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Tue, 10 Nov 2020 07:22:26 +0100 (CET) From: Markus Armbruster To: John Snow Subject: Re: QMP and the 'id' parameter References: <62700620-5228-f1cc-f0df-751c0d9f1f82@redhat.com> Date: Tue, 10 Nov 2020 07:22:26 +0100 In-Reply-To: <62700620-5228-f1cc-f0df-751c0d9f1f82@redhat.com> (John Snow's message of "Mon, 9 Nov 2020 20:47:45 -0500") Message-ID: <87361h20kd.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=63.128.21.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/10 00:21:06 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: QEMU Developers Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" John Snow writes: > The QMP specification states: > >> NOTE: Some errors can occur before the Server is able to read the "id" >> member, in these cases the "id" member will not be part of the error >> response, even if provided by the client. > > I am assuming this case ONLY occurs for Parse errors: > > {'class': 'GenericError', 'desc': 'JSON parse error, expecting value'} There are more "desc" possible, actually. The JSON parser gets fed chunks of input, and calls a callback for every full JSON value, and on parse error. QMP's callback is handle_qmp_command(). Parameter @req is the parsed JSON value, parameter @err is the (parse) error object, and exactly one of them is non-null. 1. Parse error If @err, we send an error response for it. It never has "id". See qmp_error_response() caller monitor_qmp_dispatcher_co(). The possible @err are: $ grep error_setg qobject/json-*[ch] qobject/json-parser.c: error_setg(&ctxt->err, "JSON parse error, %s", message); This is a syntax error. Search for parse_error() to see the possible @message patterns. qobject/json-streamer.c: error_setg(&err, "JSON parse error, stray '%s'", input->str); This is a lexical error. qobject/json-streamer.c: error_setg(&err, "JSON token size limit exceeded"); qobject/json-streamer.c: error_setg(&err, "JSON token count limit exceeded"); qobject/json-streamer.c: error_setg(&err, "JSON nesting depth limit exceeded"); These are (intentional) parser limits. 2. Successful parse If @req, it's a successful parse. If @req is not a JSON object, there is no "id". qmp_dispatch() reports error_setg(&err, "QMP input must be a JSON object"); If @req is a JSON object, it has "id" exactly when the client supplied one. The response mirrors @req's "id". See qmp_error_response() caller qmp_dispatch(). > And I am assuming, in the context of a client that /always/ sets an > 'id' for its execute statements, that this means that any error > response we receive without an 'id' field *must* be associated with > the most-recently-sent command. Only if the client keeps no more than one command in flight. Command responses get sent strictly in order (even parse errors), except for commands executed out-of-band. If the client sends N JSON values, and only then reads responses, and there are no parse errors, then it'll get N responses. The i-th response is for the i-th JSON value, except responses to OOB command may "jump the queue". With parse errors, it can get a different number of responses. Questions?