Address Datamax via USB VID:PID/Serial through libusb, like brother_ql_web does

Gemfile

@@ -6,5 +6,6 @@ gem 'puma'
 gem 'rqrcode'
 gem 'logger'
 gem 'ostruct'
+gem 'libusb'
 
 gem 'rackup', "~> 2.1"

app.rb

@@ -3,6 +3,7 @@ require 'sinatra'
 require "sinatra/reloader"
 require 'tmpdir'
 require 'net/http'
+require 'libusb'
 
 set :protection, :except => :frame_options
 set :environment, :production
@@ -13,7 +14,14 @@ if !File.exist?('config.json') || !File.readable?('config.json')
   raise RuntimeException, 'Could not read config.json'
 end
 
-PRINTER_DEVICE = '/dev/usb/lp_datamax' # stable udev symlink, see /etc/udev/rules.d/99-datamax.rules
+# Addressed the same way as the Brother QL printers (usb://VID:PID/Serial via
+# brother_ql_web) instead of a udev symlink -- self-contained, no reliance on
+# OS device naming/ordering.
+DATAMAX_VENDOR_ID = 0x0b0b
+DATAMAX_PRODUCT_ID = 0x106e
+DATAMAX_SERIAL = '24624'
+DATAMAX_ENDPOINT = 0x02 # bulk OUT, interface 0
+
 PRINTER_DPI = 203 # DatamaxONeil E4204B Mark III native head resolution
 PRINTER_DARKNESS = 8 # lower than factory default to stop thermal dot-gain from fusing barcode bars
 TEST_LABEL_PDF = File.join(File.dirname(__FILE__), 'test-label.pdf')
@@ -81,6 +89,22 @@ helpers do
     "^XA\n^MD#{PRINTER_DARKNESS}\n^PW#{width}\n^LL#{height}\n^FO0,0^GFA,#{total_bytes},#{total_bytes},#{bytes_per_row},#{hexdata}\n^XZ\n"
   end
 
+  def print_to_datamax(data)
+    usb = LIBUSB::Context.new
+    device = usb.devices(idVendor: DATAMAX_VENDOR_ID, idProduct: DATAMAX_PRODUCT_ID)
+                .find { |d| d.serial_number == DATAMAX_SERIAL }
+    unless device
+      raise format('Datamax printer not found (USB %04x:%04x/%s)',
+                    DATAMAX_VENDOR_ID, DATAMAX_PRODUCT_ID, DATAMAX_SERIAL)
+    end
+
+    device.open do |handle|
+      handle.claim_interface(0) do
+        handle.bulk_transfer(endpoint: DATAMAX_ENDPOINT, dataOut: data)
+      end
+    end
+  end
+
   # Transparently reverse-proxies the current request to one of the
   # brother_ql_web instances, so all three printers are reachable through
   # this single sinatra port instead of their own dedicated ports.
@@ -148,13 +172,13 @@ post '/upload' do
       when ".pdf"
         begin
           zpl = pdf_to_zpl(filepath)
-          File.open(PRINTER_DEVICE, 'wb') { |f| f.write(zpl) }
+          print_to_datamax(zpl)
           flash "PDF: Upload successful (#{filename})", "success"
         rescue => e
           flash "PDF: Failed to print (#{e.message})", "danger"
         end
       when ".zpl"
-        File.open(PRINTER_DEVICE, 'wb') { |f| f.write(File.binread(filepath)) }
+        print_to_datamax(File.binread(filepath))
         flash "ZPL: Upload successful (#{filename})", "success"
       else
         flash "#{filetype} is not supported. Only .pdf and .zpl!", "false"
@@ -171,7 +195,7 @@ end
 post '/test-print' do
   begin
     zpl = pdf_to_zpl(TEST_LABEL_PDF)
-    File.open(PRINTER_DEVICE, 'wb') { |f| f.write(zpl) }
+    print_to_datamax(zpl)
     flash 'Test label sent to printer', 'success'
   rescue => e
     flash "Test print failed (#{e.message})", 'danger'