netxlite: call getaddrinfo and handle platform-specific oddities (#764)

This commit changes our system resolver to call getaddrinfo directly when CGO is enabled. This change allows us to:

1. obtain the CNAME easily

2. obtain the real getaddrinfo retval

3. handle platform specific oddities such as `EAI_NODATA`
returned on Android devices

See https://github.com/ooni/probe/issues/2029 and https://github.com/ooni/probe/issues/2029#issuecomment-1140258729 in particular.

See https://github.com/ooni/probe/issues/2033 for documentation regarding the desire to see `getaddrinfo`'s retval.

See https://github.com/ooni/probe/issues/2118 for possible follow-up changes.

internal/netxlite/getaddrinfo_bsd.go

@@ -0,0 +1,58 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build cgo && (darwin || dragonfly || freebsd || openbsd)
+
+package netxlite
+
+/*
+#include <netdb.h>
+*/
+import "C"
+import (
+	"syscall"
+)
+
+const getaddrinfoAIFlags = (C.AI_CANONNAME | C.AI_V4MAPPED | C.AI_ALL) & C.AI_MASK
+
+// Making constants available to Go code so we can run tests
+const (
+	aiCanonname = C.AI_CANONNAME
+	aiV4Mapped  = C.AI_V4MAPPED
+	aiAll       = C.AI_ALL
+	aiMask      = C.AI_MASK
+	eaiSystem   = C.EAI_SYSTEM
+	eaiNoName   = C.EAI_NONAME
+	eaiBadFlags = C.EAI_BADFLAGS
+)
+
+// toError is the function that converts the return value from
+// the getaddrinfo function into a proper Go error.
+//
+// This function is adapted from cgoLookupIPCNAME
+// https://github.com/golang/go/blob/go1.17.6/src/net/cgo_unix.go#L145
+//
+// SPDX-License-Identifier: BSD-3-Clause.
+func (state *getaddrinfoState) toError(code int64, err error, goos string) error {
+	switch code {
+	case C.EAI_SYSTEM:
+		if err == nil {
+			// err should not be nil, but sometimes getaddrinfo returns
+			// code == C.EAI_SYSTEM with err == nil on Linux.
+			// The report claims that it happens when we have too many
+			// open files, so use syscall.EMFILE (too many open files in system).
+			// Most system calls would return ENFILE (too many open files),
+			// so at the least EMFILE should be easy to recognize if this
+			// comes up again. golang.org/issue/6232.
+			err = syscall.EMFILE
+		}
+		return newErrGetaddrinfo(code, err)
+	case C.EAI_NONAME:
+		err = ErrOODNSNoSuchHost // so it becomes FailureDNSNXDOMAIN
+		return newErrGetaddrinfo(code, err)
+	default:
+		err = ErrOODNSMisbehaving // so it becomes FailureDNSServerMisbehaving
+		return newErrGetaddrinfo(code, err)
+	}
+}