print-server: printing invoices through a spool

examples/print-server — a small business application that prints invoices. It fills an invoice template with what is typed on its page, puts the document in the queue of a print server it embeds, and shows each job as the spool sends it and the printer prints it. The page answers at once; the printing happens behind it, and survives a restart.

cd examples/print-server
go run . -fake-printer          # prints on bdf's fake IPP printer, in the same process
open http://127.0.0.1:8083/

go run . -printer ipps://printer.example.local/ipp/print   # on a printer of the network

What the server does

Fills the template. POST /invoices reads the customer, the items (a line each: name, quantity, price), a note and the user, and fills converter/xlsx/testdata/form-rows.xlsx with them (form.Fill): an Excel workbook whose tags ({{no}}, {{customer}}, {{#items}} … {{/items}}) take the data, laid out as the pages Excel prints. The result is a .bdf in memory.
Puts it in the queue. The application opens a print server from a configuration it writes itself (one printer, one queue invoices whose defaults print on both sides), with server.New, and submits the document with Spool().Submit, for the user the page names. It does not wait for the printer.
Shares the API. The print server's http.Handler is mounted under /api/: the page reads /api/v1/jobs and follows /api/v1/events, and bdf jobs -server http://127.0.0.1:8083 works on the same jobs.
Keeps the jobs. The spool is data/spool.db. Stop the program while a job is being sent and run it again: the job is checked against the printer and printed once.

Why this shape

A business system that prints should not hold a request open until the printer is done, nor lose a job because the printer was off. Embedding the print server gives the application the spool's guarantees (one file, retries, no job printed twice, accounting) without another process; a larger site runs bdf printd on its own and talks to it with print/server/client or plain HTTP, as curl does.