software/theia: generate a pseudo-random favicon

After trying to generate a random image ourselve with image magic with:

    ${imagemagick:location}/bin/convert -size 6x6 xc: +noise Random \( -clone 0 -flip \) -append \( -clone 0 -flop \) +append -scale 128x128 $${:location}

I gave up and just download an image from gravatar, but ignoring errors
if any to comply with the "instanciation should work offline" rule.

software/theia/buildout.hash.cfg

@@ -15,7 +15,7 @@
 
 [instance]
 filename = instance.cfg.in
-md5sum = 202f809fd759e9a507db7c7d33caae48
+md5sum = 1d495ab97a865b31ac7f2287cb76aed0
 
 [yarn.lock]
 filename = yarn.lock

software/theia/instance.cfg.in

@@ -53,7 +53,7 @@ template = inline:
     root $${directory:frontend-static}
     browse
     proxy / $${theia-instance:base-url} {
-      except public
+      except public $${favicon.ico:filename}
     }
     proxy /services $${theia-instance:base-url} {
       websocket
@@ -91,6 +91,29 @@ hash-files =
   $${frontend-instance:wrapper-path}
 wait-for-files = $${frontend-instance:pidfile}
 
+[favicon.ico]
+# generate a pseudo random favicon, different for each instance name.
+recipe = slapos.recipe.build
+install =
+  import hashlib, shutil
+  buildout_offline = self.buildout['buildout']['offline']
+  self.buildout['buildout']['offline'] = 'false'
+  try:
+    gravatar_url = "https://www.gravatar.com/avatar/" + hashlib.md5(
+      '''$${slap-configuration:root-instance-title}'''
+    ).hexdigest() + "?s=256&d=retro"
+    shutil.copy(self.download(gravatar_url), '''$${:location}''')
+  except Exception:
+    # Because installation should work offline, if we can't download a favicon,
+    # just ignore this step.
+    self.logger.exception("Error while downloading favicon, using empty one")
+    open('''$${:location}''', 'w').close()
+  finally:
+    self.buildout['buildout']['offline'] = buildout_offline
+
+location = $${directory:frontend-static}/$${:filename}
+filename = $${:_buildout_section_name_}
+
 [user]
 recipe = slapos.cookbook:userinfo
 

software/theia/test/test.py

@@ -74,6 +74,11 @@ class TestTheia(SlapOSInstanceTestCase):
         urljoin(authenticated_url, '/public/test_file'), verify=False)
     self.assertEqual('hello', resp.text)
 
+    # there's a (not empty) favicon
+    resp = requests.get(
+        urljoin(authenticated_url, '/favicon.ico'), verify=False)
+    self.assertEqual(requests.codes.ok, resp.status_code)
+    self.assertTrue(resp.raw)
 
   def test_theia_slapos(self):
     # Make sure we can use the shell and the integrated slapos command