net/Resolver: throw std::system_error with a new error_category

Author: MaxKellermann

src/net/Resolver.cxx

@@ -6,6 +6,7 @@
 #include "AddressInfo.hxx"
 #include "HostParser.hxx"
 #include "lib/fmt/RuntimeError.hxx"
+#include "lib/fmt/ToBuffer.hxx"
 #include "util/CharUtil.hxx"
 #include "util/StringAPI.hxx"
 
@@ -21,23 +22,35 @@
 
 #include <stdio.h>
 
+ResolverErrorCategory resolver_error_category;
+
+std::string
+ResolverErrorCategory::message(int condition) const
+{
+#ifdef _WIN32
+	return gai_strerrorA(condition);
+#else
+	return gai_strerror(condition);
+#endif
+}
+
+static std::system_error
+MakeResolverError(int error, const char *msg) noexcept
+{
+	return std::system_error{error, resolver_error_category, msg};
+}
+
 AddressInfoList
 Resolve(const char *node, const char *service,
 	const struct addrinfo *hints)
 {
 	struct addrinfo *ai;
 	int error = getaddrinfo(node, service, hints, &ai);
-	if (error != 0) {
-#ifdef _WIN32
-		const char *msg = gai_strerrorA(error);
-#else
-		const char *msg = gai_strerror(error);
-#endif
-		throw FmtRuntimeError("Failed to resolve {:?}:{:?}: {}",
-				      node == nullptr ? "" : node,
-				      service == nullptr ? "" : service,
-				      msg);
-	}
+	if (error != 0)
+		throw MakeResolverError(error,
+					FmtBuffer<512>("Failed to resolve {:?}:{:?}",
+						       node == nullptr ? "" : node,
+						       service == nullptr ? "" : service).c_str());
 
 	return AddressInfoList(ai);
 }

src/net/Resolver.hxx

@@ -4,11 +4,27 @@
 
 #pragma once
 
+#include <system_error>
+
 class AddressInfoList;
 
+class ResolverErrorCategory final : public std::error_category {
+public:
+	const char *name() const noexcept override {
+		return "gai";
+	}
+
+	std::string message(int condition) const override;
+};
+
+extern ResolverErrorCategory resolver_error_category;
+
 /**
  * Thin wrapper for getaddrinfo() which throws on error and returns a
  * RAII object.
+ *
+ * getaddrinfo() errors are thrown as std::system_error with
+ * #resolver_error_category.
  */
 AddressInfoList
 Resolve(const char *node, const char *service,
@@ -21,7 +37,8 @@ Resolve(const char *node, const char *service,
  * This is a wrapper for getaddrinfo() and it does not support local
  * sockets.
  *
- * Throws on error.
+ * Throws on error.  Resolver errors are thrown as std::system_error
+ * with #resolver_error_category.
  */
 AddressInfoList
 Resolve(const char *host_and_port, int default_port,