refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/serialresolver.go

@@ -0,0 +1,118 @@
+package netxlite
+
+import (
+	"context"
+	"errors"
+	"net"
+
+	"github.com/miekg/dns"
+	"github.com/ooni/probe-cli/v3/internal/atomicx"
+)
+
+// SerialResolver is a resolver that first issues an A query and then
+// issues an AAAA query for the requested domain.
+type SerialResolver struct {
+	Encoder     DNSEncoder
+	Decoder     DNSDecoder
+	NumTimeouts *atomicx.Int64
+	Txp         DNSTransport
+}
+
+// NewSerialResolver creates a new OONI Resolver instance.
+func NewSerialResolver(t DNSTransport) *SerialResolver {
+	return &SerialResolver{
+		Encoder:     &DNSEncoderMiekg{},
+		Decoder:     &DNSDecoderMiekg{},
+		NumTimeouts: &atomicx.Int64{},
+		Txp:         t,
+	}
+}
+
+// Transport returns the transport being used.
+func (r *SerialResolver) Transport() DNSTransport {
+	return r.Txp
+}
+
+// Network implements Resolver.Network
+func (r *SerialResolver) Network() string {
+	return r.Txp.Network()
+}
+
+// Address implements Resolver.Address
+func (r *SerialResolver) Address() string {
+	return r.Txp.Address()
+}
+
+// CloseIdleConnections closes idle connections.
+func (r *SerialResolver) CloseIdleConnections() {
+	r.Txp.CloseIdleConnections()
+}
+
+// LookupHost implements Resolver.LookupHost.
+func (r *SerialResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	var addrs []string
+	addrsA, errA := r.lookupHostWithRetry(ctx, hostname, dns.TypeA)
+	addrsAAAA, errAAAA := r.lookupHostWithRetry(ctx, hostname, dns.TypeAAAA)
+	if errA != nil && errAAAA != nil {
+		return nil, errA
+	}
+	addrs = append(addrs, addrsA...)
+	addrs = append(addrs, addrsAAAA...)
+	return addrs, nil
+}
+
+// LookupHTTPS implements Resolver.LookupHTTPS.
+func (r *SerialResolver) LookupHTTPS(
+	ctx context.Context, hostname string) (*HTTPSSvc, error) {
+	querydata, err := r.Encoder.Encode(
+		hostname, dns.TypeHTTPS, r.Txp.RequiresPadding())
+	if err != nil {
+		return nil, err
+	}
+	replydata, err := r.Txp.RoundTrip(ctx, querydata)
+	if err != nil {
+		return nil, err
+	}
+	return r.Decoder.DecodeHTTPS(replydata)
+}
+
+func (r *SerialResolver) lookupHostWithRetry(
+	ctx context.Context, hostname string, qtype uint16) ([]string, error) {
+	var errorslist []error
+	for i := 0; i < 3; i++ {
+		replies, err := r.lookupHostWithoutRetry(ctx, hostname, qtype)
+		if err == nil {
+			return replies, nil
+		}
+		errorslist = append(errorslist, err)
+		var operr *net.OpError
+		if !errors.As(err, &operr) || !operr.Timeout() {
+			// The first error is the one that is most likely to be caused
+			// by the network. Subsequent errors are more likely to be caused
+			// by context deadlines. So, the first error is attached to an
+			// operation, while subsequent errors may possibly not be. If
+			// so, the resulting failing operation is not correct.
+			break
+		}
+		r.NumTimeouts.Add(1)
+	}
+	// bugfix: we MUST return one of the errors otherwise we confuse the
+	// mechanism in errwrap that classifies the root cause operation, since
+	// it would not be able to find a child with a major operation error
+	return nil, errorslist[0]
+}
+
+// lookupHostWithoutRetry issues a lookup host query for the specified
+// qtype (dns.A or dns.AAAA) without retrying on failure.
+func (r *SerialResolver) lookupHostWithoutRetry(
+	ctx context.Context, hostname string, qtype uint16) ([]string, error) {
+	querydata, err := r.Encoder.Encode(hostname, qtype, r.Txp.RequiresPadding())
+	if err != nil {
+		return nil, err
+	}
+	replydata, err := r.Txp.RoundTrip(ctx, querydata)
+	if err != nil {
+		return nil, err
+	}
+	return r.Decoder.DecodeLookupHost(qtype, replydata)
+}