typespec1-10  Exercise 3: Checking Response Bodies Against the Contract
=========================================================================

The contract: chapter 5's books.tsp compiled to OpenAPI 3.1 JSON with seal-object-schemas: true (extra properties not allowed). The script (body_check.py):

  """Check the BODIES a running Book Service returns against the schemas in the contract.

      py body_check.py PORT
  The contract is books_sealed.json: TypeSpec's OpenAPI 3.1 output with seal-object-schemas, so extra properties are not allowed.
  """
  import json
  import pathlib
  import sys
  import urllib.error
  import urllib.request

  from jsonschema import Draft202012Validator
  from referencing import Registry, Resource

  contract = json.loads(pathlib.Path(__file__).with_name("books_sealed.json").read_text(encoding="utf8"))
  wrapper = {"$schema": "https://json-schema.org/draft/2020-12/schema", "components": contract["components"]}
  registry = Registry().with_resource("urn:contract", Resource.from_contents(wrapper))
  base = f"http://127.0.0.1:{sys.argv[1]}"


  def check(label, method, path, schema_name, data=None):
      request = urllib.request.Request(base + path, method=method, data=None if data is None else json.dumps(data).encode(),
                                       headers={"Content-Type": "application/json"})
      try:
          with urllib.request.urlopen(request) as r:
              status, body = r.status, r.read()
      except urllib.error.HTTPError as e:
          status, body = e.code, e.read()
      schema = {"$ref": f"urn:contract#/components/schemas/{schema_name}"}
      errors = list(Draft202012Validator(schema, registry=registry).iter_errors(json.loads(body)))
      print(f"{label:<28} {status}  against {schema_name:<9} {'ok' if not errors else '; '.join(e.message[:70] for e in errors)}")


  check("GET /books/1", "GET", "/books/1", "Book")
  check("GET /books/99 (missing)", "GET", "/books/99", "NotFound")
  check("POST /books", "POST", "/books", "Book", {"isbn": "9780261103573", "title": "The Hobbit", "pages": 310})

Results:

  === port 8751
  GET /books/1                 200  against Book      ok
  GET /books/99 (missing)      404  against NotFound  Unevaluated properties are not valid under the given schema ('_' was u
  POST /books                  201  against Book      ok
  === port 8741
  GET /books/1                 200  against Book      ok
  GET /books/99 (missing)      404  against NotFound  ok
  POST /books                  201  against Book      ok

THE DIFFERENCE
  Port 8741 is my faithful toy server: its 404 body is {"message": "..."} which matches NotFound.
  Port 8751 is the server generated by @typespec/http-server-js: its 404 body is {"_":404,"message":"..."}. The sealed schema rejects the '_' property ('Unevaluated properties are not valid'). The generated router wrote the whole object, including the property that carries the status code, as the body.
  Without seal-object-schemas the same check passes, because JSON Schema allows extra properties unless told otherwise: the sealing is what makes the check useful here. (I did not run it unsealed; that follows from the chapter 9 test, where an extra property was accepted by default.)
WHAT THE CHECK DOES NOT DO
  Three calls only, response bodies only: it does not check status codes (chapter 4's checker did), headers such as location, or request handling.

WHY THIS WORKS AS AN ANSWER
---------------------------
It finds a real contract violation in generated code that compiled strictly, ran, returned the right statuses and passed a status-code check. That gap is the reason for the chapter's advice to test with a body check.