Snel/src/sequencer.h

/*!
 * \file
 *   Sequencer.h
 * \brief   A basic sequence interpreter for snel
 *
 *  Created on: Feb, 2019
 *      Author: Christos Choutouridis AEM: 8997
 *      email : cchoutou@ece.auth.gr
 */
#ifndef __sequencer_h__
#define __sequencer_h__

#include <exception>
#include <string>
#include <iostream>
#include <sstream>
#include <fstream>
#include <vector>
#include <utility>
#include <algorithm>

#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>


namespace snel {

   //! file descriptor type
   using fd_t = int;

   constexpr fd_t STDIN_  = STDIN_FILENO;
   constexpr fd_t STDOUT_ = STDOUT_FILENO;
   constexpr fd_t STDERR_ = STDERR_FILENO;

   std::string filter (const std::string in);

   /*!
    * A vector based wrapper above execvp()'s `char* argv[]` interface.
    * We use a vector for convenience and resizing capabilities. We can then
    * use a pointer to underling data to pass to execvp()
    * std::vector elements guaranteed to be contiguous. To quote the standard[1]
    * \code
    *    The elements of a vector are stored contiguously, meaning that if v is
    *    a vector<T, Allocator> where T is some type other than bool, then it obeys
    *    the identity &v[n] == &v[0] + n for all 0 <= n < v.size().
    * \endcode
    *
    * See also cppreference[2].
    *
    * [1]: www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3690.pdf
    * [2]: https://en.cppreference.com/w/cpp/container/vector
    */
   struct ArgList {
      using type        = typename std::string::value_type;     // Basic data type (aka char)
      using vtype       = type*;    // Vector type
      using vtype_ptr   = vtype*;   // Pointer to vector type

      ~ArgList();
      ArgList() = default;
      ArgList(const ArgList&) = default;
      ArgList(ArgList&&) = default;

      ArgList& push_back(const std::string& item);    //!< A vector::push_back wrapper
      vtype front () { return args_.front(); }        //!< A vector::front() wrapper
      size_t size () { return args_.size(); }         //!< A vector::size() wrapper

      //! return a pointer to underling data for `execvp()`
      vtype_ptr data() noexcept { return args_.data(); }
      //! same as `data()`
      vtype_ptr operator*() noexcept { return data();  }
   private:
      // data
      std::vector<vtype> args_ {nullptr};
   };


   /*!
    * A pipe file descriptor container for each child.
    * \note
    *    We store the pipe in the first child and use the flags `from`
    *    and `to` to inform child's executer for where can find the data.
    */
   struct Pipe {
      ~Pipe();
      fd_t fd[2] {-1, -1};
      bool from  {false};
      bool to    {false};
   };

   /*!
    * An object to represent each process
    *
    * We create Child objects from command_t passed from Sequence::parse() and we
    * keep in each child informations about the file descriptors that may the process
    * will use, pipes that may need and logic flags(&&, ||) the command may have to control
    * the execution flow.
    */
   class Child {
   public:
      enum class LogicOp {
         NONE=0, OR, AND
      };
      using command_t = std::vector<std::string>;  //!< A command type
      using Error     = std::runtime_error;        //!< An error type

   public:
      ~Child ();
      Child () = default;
      Child (const command_t& c) : command{c} { }
      Child (command_t&& c) : command{std::move(c)} { }

      Child& make_arguments ();                    //!< A per child parser before execution
      //! the actual execution routine
      bool execute(std::vector<Child>::iterator it, bool first);
      Pipe& pipe() { return pipe_; }               //!< A pipe_ getter

   //! Private api
   //! @{
   private:
      void redirect_std_if(std::string fn, fd_t std_fd); //!< tool to redirect std
      void restore_std_if(fd_t std_fd);                  //!< tool to restor std redirection
   //! @}

   //! Data members
   //! \note
   //!   `arguments`, `files`, `sstd` and `pipe_` have linked resources so destructor(s) must be called.
   //!   We use RAII by having all children to static allocated vectors. '}' will do the trick even
   //!   if we throw ;)
   //! @{
   private:
      command_t command {};   //!< Each command is a vector of string as passed on from Sequence::parse()
      ArgList arguments {};   //!< The after parsing, actual null terminated arguments to pass to execvp()

      fd_t  files[3] = {-1, -1, -1};   //!< file descriptors for the user requested redirections
      fd_t  sstd [3] = {-1, -1, -1};   //!< file descriptors to store std fd's before redirection
      std::string filenames[3] = {"", "", ""};  //!< filenames of the user requested redirections
      LogicOp  logic  {LogicOp::NONE}; //!< Logic flags to store && and || command separators
      Pipe pipe_;                      //!< Pipe object for each child
   //! @}
   };

   /*!
    * The main object to represent and handle a command flow.
    *
    * We store commands as vector of vectors of commands. The inner vector
    * represent a command chain. For example the line:
    * \code ls | more && uname; cat lala \endcode
    * is an input with 2 command chains and result to a:
    * \code
    * vector<
    *    vector<(ls |), (more &&), (uname)>,
    *    vector<(cat)>
    * >
    * \endcode
    * By storing them in different vector we make the execution model simpler.
    * Each chain can stopped and the sequencer will continue with the next chain etc...
    * So in this example if `more` returns false(child::execute() returned true),
    * then `uname` will not parsed and executed and the flow will continue to `cat`
    */
   class Sequencer {
   public:
      Sequencer& parse (const std::string& input); //!< Input line parser
      Sequencer& execute (); //!< Main sequencer executor

   //! private tools
   //! @{
   private:
      //! separator trait predicate
      bool is_seperator (std::string& s) {
         return (s == "&&" || s == "||" || s == "|") ? true : false;
      }
      //! pipe trait predicate
      bool is_pipe (std::string& s) {
         return (s == "|") ? true : false;
      }
   //! @}
   private:
      //! The sequencer data representation
      std::vector <
         std::vector <Child>
      > seq_ {};
   };

}

#endif /* __sequencer_h__ */