template <typename T, long NR, long NC, typename MM, typename L>
        void interprocess_deserialize(dlib::matrix<T,NR,NC,MM,L>& item, std::istream& in)
        {
            long nr, nc;
            dlib::deserialize(nr, in);
            dlib::deserialize(nc, in);
            item.set_size(nr,nc);
            if (item.size() != 0)
                in.read((char*)&item(0,0), sizeof(T)*item.size());
            if (!in)
                throw dlib::serialization_error("Error reading matrix from interprocess iostream.");
        }

    // ----------------------------------------------------------------------------------------

        namespace impl{ std::iostream& get_data_iostream(); }

        inline void send_to_parent_process() {impl::get_data_iostream().flush();}
        template <typename U, typename ...T>
        void send_to_parent_process(U&& arg1, T&& ...args)
        /*!
            ensures
                - sends all the arguments to send_to_parent_process() to the parent process by
                  serializing them with interprocess_serialize().
        !*/
        {
            interprocess_serialize(arg1, impl::get_data_iostream());
            send_to_parent_process(std::forward<T>(args)...);
            if (!impl::get_data_iostream())
                throw dlib::error("Error sending object to parent process.");
        }

        inline void receive_from_parent_process() {}
        template <typename U, typename ...T>
        void receive_from_parent_process(U&& arg1, T&& ...args)
        /*!
            ensures
                - receives all the arguments to receive_from_parent_process() from the parent
                  process by deserializing them from interprocess_serialize().
        !*/
        {
            interprocess_deserialize(arg1, impl::get_data_iostream());
            receive_from_parent_process(std::forward<T>(args)...);
            if (!impl::get_data_iostream())
                throw dlib::error("Error receiving object from parent process.");
        }

    // ----------------------------------------------------------------------------------------

        class filestreambuf;

        class subprocess_stream : noncopyable
        {
            /*!
                WHAT THIS OBJECT REPRESENTS
                    This is a tool for spawning a subprocess and communicating with it.  Here
                    is an example:

                        subprocess_stream s("/usr/bin/some_program");
                        s.send(obj1, obj2, obj3);
                        s.receive(obj4, obj5);
                        s.wait(); // wait for sub process to terminate

                    Then in the sub process you would have:

                        receive_from_parent_process(obj1, obj2, obj3);
                        // do stuff
                        cout << "echo this text to parent cout" << endl;
                        send_to_parent_process(obj4, obj5);

                    Additionally, if the sub process writes to its standard out then that will
                    be echoed to std::cout in the parent process.  Writing to std::cerr or
                    returning a non-zero value from main will also be noted by the parent
                    process and an appropriate exception will be thrown.
            !*/

        public:

            explicit subprocess_stream(
                const char* program_name
            );
            /*!
                ensures
                    - spawns a sub process by executing the file with the given program_name.
            !*/

            ~subprocess_stream(
            );
            /*!
                ensures
                    - calls wait().  Note that the destructor never throws even though wait() can.
                      If an exception is thrown by wait() it is just logged to std::cerr.
            !*/

            void wait(
            );
            /*!
                ensures
                    - closes the input stream to the child process and then waits for the child
                      to terminate.
                    - If the child returns an error (by returning != 0 from its main) or
                      outputs to its standard error then wait() throws a dlib::error() with the
                      standard error output in it.
            !*/

            int get_child_pid() const { return child_pid; }
            /*!
                ensures
                    - returns the PID of the child process
            !*/

            template <typename U, typename ...T>
            void send(U&& arg1, T&& ...args)
            /*!
                ensures
                    - sends all the arguments to send() to the subprocess by serializing them
                      with interprocess_serialize().
            !*/
            {
                interprocess_serialize(arg1, iosub);
                send(std::forward<T>(args)...);
                if (!iosub)
                {
                    std::ostringstream sout;
                    sout << stderr.rdbuf();
                    throw dlib::error("Error sending object to child process.\n" + sout.str());
                }
            }
            void send() {iosub.flush();}

            template <typename U, typename ...T>
            void receive(U&& arg1, T&& ...args)
            /*!
                ensures
                    - receives all the arguments to receive() to the subprocess by deserializing
                      them with interprocess_deserialize().
            !*/
            {
                interprocess_deserialize(arg1, iosub);
                receive(std::forward<T>(args)...);
                if (!iosub)
                {
                    std::ostringstream sout;
                    sout << stderr.rdbuf();
                    throw dlib::error("Error receiving object from child process.\n" + sout.str() );
                }
            }
            void receive() {}

        private:

            void send_eof();

            class cpipe : noncopyable
            {
            private:
                int fd[2];
            public:
                cpipe() { if (socketpair(AF_LOCAL, SOCK_STREAM, 0, fd)) throw dlib::error("Failed to create pipe"); }
                ~cpipe() { close(); }
                int parent_fd() const { return fd[0]; }
                int child_fd() const { return fd[1]; }
                void close() { ::close(fd[0]); ::close(fd[1]); }
            };

            cpipe data_pipe;
            cpipe stdout_pipe;
            cpipe stderr_pipe;
            bool wait_called = false;
            std::unique_ptr<filestreambuf> inout_buf;
            std::unique_ptr<filestreambuf> err_buf;
            int child_pid = -1;
            std::istream stderr;
            std::iostream iosub;
        };
    }

    // ----------------------------------------------------------------------------------------

    #endif // DLIB_SUBPROCeSS_STREAM_H_