src/cgi.cpp

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/*
MIT License

Copyright (c) 2026 Poul Sander

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

*/

#ifndef _GNU_SOURCE
#define _GNU_SOURCE // for execvpe
#endif

#include "cgi.h"

#include <cstdlib>
#include <cstring>
#include <stdexcept>

#include <fcntl.h>
#include <sys/wait.h>
#include <unistd.h>

namespace {
constexpr size_t CHUNK = 64 * 1024;

// Builds a null-terminated char* array from strings for execvp/environ. The
// pointers borrow from `strings`, which must outlive the returned vector.
std::vector<char *> to_c_array(const std::vector<std::string> &strings) {
    std::vector<char *> out;
    out.reserve(strings.size() + 1);
    for (const std::string &s : strings)
        out.push_back(const_cast<char *>(s.c_str()));
    out.push_back(nullptr);
    return out;
}

// Splits one CGI header line "Name: value" into a trimmed pair. Returns false
// for malformed lines (no colon).
bool parse_header_line(const std::string &line, std::string &name,
                       std::string &value) {
    const size_t colon = line.find(':');
    if (colon == std::string::npos)
        return false;
    name = line.substr(0, colon);
    size_t start = colon + 1;
    while (start < line.size() && (line[start] == ' ' || line[start] == '\t'))
        ++start;
    value = line.substr(start);
    return true;
}

// Parses the accumulated CGI header block (already stripped of the trailing
// blank line) into `result`. A "Status: NNN ..." header sets the HTTP status;
// every other header is preserved verbatim.
void parse_headers(const std::string &block, CgiResult &result) {
    size_t pos = 0;
    while (pos < block.size()) {
        size_t end = block.find('\n', pos);
        if (end == std::string::npos)
            end = block.size();
        std::string line = block.substr(pos, end - pos);
        pos = end + 1;
        if (!line.empty() && line.back() == '\r')
            line.pop_back();
        if (line.empty())
            continue;
        std::string name, value;
        if (!parse_header_line(line, name, value))
            continue;
        if (name == "Status")
            result.status = std::atoi(value.c_str());
        else
            result.headers.emplace_back(std::move(name), std::move(value));
    }
}
} // namespace

void run_cgi(const std::vector<std::string> &argv,
             const std::vector<std::string> &env, int input_fd,
             const std::function<std::ostream &(const CgiResult &)> &on_headers) {
    int out_pipe[2];
    if (pipe(out_pipe) != 0)
        throw std::runtime_error("cgi: pipe failed");

    const pid_t pid = fork();
    if (pid < 0) {
        close(out_pipe[0]);
        close(out_pipe[1]);
        throw std::runtime_error("cgi: fork failed");
    }

    if (pid == 0) {
        // Child: wire up stdin/stdout, then exec.
        const int in = input_fd >= 0 ? input_fd : open("/dev/null", O_RDONLY);
        if (in >= 0)
            dup2(in, STDIN_FILENO);
        dup2(out_pipe[1], STDOUT_FILENO);
        close(out_pipe[0]);
        close(out_pipe[1]);

        std::vector<char *> c_argv = to_c_array(argv);
        std::vector<char *> c_env = to_c_array(env);
        execvpe(c_argv[0], c_argv.data(), c_env.data());
        _exit(127); // exec failed
    }

    // Parent: read the child's stdout.
    close(out_pipe[1]);

    std::string header_block;
    std::ostream *body = nullptr; // set once headers are parsed
    char buf[CHUNK];
    ssize_t n;
    while ((n = read(out_pipe[0], buf, sizeof(buf))) > 0) {
        if (body != nullptr) {
            body->write(buf, n);
            continue;
        }
        header_block.append(buf, n);
        const size_t sep_crlf = header_block.find("\r\n\r\n");
        const size_t sep_lf = header_block.find("\n\n");
        size_t sep = std::string::npos;
        size_t skip = 0;
        if (sep_crlf != std::string::npos &&
            (sep_lf == std::string::npos || sep_crlf <= sep_lf)) {
            sep = sep_crlf;
            skip = 4;
        } else if (sep_lf != std::string::npos) {
            sep = sep_lf;
            skip = 2;
        }
        if (sep != std::string::npos) {
            const std::string rest = header_block.substr(sep + skip);
            header_block.resize(sep);
            CgiResult result;
            parse_headers(header_block, result);
            body = &on_headers(result);
            body->write(rest.data(), rest.size());
        }
    }
    close(out_pipe[0]);

    // No blank line ever arrived: still surface whatever headers we have.
    if (body == nullptr) {
        CgiResult result;
        parse_headers(header_block, result);
        on_headers(result);
    }

    int wstatus = 0;
    waitpid(pid, &wstatus, 0);
    if (!WIFEXITED(wstatus) || WEXITSTATUS(wstatus) != 0)
        throw std::runtime_error("cgi: child exited abnormally");
}