File descriptors lifecycle

Redirection Save-Restore pattern

Example: redirecting STDOUT (target = 1) to an opened file (fd = 3):

⠀
  saved_fd = dup(target)    // dup(STDOUT) -> returns fd 5
 
  dup2(fd, target)         // fd 3 -> STDOUT
 
  close(fd)                // fd 3 no longer needed
 
  // ...
  // execute command (writes go to file via STDOUT)
  // ...
 
  dup2(saved_fd, target)   // restore STDOUT from fd 5
 
  close(saved_fd)          // fd 5 no longer needed
⠀

If dup() or dup2() fail, the function returns -1.
If setup_redirection() fails, the caller closes the original fd, thus preventing leaks.

Pipe file descriptors

⠀
  pipe(pipe_fd)             // pipe_fd[0] = read & pipe_fd[1] = write
 
  fork() left:      close(pipe_fd[0]), dup2(pipe_fd[1], STDOUT), close(pipe_fd[1])
 
  fork() right:     close(pipe_fd[1]), dup2(pipe_fd[0], STDIN),  close(pipe_fd[0])
 
  parent:           close(pipe_fd[0]), close(pipe_fd[1])
⠀

Each process closes the ends that it won't use.
The parent closes both ends after forking both children.

Heredoc pipe lifecycle

1. handle_heredoc() calls pipe(pipe_fd)
2. Lines are written to pipe_fd[1]
3. pipe_fd[1] is closed after reading completes
4. pipe_fd[0] is returned and stored in redir->heredoc_fd
5. During execution, handle_redir() consumes it by passing it to setup_redirection() which dup2's it into STDIN and closes it
6. On CTRL C, both pipe ends are closed and restore_stdin() reopens /dev/tty on fd 0
7. On AST cleanup, free_ast() closes any strictly positive heredoc_fd that was not consumed

Heredoc CTRL C (fd 0 restore)

heredoc_sigint_handler() closes STDIN_FILENO to break readline().

After pipe fds are closed, restore_stdin() opens /dev/tty.

If open() returns 0 (fd 0 was the lowest available) then STDIN is already correctly pointing to the tty, so no dup2/close needed.

The condition if (tty_fd != STDIN_FILENO) prevents the scenario of the destructive close(0).