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Recover from parse errors when parse_error() returns true (#3989)

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The return value of json_sax::parse_error() was documented both as "must
return false" and as "whether the parsing should continue", and the code
just passed it on. For JSON text, parsing stopped anyway, but sax_parse()
could report success for invalid input. The binary readers read on after
the error, looping forever on a CBOR indefinite-length array without its
end.

Now false stops parsing, and true recovers from the error:

- JSON text is repaired with the smallest local edit (insert a missing
  ',' or ':', remove a stray token, keep the readable part of a broken
  string or number, null for a value that cannot be read, close the
  innermost container at a wrong closing bracket and all of them at the
  end of the input), and parsing continues. The SAX events stay balanced,
  every key is followed by exactly one value, and each token is reported
  at most once.
- The binary formats cannot resynchronize, so they stop, but complete
  the value read so far.

sax_parse() returns false after any error. parse(), accept(), and the
from_*() functions never recover and compile to the same code as before.

Supersedes #4522.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-27 20:16:42 +02:00
parent 509c07041f
commit 9adb510a0d
21 changed files with 4242 additions and 285 deletions
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include/nlohmann/json.hpp
+23 -3
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@@ -4979,6 +4979,26 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
return parser(i.get(), nullptr, false, ignore_comments, ignore_trailing_commas, true).accept(true);
}
private:
/// read a binary format and pass it to a SAX parser; if the SAX parser
/// asks to recover from an error, the value read so far is completed
/// (see detail::json_sax_salvager and #3989)
template<typename InputAdapterType, typename SAX>
static bool sax_parse_binary(InputAdapterType ia, SAX* sax,
const input_format_t format, const bool strict)
{
(void)detail::is_sax_static_asserts<SAX, basic_json> {};
using salvager_t = detail::json_sax_salvager<basic_json, SAX>;
salvager_t salvager(sax);
const bool result = detail::binary_reader<basic_json, InputAdapterType, salvager_t>(std::move(ia), format).sax_parse(format, &salvager, strict);
if (!result)
{
salvager.close_open_containers();
}
return result;
}
public:
/// @brief generate SAX events
/// @sa https://json.nlohmann.me/api/basic_json/sax_parse/
template <typename InputType, typename SAX>
@@ -4992,7 +5012,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
auto ia = detail::input_adapter(std::forward<InputType>(i));
return format == input_format_t::json
? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
: detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
: sax_parse_binary(std::move(ia), sax, format, strict);
}
/// @brief generate SAX events (iterator pair, or iterator+sentinel pair for C++20 ranges support)
@@ -5009,7 +5029,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
auto ia = detail::input_adapter(std::move(first), std::move(last));
return format == input_format_t::json
? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
: detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
: sax_parse_binary(std::move(ia), sax, format, strict);
}
/// @brief generate SAX events
@@ -5031,7 +5051,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
// NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
: detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
: sax_parse_binary(std::move(ia), sax, format, strict);
}
#ifndef JSON_NO_IO
/// @brief deserialize from stream