Repository: bitcoin/bitcoin
File: src/common/url.cpp (C++)

Code before:
// Copyright (c) 2015-2022 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <common/url.h>

#include <event2/http.h>

#include <cstdlib>
#include <string>

std::string urlDecode(const std::string &urlEncoded) {
    std::string res;
    if (!urlEncoded.empty()) {
        char *decoded = evhttp_uridecode(urlEncoded.c_str(), false, nullptr);
        if (decoded) {
            res = std::string(decoded);
            free(decoded);
        }
    }
    return res;
}


Review comment (suggestion):
it seems to me that `&& i + 2 < url_encoded.size()` part isn't checked by the tests, consider adding a few more corner cases, e.g. this is what I tested it with:
```C++
BOOST_AUTO_TEST_CASE(decode_malformed_test) {
    BOOST_CHECK_EQUAL(urlDecode("%%xhello th+ere \xff"), "%%xhello th+ere \xff");

    BOOST_CHECK_EQUAL(urlDecode("%"), "%");
    BOOST_CHECK_EQUAL(urlDecode("%%"), "%%");
    BOOST_CHECK_EQUAL(urlDecode("%%%"), "%%%");
    BOOST_CHECK_EQUAL(urlDecode("%%%%"), "%%%%");

    BOOST_CHECK_EQUAL(urlDecode("+"), "+");
    BOOST_CHECK_EQUAL(urlDecode("++"), "++");

    BOOST_CHECK_EQUAL(urlDecode("?"), "?");
    BOOST_CHECK_EQUAL(urlDecode("??"), "??");

    BOOST_CHECK_EQUAL(urlDecode("%G1"), "%G1");
    BOOST_CHECK_EQUAL(urlDecode("%2"), "%2");
    BOOST_CHECK_EQUAL(urlDecode("%ZX"), "%ZX");

    BOOST_CHECK_EQUAL(urlDecode("valid%20string%G1"), "valid string%G1");
    BOOST_CHECK_EQUAL(urlDecode("%20invalid%ZX"), " invalid%ZX");
    BOOST_CHECK_EQUAL(urlDecode("%20%G1%ZX"), " %G1%ZX");
}
```

Since the original implementation seems to include a case for handling `?` and `+` as well, I've added tests to make sure they're kept.

Code after:
// Copyright (c) 2015-2022 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <common/url.h>

#include <charconv>
#include <string>
#include <string_view>
#include <system_error>

std::string UrlDecode(std::string_view url_encoded)
{
    std::string res;
    res.reserve(url_encoded.size());

    for (size_t i = 0; i < url_encoded.size(); ++i) {
        char c = url_encoded[i];
        // Special handling for percent which should be followed by two hex digits
        // representing an octet values, see RFC 3986, Section 2.1 Percent-Encoding
        if (c == '%' && i + 2 < url_encoded.size()) {
            unsigned int decoded_value{0};
            auto [p, ec] = std::from_chars(url_encoded.data() + i + 1, url_encoded.data() + i + 3, decoded_value, 16);

            // Only if there is no error and the pointer is set to the end of
            // the string, we can be sure both characters were valid hex
            if (ec == std::errc{} && p == url_encoded.data() + i + 3) {
                res += static_cast<char>(decoded_value);
                // Next two characters are part of the percent encoding
                i += 2;
                continue;
            }
            // In case of invalid percent encoding, add the '%' and continue
        }
        res += c;
    }

    return res;
}