#3 - DNS support + IPv6

modules/io/include/ResolutionError.hpp

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <cerrno>
+#include <cstring>
+#include <netdb.h>
+#include <string>
+
+namespace tw::net {
+
+/**
+ * A failed host lookup.
+ *
+ * Kept apart from NetworkError because getaddrinfo reports EAI_ codes, which
+ * are their own mostly-negative space: sharing one enum would map a lookup
+ * failure onto whichever errno happened to carry the same number.
+ */
+struct ResolutionError {
+    int m_code;
+    int m_errno;
+
+public:
+    /**
+     * Only EAI_SYSTEM defers to errno, and errno will not have survived by the
+     * time message() runs, so it is captured here.
+     */
+    static ResolutionError from_gai(int code) {
+        return { code, errno };
+    }
+
+    std::string message() const {
+        switch (m_code) {
+            case EAI_NONAME:
+                return "The host name is not known.";
+            case EAI_AGAIN:
+                return "The name server is unreachable or busy; the lookup may succeed later.";
+            case EAI_FAIL:
+                return "The name server returned a permanent failure.";
+            case EAI_FAMILY:
+                return "The requested address family is not supported.";
+            case EAI_SERVICE:
+                return "The requested port is not available for this socket type.";
+            case EAI_MEMORY:
+                return "Insufficient memory was available to complete the lookup.";
+            case EAI_SYSTEM:
+                return std::string(strerror(m_errno));
+            default:
+                return std::string(gai_strerror(m_code));
+        }
+    }
+};
+
+}

modules/io/include/io/HostResolver.hpp

@@ -0,0 +1,57 @@
+#pragma once
+
+#include "ResolutionError.hpp"
+#include "tl/expected.hpp"
+
+#include <cstring>
+#include <netdb.h>
+#include <string>
+#include <sys/socket.h>
+
+namespace tw::net {
+
+/**
+ * Turns a host name or an address literal into a socket address.
+ *
+ * `family` is the family of the socket the result will be given to. AF_INET6
+ * asks for IPv4-only names as ::ffff: mapped addresses, so that one dual-stack
+ * socket reaches both; AF_UNSPEC takes the name as it comes and suits addresses
+ * that are only being validated, displayed or stored.
+ *
+ * Blocks for the length of a DNS round trip when the name is not already known,
+ * so it belongs at connect time rather than anywhere periodic.
+ */
+inline tl::expected<sockaddr_storage, ResolutionError>
+resolve_host(const std::string& host, int port, sa_family_t family = AF_UNSPEC) {
+    addrinfo hints {};
+    hints.ai_family   = family;
+    hints.ai_socktype = SOCK_DGRAM;
+
+    // AI_ADDRCONFIG is deliberately absent. Together with AF_INET6 it discards
+    // every result on a host that carries no global IPv6 address, which is the
+    // default state of a container on a bridge network.
+    if(family == AF_INET6) {
+        hints.ai_flags = AI_V4MAPPED | AI_ALL;
+    }
+
+    // Passing the port as the service spares us setting sin_port or sin6_port
+    // by hand once the family of the answer is known.
+    const std::string service = std::to_string(port);
+
+    addrinfo* results = nullptr;
+    const int rc = ::getaddrinfo(host.c_str(), service.c_str(), &hints, &results);
+    if(rc != 0) {
+        return tl::make_unexpected(ResolutionError::from_gai(rc));
+    }
+
+    // The list arrives ordered by RFC 6724, so the head is the address the
+    // system would have picked for itself.
+    sockaddr_storage storage {};
+    std::memcpy(&storage, results->ai_addr, results->ai_addrlen);
+
+    ::freeaddrinfo(results);
+
+    return storage;
+}
+
+}