Location list entries for unsaved buffer open a new buffer

I have a function that pipes a buffer into an external program, then
parses its output in a callback and adds the parsed results to
a location list. Simplified code for the callback:

def Callback(channel: job, msg: string, winid: number, efm: string, cwd: string)
silent execute "lcd" cwd
var what = getqflist({"lines": [msg], "efm": efm})
silent lcd -
setloclist(winid, what.items, "a")
enddef

The lines to be parsed have a pretty standard format:

filename:line:col message

The program reads from stdin, hence `filename` is passed as an argument
to the program. The program simply puts whatever filename it gets into
its output messages.

This works fine for buffers backed up by a file: I pass the buffer's
name as the filename argument. My problem is that I don't know how to
create an entry in the location list for an unsaved buffer. Passing
something like '[No Name]', 'unnamed', or similar does not work. The
location list ends up having entries that look like this:

unnamed|1 col 8 E123| some message

If the unsaved buffer ia buffer 1, then the message above is parsed as:

{'items': [{'bufnr': 2, 'lnum': 1, 'col': 8, 'text': 'some message', ...}]}

Therefore, selecting such entries causes a new buffer 2 to be created.

How do I generate location list entries that refer to the proper unsaved
buffer?

Thanks,
Life.

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