Set page size after issuing painting/show operators - ghostscript

Question

I'm developing a printer emulator (partly) in PostScript. The command system I want to emulate is designed for roll paper thermal printers, mostly used for printing Point of Sale receipts. I'd prefer to execute the operations in the same order as the real printer would do, which means my program doesn't know the page size until it encounters a "cut" command. I want the program to work with variable paper size CUPS printers¹.
Can I change the page height after I issued painting/show commands without the filled parts of the document disappearing?
I have tried modifying the page device dictionary at the end of the document, but if I change the PageSize array, everything on the document disappears.
For example if I run the following program:
<< /PageSize [ 100 30 ] >> setpagedevice
0 0 moveto
(Text) show
showpage
I get the output:
But when I modify the code to adjust the page size right before the showpage command:
<< /PageSize [ 100 30 ] >> setpagedevice
0 0 moveto
(Text) show
<< /PageSize [ 100 100 ] >> setpagedevice
showpage
I only get a blank image:
I'm aware I can defer the execution of the painting/show operators, so my program calculates the document size before painting, and only executes the operations when it encounters the cut command. I would be able to implement that by myself, I don't need help with such a solution currently. I'm rather wondering, whether a simpler solution is possible for cutting an already drawn document to a calculated page size.

Answer 1

You cannot use marking operations and then select the page size in PostScript. Setting the media size in PostScript executes an implicit erasepage which clears any marks on the page.
See the note on page 408 of the 3rd edition PLRM (Section 6.1.1 PageDevice Dictionary):
Note: setpagedevice is a page-oriented operator used to control the
output processing of one or more pages of a page description. Any call
to setpagedevice implicitly invokes erasepage and initgraphics, and
thus must precede the descriptions of the pages to be affected.

Answer 2

Thanks to KenS I realized what was wrong with my original approach so I came up with an alternative solution, deferring the execution of the marking operators and calculating the page height beforehand. Below is an incomplete proof of concept of my method of implementing a simple receipt printer with variable page size:
%!
/feed {
0 % return to left margin
currentpoint exch pop % y coordinate
20 sub moveto % feed 20 points
} def
% deferred feed
/_feed {
/feed cvx % push executable name on stack
dup exec % execute procedure, to save position in current point
} def
% deferred show
/_show {
dup % duplicate string
stringwidth rmoveto % simulate position change
/show cvx % push show operator on stack
2 array
astore cvx % create procedure for showing the text
} def
Each time I run the underscore procedures, they will push a procedure on the operand stack, but apply all position changes that will happen during the final execution.
% Set font
/DejaVuSansMono findfont
16 scalefont
setfont
% The receipt itself
0 0 moveto
(text) _show
_feed
(text) _show
_feed
% Save coordinates
currentpoint
/y exch def
/x exch def
% Calculate and set document height based on position
/pageheight y neg def
<< /PageSize [ 100 pageheight ] >> setpagedevice
% Translate the negative y coordinates
0 pageheight 16 sub translate
% reset position
0 0 moveto
% Execute all procedures on the operand stack
count array astore { exec } forall
showpage
The output of this script: the word "text" appearing twice, the image is automatically cut to the right height
It is incomplete of course, but I wanted to demonstrate it the simplest possible way. I hope some will find it interesting/useful.