Re: WIP Incremental JSON Parser

Some more observations as I make my way through the patch:

In src/common/jsonapi.c,

> +#define JSON_NUM_NONTERMINALS 6

Should this be 5 now?

> +       res = pg_parse_json_incremental(&(incstate->lex), &(incstate->sem),
> +                                                                       chunk, size, is_last);
> +
> +       expected = is_last ? JSON_SUCCESS : JSON_INCOMPLETE;
> +
> +       if (res != expected)
> +               json_manifest_parse_failure(context, "parsing failed");

This leads to error messages like

    pg_verifybackup: error: could not parse backup manifest: parsing failed

which I would imagine is going to lead to confused support requests in
the event that someone does manage to corrupt their manifest. Can we
make use of json_errdetail() and print the line and column numbers?
Patch 0001 over at [1] has one approach to making json_errdetail()
workable in frontend code.

Top-level scalars like `false` or `12345` do not parse correctly if
the chunk size is too small; instead json_errdetail() reports 'Token
"" is invalid'. With small chunk sizes, json_errdetail() additionally
segfaults on constructions like `[tru]` or `12zz`.

For my local testing, I'm carrying the following diff in
001_test_json_parser_incremental.pl:

> - ok($stdout =~ /SUCCESS/, "chunk size $size: test succeeds");
> - ok(!$stderr, "chunk size $size: no error output");
> + like($stdout, qr/SUCCESS/, "chunk size $size: test succeeds");
> + is($stderr, "", "chunk size $size: no error output");

This is particularly helpful when a test fails spuriously due to code
coverage spray on stderr.

Thanks,
--Jacob

[1] https://www.postgresql.org/message-id/CAOYmi%2BmSSY4SvOtVN7zLyUCQ4-RDkxkzmTuPEN%2Bt-PsB7GHnZA%40mail.gmail.com