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module;
#ifndef _WIN32
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>
#include <poll.h>
#endif
export module mcpplibs.tinyhttps:platform;
import std;
// mcpplibs.tinyhttps:platform — the one place OS-specific networking quirks
// live. Everything else (socket.cppm, http.cppm…) stays portable and branches
// on `platform::is_windows` with `if constexpr`, never raw #ifdef.
//
// Sole concern today: hostname resolution on platforms where libc can't do it.
// A musl-static binary on Android/Termux can't resolve through getaddrinfo() —
// libc reads /etc/resolv.conf, but Android's /etc is read-only with no
// resolv.conf; Termux keeps its nameservers in $PREFIX/etc/resolv.conf, which
// libc never consults. So getaddrinfo stalls on a dead 127.0.0.1:53 and every
// HTTPS fetch (github, gitcode, anything) ends in "Connection failed". We read
// the relocatable resolv.conf ourselves and run a UDP DNS query. Self-contained:
// no shelling out to curl/getprop, no extra deps.
namespace mcpplibs::tinyhttps::platform {
// Compile-time platform flag for `if constexpr` at call sites.
export inline constexpr bool is_windows =
#ifdef _WIN32
true;
#else
false;
#endif
#ifndef _WIN32
namespace {
inline bool is_numeric_host(const char* host) {
// An IPv4/IPv6 literal is digits, dots, colons and hex letters only. This is
// a fast pre-filter; getaddrinfo(AI_NUMERICHOST) downstream is the real check.
if (host == nullptr || host[0] == '\0') return false;
for (const char* p = host; *p; ++p) {
char c = *p;
bool ok = (c >= '0' && c <= '9') || c == '.' || c == ':'
|| (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F');
if (!ok) return false;
}
return true;
}
// Nameservers from the first readable resolv.conf, $PREFIX (Termux) first.
inline std::vector<std::string> resolv_nameservers() {
std::vector<std::string> servers;
std::vector<std::string> paths;
if (const char* prefix = std::getenv("PREFIX"); prefix && *prefix) {
paths.emplace_back(std::string(prefix) + "/etc/resolv.conf");
}
paths.emplace_back("/data/data/com.termux/files/usr/etc/resolv.conf");
paths.emplace_back("/etc/resolv.conf");
for (const auto& path : paths) {
std::ifstream f(path);
if (!f) continue;
std::string line;
while (std::getline(f, line)) {
if (auto hash = line.find('#'); hash != std::string::npos) line.erase(hash);
std::istringstream is(line);
std::string kw, val;
if ((is >> kw >> val) && kw == "nameserver" && !val.empty()) {
servers.push_back(val);
}
}
if (!servers.empty()) break;
}
return servers;
}
// Query one nameserver for A records of `host`. Returns dotted-quad IPv4s.
inline std::vector<std::string> dns_query_a(const std::string& server, const char* host, int timeoutMs) {
std::vector<std::string> ips;
unsigned char q[512];
size_t n = 0;
q[n++] = 0x12; q[n++] = 0x34; // id
q[n++] = 0x01; q[n++] = 0x00; // flags: recursion desired
q[n++] = 0x00; q[n++] = 0x01; // qdcount = 1
q[n++] = 0x00; q[n++] = 0x00; // ancount
q[n++] = 0x00; q[n++] = 0x00; // nscount
q[n++] = 0x00; q[n++] = 0x00; // arcount
for (const char* p = host; *p; ) { // qname: length-prefixed labels
const char* dot = p;
while (*dot && *dot != '.') ++dot;
size_t len = static_cast<size_t>(dot - p);
if (len == 0 || len > 63 || n + len + 1 >= sizeof(q)) return ips;
q[n++] = static_cast<unsigned char>(len);
for (size_t i = 0; i < len; ++i) q[n++] = static_cast<unsigned char>(p[i]);
p = (*dot == '.') ? dot + 1 : dot;
}
q[n++] = 0x00; // root label
q[n++] = 0x00; q[n++] = 0x01; // qtype A
q[n++] = 0x00; q[n++] = 0x01; // qclass IN
struct addrinfo hints{};
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_NUMERICHOST;
