Update to Flask 3.1 and Werkzeug 3.1 and keep the size limits of MSColab requests

docs/mscolab.rst

@@ -306,7 +306,10 @@ You can use the `Preview` button to see how your text is formatted before sendin
 
 There is also support for image/document upload. You can set the upload size limit with ``MAX_UPLOAD_SIZE`` in the
 mscolab_settings.py file. The default limit is 2 MiB. Any other request, e.g. creating an operation from a large
-flight track, is limited by ``MAX_CONTENT_LENGTH``, by default 16 MiB.
+flight track, is limited by ``MAX_CONTENT_LENGTH``, by default 16 MiB. A single form field is limited by Flask's
+``MAX_FORM_MEMORY_SIZE``, by default 500 kB; only the flight track of a new operation may be as large as the whole
+request. Without a ``MAX_CONTENT_LENGTH`` (``None``) requests are not limited, but form fields, also the flight
+track, are limited by ``MAX_FORM_MEMORY_SIZE``.
 
 Only files with an extension of ``MSCOLAB_ATTACHMENT_EXTENSIONS`` can be sent as chat attachments. By default these are
 flight tracks (ftml, csv, txt, kml, gpx), science data (nc, nc4, h5, json), documents (pdf, md, docx, xlsx, pptx, odt,

mslib/mscolab/app/__init__.py

@@ -35,7 +35,7 @@
 from flask_mail import Mail
 
 from flask_migrate import Migrate
-from flask import Flask, Request, current_app, jsonify, request, url_for
+from flask import Flask, current_app, jsonify, request, url_for
 
 import mslib
 
@@ -248,25 +248,39 @@ def repair_schema():
             add_token_nonce(Operations(MigrationContext.configure(conn)), conn)
 
 
-class MSColabRequest(Request):
+# endpoints of uploads, a request to them is limited to MAX_UPLOAD_SIZE instead of MAX_CONTENT_LENGTH
+UPLOAD_ENDPOINTS = ("chat.message_attachment", "user.upload_profile_image")
+# endpoints with a large form field, the flight track of a new operation; it may be as large as the whole request
+LARGE_FORM_FIELD_ENDPOINTS = ("operation.create_operation",)
+
+
+def limit_request_size():
     """
     Limits the size of an upload to MAX_UPLOAD_SIZE and of any other request to MAX_CONTENT_LENGTH
 
-    Flask refuses a larger request with 413 before reading it.
+    A form field is limited to Flask's MAX_FORM_MEMORY_SIZE (by default 500 kB), also on the endpoints anyone can
+    reach, e.g. /token. Only on LARGE_FORM_FIELD_ENDPOINTS it may be as large as the whole request. Flask refuses a
+    larger request with 413 before reading it.
     """
-    UPLOAD_ENDPOINTS = ("chat.message_attachment", "user.upload_profile_image")
-
-    @property
-    def max_content_length(self):
-        if current_app and self.endpoint in self.UPLOAD_ENDPOINTS:
-            return current_app.config["MAX_UPLOAD_SIZE"]
-        return super().max_content_length
+    if request.endpoint in UPLOAD_ENDPOINTS:
+        request.max_content_length = current_app.config["MAX_UPLOAD_SIZE"]
+    # without a MAX_CONTENT_LENGTH (None), Flask keeps MAX_FORM_MEMORY_SIZE
+    if request.endpoint in LARGE_FORM_FIELD_ENDPOINTS and request.max_content_length is not None:
+        request.max_form_memory_size = request.max_content_length
 
 
 # 413: Payload Too Large
 def error413(error):
-    limit = request.max_content_length / 1024 / 1024
-    return jsonify({"success": False, "message": f"Request too large. The limit is {limit:.1f} MiB."}), 413
+    """
+    Answers a request that is too large: the whole request (MAX_CONTENT_LENGTH or MAX_UPLOAD_SIZE), a form field
+    (MAX_FORM_MEMORY_SIZE) or the number of form parts (MAX_FORM_PARTS); Werkzeug doesn't say which
+    """
+    limit = request.max_content_length
+    if limit is not None and (request.content_length is None or request.content_length > limit):
+        message = f"Request too large. The limit is {limit / 1024 / 1024:.1f} MiB."
+    else:
+        message = "A form field of the request is too large, or the request has too many form parts."
+    return jsonify({"success": False, "message": message}), 413
 
 
 class InsecureSecretError(RuntimeError):
@@ -361,7 +375,7 @@ def create_app(config_object=mscolab_settings):
     check_basic_auth_setting(config_object, "mscolab_settings")
     app.register_error_handler(PasswordCheckBusy, lambda error: (str(error), 503, {"Retry-After": "10"}))
     # uploads are limited to MAX_UPLOAD_SIZE, other requests to MAX_CONTENT_LENGTH
-    app.request_class = MSColabRequest
+    app.before_request(limit_request_size)
     app.register_error_handler(413, error413)
     # Expose docs path for callers/tests and make it part of Flask config for consistency.
     app.config['DOCS_SERVER_PATH'] = DOCS_SERVER_PATH