path: root/qga/main.c
diff options
authorMarkus Armbruster <armbru@redhat.com>2018-08-23 18:40:01 +0200
committerMarkus Armbruster <armbru@redhat.com>2018-08-24 20:26:37 +0200
commit62815d85aed71eff7b6c3a524705180fb04f5d30 (patch)
treeef0327f940ea24f3ff8093292e17a4bd63259d6c /qga/main.c
parent037f2440888a22bd00ea0d8e37a1b7ed7d2bba88 (diff)
json: Redesign the callback to consume JSON values
The classical way to structure parser and lexer is to have the client call the parser to get an abstract syntax tree, the parser call the lexer to get the next token, and the lexer call some function to get input characters. Another way to structure them would be to have the client feed characters to the lexer, the lexer feed tokens to the parser, and the parser feed abstract syntax trees to some callback provided by the client. This way is more easily integrated into an event loop that dispatches input characters as they arrive. Our JSON parser is kind of between the two. The lexer feeds tokens to a "streamer" instead of a real parser. The streamer accumulates tokens until it got the sequence of tokens that comprise a single JSON value (it counts curly braces and square brackets to decide). It feeds those token sequences to a callback provided by the client. The callback passes each token sequence to the parser, and gets back an abstract syntax tree. I figure it was done that way to make a straightforward recursive descent parser possible. "Get next token" becomes "pop the first token off the token sequence". Drawback: we need to store a complete token sequence. Each token eats 13 + input characters + malloc overhead bytes. Observations: 1. This is not the only way to use recursive descent. If we replaced "get next token" by a coroutine yield, we could do without a streamer. 2. The lexer reports errors by passing a JSON_ERROR token to the streamer. This communicates the offending input characters and their location, but no more. 3. The streamer reports errors by passing a null token sequence to the callback. The (already poor) lexical error information is thrown away. 4. Having the callback receive a token sequence duplicates the code to convert token sequence to abstract syntax tree in every callback. 5. Known bug: the streamer silently drops incomplete token sequences. This commit rectifies 4. by lifting the call of the parser from the callbacks into the streamer. Later commits will address 3. and 5. The lifting removes a bug from qjson.c's parse_json(): it passed a pointer to a non-null Error * in certain cases, as demonstrated by check-qjson.c. json_parser_parse() is now unused. It's a stupid wrapper around json_parser_parse_err(). Drop it, and rename json_parser_parse_err() to json_parser_parse(). Signed-off-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Message-Id: <20180823164025.12553-35-armbru@redhat.com>
Diffstat (limited to 'qga/main.c')
1 files changed, 3 insertions, 9 deletions
diff --git a/qga/main.c b/qga/main.c
index 87372d40ef..2fc49d00d8 100644
--- a/qga/main.c
+++ b/qga/main.c
@@ -19,7 +19,6 @@
#include <sys/wait.h>
#include "qapi/qmp/json-streamer.h"
-#include "qapi/qmp/json-parser.h"
#include "qapi/qmp/qdict.h"
#include "qapi/qmp/qjson.h"
#include "qapi/qmp/qstring.h"
@@ -597,18 +596,13 @@ static void process_command(GAState *s, QDict *req)
/* handle requests/control events coming in over the channel */
-static void process_event(JSONMessageParser *parser, GQueue *tokens)
+static void process_event(void *opaque, QObject *obj, Error *err)
- GAState *s = container_of(parser, GAState, parser);
- QObject *obj;
+ GAState *s = opaque;
QDict *req, *rsp;
- Error *err = NULL;
int ret;
- g_assert(s && parser);
g_debug("process_event: called");
- obj = json_parser_parse_err(tokens, NULL, &err);
if (err) {
goto err;
@@ -1320,7 +1314,7 @@ static int run_agent(GAState *s, GAConfig *config, int socket_activation)
s->command_state = ga_command_state_new();
ga_command_state_init(s, s->command_state);
- json_message_parser_init(&s->parser, process_event);
+ json_message_parser_init(&s->parser, process_event, s, NULL);
#ifndef _WIN32
if (!register_signal_handlers()) {