def _setupChild(self, fdmap):
        """
        fdmap[childFD] = parentFD

        The child wants to end up with 'childFD' attached to what used to be
        the parent's parentFD. As an example, a bash command run like
        'command 2>&1' would correspond to an fdmap of {0:0, 1:1, 2:1}.
        'command >foo.txt' would be {0:0, 1:os.open('foo.txt'), 2:2}.

        This is accomplished in two steps::

            1. close all file descriptors that aren't values of fdmap.  This
               means 0 .. maxfds (or just the open fds within that range, if
               the platform supports '/proc/<pid>/fd').

            2. for each childFD::

                 - if fdmap[childFD] == childFD, the descriptor is already in
                   place.  Make sure the CLOEXEC flag is not set, then delete
                   the entry from fdmap.

                 - if childFD is in fdmap.values(), then the target descriptor
                   is busy. Use os.dup() to move it elsewhere, update all
                   fdmap[childFD] items that point to it, then close the
                   original. Then fall through to the next case.

                 - now fdmap[childFD] is not in fdmap.values(), and is free.
                   Use os.dup2() to move it to the right place, then close the
                   original.
        """
        debug = self.debug_child
        if debug:
            errfd = sys.stderr
            errfd.write("starting _setupChild\n")

        destList = fdmap.values()
        for fd in _listOpenFDs():
            if fd in destList:
                continue
            if debug and fd == errfd.fileno():
                continue
            try:
                os.close(fd)
            except:
                pass

        # at this point, the only fds still open are the ones that need to
        # be moved to their appropriate positions in the child (the targets
        # of fdmap, i.e. fdmap.values() )

        if debug: print("fdmap", fdmap, file=errfd)
        for child in sorted(fdmap.keys()):
            target = fdmap[child]
            if target == child:
                # fd is already in place
                if debug: print("%d already in place" % target, file=errfd)
                fdesc._unsetCloseOnExec(child)
            else:
                if child in fdmap.values():
                    # we can't replace child-fd yet, as some other mapping
                    # still needs the fd it wants to target. We must preserve
                    # that old fd by duping it to a new home.
                    newtarget = os.dup(child) # give it a safe home
                    if debug: print("os.dup(%d) -> %d" % (child, newtarget),
                                    file=errfd)
                    os.close(child) # close the original
                    for c, p in items(fdmap):
                        if p == child:
                            fdmap[c] = newtarget # update all pointers
                # now it should be available
                if debug: print("os.dup2(%d,%d)" % (target, child), file=errfd)
                os.dup2(target, child)

        # At this point, the child has everything it needs. We want to close
        # everything that isn't going to be used by the child, i.e.
        # everything not in fdmap.keys(). The only remaining fds open are
        # those in fdmap.values().

        # Any given fd may appear in fdmap.values() multiple times, so we
        # need to remove duplicates first.

        old = []
        for fd in fdmap.values():
            if not fd in old:
                if not fd in fdmap.keys():
                    old.append(fd)
        if debug: print("old", old, file=errfd)
        for fd in old:
            os.close(fd)

        self._resetSignalDisposition()

        fdmap[childFD] = parentFD

        The child wants to end up with 'childFD' attached to what used to be
        the parent's parentFD. As an example, a bash command run like
        'command 2>&1' would correspond to an fdmap of {0:0, 1:1, 2:1}.
        'command >foo.txt' would be {0:0, 1:os.open('foo.txt'), 2:2}.

        This is accomplished in two steps::

            1. close all file descriptors that aren't values of fdmap.  This
               means 0 .. maxfds (or just the open fds within that range, if
               the platform supports '/proc/<pid>/fd').

            2. for each childFD::

                 - if fdmap[childFD] == childFD, the descriptor is already in
                   place.  Make sure the CLOEXEC flag is not set, then delete
                   the entry from fdmap.

                 - if childFD is in fdmap.values(), then the target descriptor
                   is busy. Use os.dup() to move it elsewhere, update all
                   fdmap[childFD] items that point to it, then close the
                   original. Then fall through to the next case.

                 - now fdmap[childFD] is not in fdmap.values(), and is free.
                   Use os.dup2() to move it to the right place, then close the
                   original.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/process.py