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[Qemu-devel] [PATCH v3 01/58] docs/interop/qmp-spec: How to force known
From: |
Markus Armbruster |
Subject: |
[Qemu-devel] [PATCH v3 01/58] docs/interop/qmp-spec: How to force known good parser state |
Date: |
Thu, 23 Aug 2018 18:39:28 +0200 |
Section "QGA Synchronization" specifies that sending "a raw 0xFF
sentinel byte" makes the server "reset its state and discard all
pending data prior to the sentinel." What actually happens there is a
lexical error, which will produce one or more error responses.
Moreover, it's not specific to QGA.
Create new section "Forcing the JSON parser into known-good state" to
document the technique properly. Rewrite section "QGA
Synchronization" to document just the other direction, i.e. command
guest-sync-delimited.
Section "Protocol Specification" mentions "synchronization bytes
(documented below)". Delete that.
While there, fix it not to claim '"Server" is QEMU itself', but
'"Server" is either QEMU or the QEMU Guest Agent'.
Signed-off-by: Markus Armbruster <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
---
docs/interop/qmp-spec.txt | 42 ++++++++++++++++++++++++++-------------
1 file changed, 28 insertions(+), 14 deletions(-)
diff --git a/docs/interop/qmp-spec.txt b/docs/interop/qmp-spec.txt
index 1566b8ae5e..8f7da0245d 100644
--- a/docs/interop/qmp-spec.txt
+++ b/docs/interop/qmp-spec.txt
@@ -20,9 +20,9 @@ operating system.
2. Protocol Specification
=========================
-This section details the protocol format. For the purpose of this document
-"Client" is any application which is using QMP to communicate with QEMU and
-"Server" is QEMU itself.
+This section details the protocol format. For the purpose of this
+document, "Server" is either QEMU or the QEMU Guest Agent, and
+"Client" is any application communicating with it via QMP.
JSON data structures, when mentioned in this document, are always in the
following format:
@@ -34,9 +34,8 @@ by the JSON standard:
http://www.ietf.org/rfc/rfc7159.txt
-The protocol is always encoded in UTF-8 except for synchronization
-bytes (documented below); although thanks to json-string escape
-sequences, the server will reply using only the strict ASCII subset.
+The server expects its input to be encoded in UTF-8, and sends its
+output encoded in ASCII.
For convenience, json-object members mentioned in this document will
be in a certain order. However, in real protocol usage they can be in
@@ -215,16 +214,31 @@ Some events are rate-limited to at most one per second.
If additional
dropped, and the last one is delayed. "Similar" normally means same
event type. See qmp-events.txt for details.
-2.6 QGA Synchronization
+2.6 Forcing the JSON parser into known-good state
+-------------------------------------------------
+
+Incomplete or invalid input can leave the server's JSON parser in a
+state where it can't parse additional commands. To get it back into
+known-good state, the client should provoke a lexical error.
+
+The cleanest way to do that is sending an ASCII control character
+other than '\t' (horizontal tab), '\r' (carriage return), or '\n' (new
+line).
+
+Sadly, older versions of QEMU can fail to flag this as an error. If a
+client needs to deal with them, it should send a 0xFF byte.
+
+2.7 QGA Synchronization
-----------------------
-When using QGA, an additional synchronization feature is built into
-the protocol. If the Client sends a raw 0xFF sentinel byte (not valid
-JSON), then the Server will reset its state and discard all pending
-data prior to the sentinel. Conversely, if the Client makes use of
-the 'guest-sync-delimited' command, the Server will send a raw 0xFF
-sentinel byte prior to its response, to aid the Client in discarding
-any data prior to the sentinel.
+When a client connects to QGA over a transport lacking proper
+connection semantics such as virtio-serial, QGA may have read partial
+input from a previous client. The client needs to force QGA's parser
+into known-good state using the previous section's technique.
+Moreover, the client may receive output a previous client didn't read.
+To help with skipping that output, QGA provides the
+'guest-sync-delimited' command. Refer to its documentation for
+details.
3. QMP Examples
--
2.17.1
- [Qemu-devel] [PATCH v3 32/58] json-parser: simplify and avoid JSONParserContext allocation, (continued)
- [Qemu-devel] [PATCH v3 32/58] json-parser: simplify and avoid JSONParserContext allocation, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 30/58] check-qjson: Fix and enable utf8_string()'s disabled part, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 08/58] check-qjson: Cover escaped characters more thoroughly, part 1, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 07/58] test-qga: Clean up how we test QGA synchronization, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 15/58] check-qjson qmp-test: Cover control characters more thoroughly, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 24/58] json: Leave rejecting invalid UTF-8 to parser, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 05/58] qmp-cmd-test: Split off qmp-test, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 23/58] json: Report first rather than last parse error, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 14/58] check-qjson: Fix utf8_string() to test all invalid sequences, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 11/58] check-qjson: Consolidate partly redundant string tests, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 01/58] docs/interop/qmp-spec: How to force known good parser state,
Markus Armbruster <=
- [Qemu-devel] [PATCH v3 20/58] json: Tighten and simplify qstring_from_escaped_str()'s loop, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 09/58] check-qjson: Streamline escaped_string()'s test strings, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 02/58] check-qjson: Cover multiple JSON objects in same string, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 03/58] check-qjson: Cover blank and lexically erroneous input, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 06/58] qmp-test: Cover syntax and lexical errors, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 49/58] json: Streamline json_message_process_token(), Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 47/58] qjson: Have qobject_from_json() & friends reject empty and blank, Markus Armbruster, 2018/08/23
- [Qemu-devel] [PATCH v3 46/58] json: Assert json_parser_parse() consumes all tokens on success, Markus Armbruster, 2018/08/23