Explicitly allow requests from public websites via new header

- Chrome now enforces that servers on private networks explicitly
  grant access to public websites using a new header
  "Access-Control-Allow-Private-Network" that should be sent in
  responses to preflight OPTIONS requests.
- This change implements special handling for OPTIONS requests by
  sending all the existing CORS headers along with the new
  Access-Control-Allow-Private-Network header if private network
  access is being requested.
- See https://developer.chrome.com/blog/private-network-access-preflight/
  for more info.

plugin/web.py

@@ -24,7 +24,8 @@
 #
 
 class WebRequest:
-    def __init__(self, headers, body):
+    def __init__(self, method, headers, body):
+        self.method = method
         self.headers = headers
         self.body = body
 
@@ -95,8 +96,15 @@ def parseRequest(self, data):
         if len(parts) == 1:
             return None, 0
 
+        lines = parts[0].split('\r\n'.encode('utf-8'))
+        method = None
+
+        if len(lines) > 0:
+            request_line_parts = lines[0].split(' '.encode('utf-8'))
+            method = request_line_parts[0].upper() if len(request_line_parts) > 0 else None
+
         headers = {}
-        for line in parts[0].split('\r\n'.encode('utf-8')):
+        for line in lines[1:]:
             pair = line.split(': '.encode('utf-8'))
             headers[pair[0].lower()] = pair[1] if len(pair) > 1 else None
 
@@ -108,8 +116,7 @@ def parseRequest(self, data):
             return None, 0
 
         body = data[headerLength : totalLength]
-        return WebRequest(headers, body), totalLength
-
+        return WebRequest(method, headers, body), totalLength
 
 #
 # WebServer
@@ -154,7 +161,54 @@ def listen(self):
 
 
     def handlerWrapper(self, req):
+        allowed, corsOrigin = self.allowOrigin(req)
+
+        if req.method == b'OPTIONS':
+            body = ''.encode('utf-8')
+            headers = self.buildHeaders(corsOrigin, body)
+
+            if b'access-control-request-private-network' in req.headers and (
+            req.headers[b'access-control-request-private-network'] == b'true'):
+                # include this header so that if a public origin is included in the whitelist,
+                # then browsers won't fail requests due to the private network access check
+                headers.append(['Access-Control-Allow-Private-Network', 'true'])
+
+            return self.buildResponse(headers, body)
+
+        paramsError = False
+
+        try:
+            params = json.loads(req.body.decode('utf-8'))
+        except ValueError:
+            body = json.dumps(None).encode('utf-8')
+            paramsError = True
+
+        if allowed or not paramsError and params.get('action', '') == 'requestPermission':
+            if len(req.body) == 0:
+                body = 'AnkiConnect v.{}'.format(util.setting('apiVersion')).encode('utf-8')
+            else:
+                if params.get('action', '') == 'requestPermission':
+                    params['params'] = params.get('params', {})
+                    params['params']['allowed'] = allowed
+                    params['params']['origin'] = b'origin' in req.headers and req.headers[b'origin'].decode() or ''
+                    if not allowed :
+                        corsOrigin = params['params']['origin']
+
+                body = json.dumps(self.handler(params)).encode('utf-8')
+
+            headers = self.buildHeaders(corsOrigin, body)
+        else :
+            headers = [
+                ['HTTP/1.1 403 Forbidden', None],
+                ['Access-Control-Allow-Origin', corsOrigin],
+                ['Access-Control-Allow-Headers', '*']
+            ]
+            body = ''.encode('utf-8')
 
+        return self.buildResponse(headers, body)
+
+
+    def allowOrigin(self, req):
         # handle multiple cors origins by checking the 'origin'-header against the allowed origin list from the config
         webCorsOriginList = util.setting('webCorsOriginList')
 
@@ -183,43 +237,22 @@ def handlerWrapper(self, req):
                 allowed = True
         else:
             allowed = True
+
+        return allowed, corsOrigin
+
 
-        resp = bytes()
-        paramsError = False
-        try:
-            params = json.loads(req.body.decode('utf-8'))
-        except ValueError:
-            body = json.dumps(None).encode('utf-8')
-            paramsError = True
+    def buildHeaders(self, corsOrigin, body):
+        return [
+            ['HTTP/1.1 200 OK', None],
+            ['Content-Type', 'text/json'],
+            ['Access-Control-Allow-Origin', corsOrigin],
+            ['Access-Control-Allow-Headers', '*'],
+            ['Content-Length', str(len(body))]
+        ]
 
-        if allowed or not paramsError and params.get('action', '') == 'requestPermission':
-            if len(req.body) == 0:
-                body = 'AnkiConnect v.{}'.format(util.setting('apiVersion')).encode('utf-8')
-            else:
-                if params.get('action', '') == 'requestPermission':
-                    params['params'] = params.get('params', {})
-                    params['params']['allowed'] = allowed
-                    params['params']['origin'] = b'origin' in req.headers and req.headers[b'origin'].decode() or ''
-                    if not allowed :
-                        corsOrigin = params['params']['origin']
-
-                body = json.dumps(self.handler(params)).encode('utf-8')
-
-            headers = [
-                ['HTTP/1.1 200 OK', None],
-                ['Content-Type', 'text/json'],
-                ['Access-Control-Allow-Origin', corsOrigin],
-                ['Access-Control-Allow-Headers', '*'],
-                ['Content-Length', str(len(body))]
-            ]
-        else :
-            headers = [
-                ['HTTP/1.1 403 Forbidden', None],
-                ['Access-Control-Allow-Origin', corsOrigin],
-                ['Access-Control-Allow-Headers', '*']
-            ]
-            body = ''.encode('utf-8')
 
+    def buildResponse(self, headers, body):
+        resp = bytes()
         for key, value in headers:
             if value is None:
                 resp += '{}\r\n'.format(key).encode('utf-8')
@@ -228,7 +261,6 @@ def handlerWrapper(self, req):
 
         resp += '\r\n'.encode('utf-8')
         resp += body
-
         return resp