HTTP 幂等让重试变安全

from http.client import HTTPConnection, RemoteDisconnected
from http.server import BaseHTTPRequestHandler, HTTPServer
from threading import Thread


state = {"body": None, "version": 0, "orders": [], "requests": []}


def etag():
    return '"v%d"' % state["version"]


class Handler(BaseHTTPRequestHandler):
    def handle_request(self):
        state["requests"].append((self.command, self.path))
        body = self.rfile.read(int(self.headers.get("Content-Length", "0")))
        status, answer = 404, b""
        if self.path == "/item":
            if self.command in ("GET", "HEAD"):
                if state["body"] is not None:
                    status, answer = 200, state["body"]
            elif self.command == "PUT":
                condition = self.headers.get("If-Match")
                # The fixture accepts one exact strong entity tag, not the full grammar.
                if condition is not None and (state["body"] is None or condition != etag()):
                    status = 412
                else:
                    status = 201 if state["body"] is None else 204
                    if body != state["body"]:
                        state["version"] += 1
                    state["body"] = body
            elif self.command == "DELETE":
                status = 404 if state["body"] is None else 204
                state["body"] = None
        elif self.path == "/orders" and self.command == "POST":
            state["orders"].append(body)
            status, answer = 201, str(len(state["orders"])).encode()
        if self.headers.get("X-Fixture-Drop-Reply") == "yes" and 200 <= status < 300:
            self.close_connection = True  # Apply the change, then close without a response.
            return
        self.send_response(status)
        if self.path == "/item" and state["body"] is not None:
            self.send_header("ETag", etag())
        if status != 204:
            self.send_header("Content-Length", str(len(answer)))
        self.end_headers()
        if self.command != "HEAD":
            self.wfile.write(answer)

    do_GET = do_HEAD = do_PUT = do_DELETE = do_POST = handle_request

    def log_message(self, *_):
        pass


with HTTPServer(("127.0.0.1", 0), Handler) as server:
    thread = Thread(target=server.serve_forever, kwargs={"poll_interval": 0.05})
    thread.start()

    def request(method, route="/item", body=b"", headers=None):
        client = HTTPConnection(*server.server_address, timeout=2)
        try:
            client.request(method, route, body=body, headers=headers or {})
            response = client.getresponse()
            return response.status, response.read(), response.getheader("ETag")
        finally:
            client.close()

    def lost_put(body, headers=None):
        try:
            request("PUT", body=body, headers={**(headers or {}), "X-Fixture-Drop-Reply": "yes"})
        except RemoteDisconnected:
            return
        raise AssertionError("the fixture should close before sending a response")

    try:
        assert request("PUT", body=b"blue")[0] == 201
        assert request("PUT", body=b"blue")[0] == 204
        before = len(state["requests"])
        assert request("GET")[1] == request("GET")[1] == b"blue"
        assert len(state["requests"]) == before + 2 and state["body"] == b"blue"
        assert request("HEAD")[1] == b""
        assert [request("DELETE")[0], request("DELETE")[0]] == [204, 404]
        print("PUT returns 201 then 204; DELETE returns 204 then 404; intended state is unchanged by repeats")
        print("GET appends separate diagnostic records without changing the requested resource")

        lost_put(b"blue")
        assert request("GET")[1] == b"blue"
        assert request("PUT", body=b"blue")[0] == 204
        old_tag = request("GET")[2]
        lost_put(b"green", {"If-Match": old_tag})
        assert request("PUT", body=b"green", headers={"If-Match": old_tag})[0] == 412
        assert request("GET")[1] == b"green"
        print("a lost reply can follow a committed PUT; replaying its old precondition can return 412")

        tag = request("GET")[2]
        assert request("PUT", body=b"other-writer")[0] == 204
        assert request("PUT", body=b"green", headers={"If-Match": tag})[0] == 412
        assert request("GET")[1] == b"other-writer"
        assert request("PUT", body=b"green")[0] == 204
        assert request("GET")[1] == b"green"
        print("a stale conditional PUT is refused; an unconditional repeat overwrites the intervening writer")

        first = request("POST", "/orders", b"same order")
        second = request("POST", "/orders", b"same order")
        assert first[:2] == (201, b"1") and second[:2] == (201, b"2")
        assert state["orders"] == [b"same order", b"same order"]
        print("two identical POST bodies create two orders in this endpoint's contract")
    finally:
        server.shutdown()
        thread.join(timeout=2)
        assert not thread.is_alive()