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NAME
    AnyEvent::Handle::Writer - Extended version of AnyEvent::Handle with
    additional write options

SYNOPSIS
       use AnyEvent;
       use AnyEvent::Handle::Writer;

       my $hdl; $hdl = AnyEvent::Handle::Writer->new(
          fh => $fh,
          on_error => sub {
             my ($hdl, $fatal, $msg) = @_;
             warn "got error $msg\n";
             $hdl->destroy;
          }
       );

       # Custom writer
       $hdl->push_write(sub {
          my $h = shift;
          if ($have_data) {
             $h->unshift_write($data);
             return 1; # Work done
          } else {
             return 0; # Work not done, call me again
          }
       });

       # sendfile
       $hdl->push_sendfile('/path/to/file', 1024);

RATIONALE
    We have great l<AnyEvent::Handle>. But it have only raw write queue.
    This module extends it with callbacks in write queue and adds a
    "push_sendfile()" call, which would be processed at correct time

METHODS
  push_write($data)
    "WRITE_QUEUE" in AnyEvent::Handle

  push_write(type => @args)
    "WRITE_QUEUE" in AnyEvent::Handle

  push_write($cb->($handle))
    This version of call allow to push a callback, which will be invoked
    when the write queue before it became empty.

       Callback should return:
          true  - when there is no work to be done with this callback.
                  it will be removed from queue and continue
          false - when it want to be called again (i.e. not all work was done)
                  it will be kept in queue and called on next drain

    This call allow us to implement such a thing:

       $handle->push_write("HTTP/1.1 200 OK\012\015\012\015");
       $handle->push_write(sub {
          # Manual work on handle
          my $h = shift;
          my $len = syswrite($h->{fh}, $data); # Here may be also sendfile work
          if (defined $len) {
             diag "written $len";
             substr $data, 0, $len, "";
             if (length $data) {
                return 0; # want be called again
             } else {
                return 1; # all done
             }
          } elsif (!$!{EAGAIN} and !$!{EINTR} and !$!{WSAEWOULDBLOCK}) {
             $h->_error ($!, 1);
             return 1; # No more requests to me, got an error
          }
          else { return 0; }
       });
       $handle->push_write("HTTP/1.1 200 OK\012\015\012\015");
       $handle->push_write("Common response");

  unshift_write($data)
=head2 unshift_write(type => @args)
=head2 unshift_write($cb->($handle))
    Analogically to "unshift_read", it unshift write data at the beginngin
    of queue. The only recommended usage is from write callback

       $handle->push_write("1")
       $handle->push_write(sub {
          my $h = shift;
          $h->unshift_write("2");
          return 1;
       });
       $handle->push_write("3");
       
   # The output will be "123"

  push_sendfile($filename, [$size, [$offset]]);