From 62815d85aed71eff7b6c3a524705180fb04f5d30 Mon Sep 17 00:00:00 2001 From: Markus Armbruster Date: Thu, 23 Aug 2018 18:40:01 +0200 Subject: 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 Reviewed-by: Eric Blake Message-Id: <20180823164025.12553-35-armbru@redhat.com> --- include/qapi/qmp/json-parser.h | 3 +-- include/qapi/qmp/json-streamer.h | 8 ++++++-- 2 files changed, 7 insertions(+), 4 deletions(-) (limited to 'include') diff --git a/include/qapi/qmp/json-parser.h b/include/qapi/qmp/json-parser.h index 102f5c0068..a34209db7a 100644 --- a/include/qapi/qmp/json-parser.h +++ b/include/qapi/qmp/json-parser.h @@ -16,7 +16,6 @@ #include "qemu-common.h" -QObject *json_parser_parse(GQueue *tokens, va_list *ap); -QObject *json_parser_parse_err(GQueue *tokens, va_list *ap, Error **errp); +QObject *json_parser_parse(GQueue *tokens, va_list *ap, Error **errp); #endif diff --git a/include/qapi/qmp/json-streamer.h b/include/qapi/qmp/json-streamer.h index 7922e185a5..e162fd01da 100644 --- a/include/qapi/qmp/json-streamer.h +++ b/include/qapi/qmp/json-streamer.h @@ -25,7 +25,9 @@ typedef struct JSONToken { typedef struct JSONMessageParser { - void (*emit)(struct JSONMessageParser *parser, GQueue *tokens); + void (*emit)(void *opaque, QObject *json, Error *err); + void *opaque; + va_list *ap; JSONLexer lexer; int brace_count; int bracket_count; @@ -37,7 +39,9 @@ void json_message_process_token(JSONLexer *lexer, GString *input, JSONTokenType type, int x, int y); void json_message_parser_init(JSONMessageParser *parser, - void (*func)(JSONMessageParser *, GQueue *)); + void (*emit)(void *opaque, QObject *json, + Error *err), + void *opaque, va_list *ap); void json_message_parser_feed(JSONMessageParser *parser, const char *buffer, size_t size); -- cgit v1.2.3