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Fri, 30 Jan 2026 13:00:13 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.22]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id CBC741800965 for ; Fri, 30 Jan 2026 13:00:12 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 416CF21E692D; Fri, 30 Jan 2026 14:00:10 +0100 (CET) From: Markus Armbruster To: Paolo Bonzini Cc: qemu-devel@nongnu.org, armbru@redhat.com Subject: Re: [PATCH 0/5] qobject: switch JSON parser to push In-Reply-To: <20260107084840.150843-1-pbonzini@redhat.com> (Paolo Bonzini's message of "Wed, 7 Jan 2026 09:48:35 +0100") References: <20260107084840.150843-1-pbonzini@redhat.com> Date: Fri, 30 Jan 2026 14:00:10 +0100 Message-ID: <87ldhffe79.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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_H2=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-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: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Paolo Bonzini writes: > This rewrites the json-parser to use a push parser aka state machine. > While push parsers are inherently more complex than recursive descent, > the grammar for JSON is simple enough that the parser remains readable. > There is therefore no need to use e.g. QEMU coroutines. > > Unlike the suggestion in commit 62815d85aed ("json: Redesign the callback > to consume JSON values", 2018-08-24), I kept the json-streamer concept. > It helps in handling input limits, it performs error recovery, and it > converts the token-at-a-time push interface to callbacks---all things > that are more easily done in a separate layer to keep the parser clean. > However, there is no need anymore for it to store partial JSON objects > in tokenized form. > > Another benefit is that QEMU can report the first parsing error > immediately, without waiting for delimiters to be balanced. Sounds promising! Let's see... Before the series: $ socat "READLINE,prompt=QMP> " UNIX-CONNECT:$HOME/work/images/test-qmp {"QMP": {"version": {"qemu": {"micro": 50, "minor": 2, "major": 10}, "package": "v10.2.0-567-gfb6b66de43-dirty"}, "capabilities": ["oob"]}} QMP> [{"a"] Parse error not diagnosed right away, but ... QMP> } {"error": {"class": "GenericError", "desc": "JSON parse error, missing : in object pair"}} .... only when the streamer decides the expression is complete. After the series: QMP> [{"a"] {"error": {"class": "GenericError", "desc": "JSON parse error at line 1, column 6, expecting ':'"}} Cool! However, if I do it again, things fall apart: QMP> [{"a" QMP> } QMP> } QMP> } QMP> ] QMP> ] {"error": {"class": "GenericError", "desc": "JSON parse error at line 7, column 1, expecting value"}} Parse error recovery not quite right? > On top of the benefits intrinsic in the push architecture, it so happens > that it's really easy to add a location to JSON parsing errors now, so > do that as well. > > Paolo