struct addrinfo* sres = nullptr;
if (::getaddrinfo(server.c_str(), "53", &hints, &sres) != 0 || sres == nullptr) return ips;
int fd = ::socket(sres->ai_family, sres->ai_socktype, sres->ai_protocol);
if (fd < 0) { ::freeaddrinfo(sres); return ips; }
bool sent = ::sendto(fd, q, n, 0, sres->ai_addr, sres->ai_addrlen) == static_cast<ssize_t>(n);
::freeaddrinfo(sres);
if (!sent) { ::close(fd); return ips; }
struct pollfd pfd{};
pfd.fd = fd;
pfd.events = POLLIN;
if (::poll(&pfd, 1, timeoutMs) <= 0) { ::close(fd); return ips; }
unsigned char resp[1024];
ssize_t rn = ::recv(fd, resp, sizeof(resp), 0);
::close(fd);
if (rn < 12) return ips;
auto R = static_cast<size_t>(rn);
unsigned ancount = (static_cast<unsigned>(resp[6]) << 8) | resp[7];
size_t off = 12;
while (off < R && resp[off] != 0) off += resp[off] + 1; // question qname (no compression)
off += 1 + 4; // root label + qtype + qclass
for (unsigned a = 0; a < ancount && off + 12 <= R; ++a) {
if ((resp[off] & 0xc0) == 0xc0) { // compressed name pointer
off += 2;
} else {
while (off < R && resp[off] != 0) off += resp[off] + 1;
off += 1;
}
if (off + 10 > R) break;
unsigned type = (static_cast<unsigned>(resp[off]) << 8) | resp[off + 1];
unsigned rdlen = (static_cast<unsigned>(resp[off + 8]) << 8) | resp[off + 9];
off += 10;
if (off + rdlen > R) break;
if (type == 1 && rdlen == 4) {
ips.push_back(std::format("{}.{}.{}.{}",
resp[off], resp[off + 1], resp[off + 2], resp[off + 3]));
}
off += rdlen;
}
return ips;
}
} // anonymous namespace
#endif // !_WIN32
// True when libc's own resolver has a usable config (/etc/resolv.conf). When
// false, callers should prefer resolve_fallback() to avoid a multi-second stall
// on a dead 127.0.0.1:53. Always true on Windows (the OS resolver is reliable).
//
// Note: the bodies branch with #ifdef, not `if constexpr` — a non-template
// `if constexpr` still compiles its discarded branch, which would reference the
// POSIX-only helpers above that don't exist on Windows. Concentrating that
// preprocessor divergence here is exactly why this platform module exists; call
// sites elsewhere branch on `is_windows` with `if constexpr`.
export bool system_resolver_configured() {
#ifdef _WIN32
return true;
#else
std::error_code ec;
return std::filesystem::exists("/etc/resolv.conf", ec);
#endif
}
// Resolve a hostname to IPv4 literals WITHOUT relying on libc's resolver, by
// reading the system nameservers ($PREFIX/etc/resolv.conf on Termux) and doing
// a UDP DNS query. Returns {} on Windows, when no nameserver is configured, or
// on query failure. A numeric host is returned unchanged.
export std::vector<std::string> resolve_fallback([[maybe_unused]] const char* host,
[[maybe_unused]] int timeoutMs) {
#ifdef _WIN32
return {};
#else
if (is_numeric_host(host)) return { std::string(host) };
// Process-wide cache of successful resolutions. A single download performs
// several connects to the same host (HEAD probe, GET, redirect targets); on
// Termux each would otherwise re-run a UDP query to 8.8.8.8, and one dropped
// packet stalls the whole transfer. Cache once, reuse for the process.
static std::mutex cache_mutex;
static std::map<std::string, std::vector<std::string>> cache;
{
std::lock_guard<std::mutex> lock(cache_mutex);
if (auto it = cache.find(host); it != cache.end()) return it->second;
}
for (const auto& ns : resolv_nameservers()) {
auto got = dns_query_a(ns, host, timeoutMs);
if (!got.empty()) {
std::lock_guard<std::mutex> lock(cache_mutex);
cache.emplace(std::string(host), got);
return got;
}
}
return {};
#endif
}
} // namespace mcpplibs::tinyhttps::platform