#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));
+        }
+    }
+};
+
+}