json_writer emits invalid JSON for string vectors (elements unquoted, last element unescaped)

[sims1253]

stan/src/stan/callbacks/json_writer.hpp

Line 361 in 8ad5c98

void write(const std::string& key, const std::vector<std::string>& values) {

write(key, const std::vector<std::string>&) writes elements without surrounding quotes, and the final write bypasses process_string

stan/src/stan/callbacks/json_writer.hpp

Line 376 in 8ad5c98

*output_ << values.back() << " ]";

stan::callbacks::json_writer<std::stringstream> w(std::move(owned));
w.begin_record();
w.write("quoted", std::vector<std::string>{"a\"b", "c\"d"});
w.end_record();
// output: "quoted" : [ a\"b, c"d ]

Gives invalid JSON for ordinary input. The final write also sits outside the if (values.size() > 0) guard the int overload has the same shape so an empty vector hits values.back() on an empty vector. The double overload keeps its final write inside the guard.

stan/src/stan/callbacks/json_writer.hpp

Lines 409 to 425 in 8ad5c98

	void write(const std::string& key, const std::vector<int>& values) {
	if (output_ == nullptr) {
	return;
	}
	write_sep();
	write_key(key);
	*output_ << "[ ";
	if (values.size() > 0) {
	auto last = values.end();
	--last;
	for (auto it = values.begin(); it != last; ++it) {
	*output_ << *it << ", ";
	}
	}
	*output_ << values.back() << " ]";
	}

stan/src/stan/callbacks/json_writer.hpp

Line 424 in 8ad5c98

*output_ << values.back() << " ]";

stan/src/stan/callbacks/json_writer.hpp

Lines 384 to 399 in 8ad5c98

	void write(const std::string& key, const std::vector<double>& values) {
	if (output_ == nullptr) {
	return;
	}
	write_sep();
	write_key(key);
	*output_ << "[ ";
	if (values.size() > 0) {
	auto last = values.end();
	--last;
	for (auto it = values.begin(); it != last; ++it) {
	write_value(*it);
	*output_ << ", ";
	}
	write_value(values.back());

Probably just needs to mirror the double overload and quote and escape string elements including the last.

Environment:

• Ubuntu 22.04.5 LTS (WSL2, kernel 6.18.33.2-microsoft-standard-WSL2), x86_64
• stan-dev/stan develop @ 8ad5c98
• g++ 11.4.0 (Ubuntu 22.04), -std=c++17

[WardBrian]

Because we are the only consumer of this type, I’m not sure if any of these bugs are actually reachable for users (I doubt it), but they all seem like reasonable fixes to apply

[sims1253]

Iirc this was one of the things that fell out of an agent wanting to debug something and accidentally running into this issue.