Re: JSON/YAML/TOML/etc. parsing performance

[Paul Eggert <eggert@cs.ucla.edu>]

On 10/03/2017 05:26 AM, Philipp Stephani wrote:

> > +#if __has_attribute (warn_unused_result)
> > +# define ATTRIBUTE_WARN_UNUSED_RESULT __attribute__ 
> ((__warn_unused_result__))
> > +#else
> > +# define ATTRIBUTE_WARN_UNUSED_RESULT
> > +#endif
>
> Hmm... why do we need this attribute?  You use it with 2 static
> functions, so this sounds like a left-over from the development stage?
>
> It's not strictly needed (and if you don't like it, I can remove it), 
> but it helps catching memory leaks.

I've found __warn_unused_result__ to be more trouble than it's worth in 
library functions. Emacs has lib/ignore-value.h in order to work around 
__warn_unused_result__ brain damage, for example. For static functions 
the problem is less, but still, I mildly favor leaving it out.

>
>     And btw, how can size be greater than SIZE_MAX in this case? This is
>     a valid Lisp object, isn't it?  (There are more such tests in the
>     patch, e.g. in lisp_to_json, and I think they, too, are redundant.)
>
>
> Depends on the range of ptrdiff_t and size_t. IIUC nothing in the C 
> standard guarantees PTRDIFF_MAX <= SIZE_MAX. If we want to guarantee 
> that, we should probably add at least a static assertion.

There should be no need for that. No Lisp object can exceed min 
(PTRDIFF_MAX, SIZE_MAX) bytes; alloc.c guarantees this, so that Emacs 
should work even on oddball platforms where SIZE_MAX < PTRDIFF_MAX, and 
there is no need for a runtime check here.

The main practical problem here, by the way, is objects whose sizes 
exceed PTRDIFF_MAX on mainstream 32-bit platforms. This Does Not Work 
because you cannot subtract pointers within such objects reliably, 
sometimes even when the true difference is representable as ptrdiff_t! 
This is the main reason alloc.c prohibits creating such large objects, 
and that Emacs should reject any attempt to support such objects (even 
non-Lisp objects).


> > +  if (buffer_and_size->size > PTRDIFF_MAX)
> > +    xsignal1 (Qoverflow_error, build_string ("buffer too large"));
> > +  insert (buffer_and_size->buffer, buffer_and_size->size);
>
> I don't think we need this test here, as 'insert' already has the
> equivalent test in one of its subroutines.
>
> It can't, because it takes the byte length as ptrdiff_t. We need to 
> check before whether the size is actually in the valid range of ptrdiff_t.

A PTRDIFF_MAX test is needed if the JSON library can return strings 
longer than PTRDIFF_MAX. Please just use buffer_overflow () to signal 
the error, though.

> > +    case JSON_INTEGER:
> > +      {
> > +        json_int_t value = json_integer_value (json);
> > +        if (FIXNUM_OVERFLOW_P (value))
> > +          xsignal1 (Qoverflow_error,
> > +                    build_string ("JSON integer is too large"));
> > +        return make_number (value);
>
> This overflow test is also redundant, as make_number already does it.
>
> It can't, because json_int_t can be larger than EMACS_INT. Also, 
> make_number doesn't contain any checks.

You're right that a test is needed. However, elsewhere we just use 
xsignal0 (Qoverflow_error) for this sort of thing, and I suggest being 
consistent and doing that here as well. Similarly for other calls to 
xsignal1 with Qoverflow_error.

> > +    case JSON_STRING:
> > +      {
> > +        size_t size = json_string_length (json);
> > +        if (FIXNUM_OVERFLOW_P (size))
> > +          xsignal1 (Qoverflow_error, build_string ("JSON string is 
> too long"));
> > +        return json_make_string (json_string_value (json), size);
>
> Once again, the overflow test is redundant, as make_specified_string
> (called by json_make_string) already includes an equivalent test.
>
> And once again, we need to check at least whether the size fits into 
> ptrdiff_t.
You're right, a test is needed. However, I suggest using string_overflow 
() to signal string overflows.