[FIX] snailmail: mandatory cover page + handle cover page generation

Pingen's v2 API does not offer an endpoint to generate a Cover Page automatically with an API call anymore, thus the need to generate it and append it to the invoice from the client-side beforehand.
Also make the Cover Page option mandatory due to the invoice being near
impossible to format for Pingen's validation needs. (Odoo 13 Only)

closes odoo/odoo#103385

Signed-off-by: Louis Baudoux (lba) <lba@odoo.com>

addons/snailmail/models/snailmail_letter.py

@@ -2,9 +2,16 @@
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 import re
 import base64
-import datetime
+import io
 
-from odoo import fields, models, api, _, tools
+from PyPDF2 import PdfFileReader, PdfFileMerger
+from reportlab.platypus import Frame, Paragraph, KeepInFrame
+from reportlab.lib.units import mm
+from reportlab.lib.pagesizes import A4
+from reportlab.lib.styles import getSampleStyleSheet
+from reportlab.pdfgen.canvas import Canvas
+
+from odoo import fields, models, api, _
 from odoo.addons.iap import jsonrpc
 from odoo.exceptions import UserError, AccessError
 from odoo.tools.safe_eval import safe_eval
@@ -120,7 +127,11 @@ class SnailmailLetter(models.Model):
             else:
                 report_name = 'Document'
             filename = "%s.%s" % (report_name, "pdf")
-            pdf_bin, _ = report.with_context(snailmail_layout=not self.cover).render_qweb_pdf(self.res_id)
+            if not self.cover:
+                raise UserError(_("Snailmails without covers are no longer supported in Odoo 13.\nPlease enable the 'Add a Cover Page' option in your Invoicing settings or upgrade your Odoo."))
+            pdf_bin, unused_filetype = report.with_context(snailmail_layout=not self.cover).render_qweb_pdf(self.res_id)
+            if self.cover:
+                pdf_bin = self._append_cover_page(pdf_bin)
             attachment = self.env['ir.attachment'].create({
                 'name': filename,
                 'datas': base64.b64encode(pdf_bin),
@@ -183,7 +194,6 @@ class SnailmailLetter(models.Model):
         dbuuid = self.env['ir.config_parameter'].sudo().get_param('database.uuid')
         documents = []
 
-        batch = len(self) > 1
         for letter in self:
             document = {
                 # generic informations to send
@@ -398,3 +408,33 @@ class SnailmailLetter(models.Model):
             }
             failures_infos.append(info)
         return failures_infos
+
+    def _append_cover_page(self, invoice_bin: bytes):
+        address = self.partner_id.contact_address.replace('\n', '<br/>')
+        address_x = 118 * mm
+        address_y = 60 * mm
+        frame_width = 85.5 * mm
+        frame_height = 25.5 * mm
+
+        cover_buf = io.BytesIO()
+        canvas = Canvas(cover_buf, pagesize=A4)
+        styles = getSampleStyleSheet()
+
+        frame = Frame(address_x, A4[1] - address_y - frame_height, frame_width, frame_height)
+        story = [Paragraph(address, styles['Normal'])]
+        address_inframe = KeepInFrame(0, 0, story)
+        frame.addFromList([address_inframe], canvas)
+        canvas.save()
+        cover_buf.seek(0)
+
+        invoice = PdfFileReader(io.BytesIO(invoice_bin))
+        cover_bin = io.BytesIO(cover_buf.getvalue())
+        cover_file = PdfFileReader(cover_bin)
+        merger = PdfFileMerger()
+
+        merger.append(cover_file, import_bookmarks=False)
+        merger.append(invoice, import_bookmarks=False)
+
+        out_buff = io.BytesIO()
+        merger.write(out_buff)
+        return out_buff.getvalue()