Re: [Qemu-devel] [PATCH 4/6] json: Nicer recovery from lexical errors

[Eric Blake <eblake@redhat.com>]

On 08/27/2018 02:00 AM, Markus Armbruster wrote:
> When the lexer chokes on an input character, it consumes the
> character, emits a JSON error token, and enters its start state.  This
> can lead to suboptimal error recovery.  For instance, input
> 
>      0123 ,
> 
> produces the tokens
> 
>      JSON_ERROR    01
>      JSON_INTEGER  23
>      JSON_COMMA    ,
> 
> Make the lexer skip characters after a lexical error until a
> structural character ('[', ']', '{', '}', ':', ','), an ASCII control
> character, or '\xFE', or '\xFF'.
> 
> Note that we must not skip ASCII control characters, '\xFE', '\xFF',
> because those are documented to force the JSON parser into known-good
> state, by docs/interop/qmp-spec.txt.
> 
> The lexer now produces
> 
>      JSON_ERROR    01
>      JSON_COMMA    ,

So the lexer has now completely skipped the intermediate input, but the 
resulting error message need only point at the start of where input went 
wrong, and skipping to a sane point results in fewer error tokens to be 
reported.  Makes sense.

> 
> Update qmp-test for the nicer error recovery: QMP now report just one

s/report/reports/

> error for input %p instead of two.  Also drop the newline after %p; it
> was needed to tease out the second error.

That's because pre-patch, 'p' is one of the input characters that 
requires lookahead to determine if it forms a complete token (and the 
newline provides the transition needed to consume it); now post-patch, 
the 'p' is consumed as part of the junk after the error is first 
detected at the '%'.

And to my earlier complaint about 0x1a resulting in JSON_ERROR then 
JSON_INTEGER then JSON_KEYWORD, that sequence is likewise now identified 
as a single JSON_ERROR at the 'x', with the rest of the attempted hex 
number (invalid in JSON) silently skipped.  Nice.

> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>   qobject/json-lexer.c | 43 +++++++++++++++++++++++++++++--------------
>   tests/qmp-test.c     |  6 +-----
>   2 files changed, 30 insertions(+), 19 deletions(-)
> 
> diff --git a/qobject/json-lexer.c b/qobject/json-lexer.c
> index 28582e17d9..39c7ce7adc 100644
> --- a/qobject/json-lexer.c
> +++ b/qobject/json-lexer.c
> @@ -101,6 +101,7 @@
>   
>   enum json_lexer_state {
>       IN_ERROR = 0,               /* must really be 0, see json_lexer[] */
> +    IN_RECOVERY,
>       IN_DQ_STRING_ESCAPE,
>       IN_DQ_STRING,
>       IN_SQ_STRING_ESCAPE,
> @@ -130,6 +131,28 @@ QEMU_BUILD_BUG_ON(IN_START_INTERP != IN_START + 1);
>   static const uint8_t json_lexer[][256] =  {
>       /* Relies on default initialization to IN_ERROR! */
>   
> +    /* error recovery */
> +    [IN_RECOVERY] = {
> +        /*
> +         * Skip characters until a structural character, an ASCII
> +         * control character other than '\t', or impossible UTF-8
> +         * bytes '\xFE', '\xFF'.  Structural characters and line
> +         * endings are promising resynchronization points.  Clients
> +         * may use the others to force the JSON parser into known-good
> +         * state; see docs/interop/qmp-spec.txt.
> +         */
> +        [0 ... 0x1F] = IN_START | LOOKAHEAD,

And here's where the LOOKAHEAD bit has to be separate, because you are 
now assigning semantics to the transition on '\0' that are distinct from 
either of the two roles previously enumerated as possible.

> +        [0x20 ... 0xFD] = IN_RECOVERY,
> +        [0xFE ... 0xFF] = IN_START | LOOKAHEAD,
> +        ['\t'] = IN_RECOVERY,
> +        ['['] = IN_START | LOOKAHEAD,
> +        [']'] = IN_START | LOOKAHEAD,
> +        ['{'] = IN_START | LOOKAHEAD,
> +        ['}'] = IN_START | LOOKAHEAD,
> +        [':'] = IN_START | LOOKAHEAD,
> +        [','] = IN_START | LOOKAHEAD,
> +    },

It took me a couple of reads before I was satisfied that everything is 
initialized as desired (range assignments followed by more-specific 
re-assignment works, but isn't common), but this looks right.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org