feat(netxlite): implement parallel resolver (#724)

This diff imports the parallel resolver from websteps winter 2022
edition, which was originally implemented here:

bassosimone/websteps-illustrated@55231d7

See ooni/probe#2096

internal/netxlite/parallelresolver.go

@@ -0,0 +1,131 @@
+package netxlite
+
+//
+// Parallel resolver implementation
+//
+
+import (
+	"context"
+
+	"github.com/miekg/dns"
+	"github.com/ooni/probe-cli/v3/internal/atomicx"
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// ParallelResolver uses a transport and performs a LookupHost
+// operation in a parallel fashion, hence its name.
+//
+// You should probably use NewUnwrappedParallelResolver to
+// create a new instance of this type.
+type ParallelResolver struct {
+	// Encoder is the MANDATORY encoder to use.
+	Encoder model.DNSEncoder
+
+	// Decoder is the MANDATORY decoder to use.
+	Decoder model.DNSDecoder
+
+	// NumTimeouts is MANDATORY and counts the number of timeouts.
+	NumTimeouts *atomicx.Int64
+
+	// Txp is the MANDATORY underlying DNS transport.
+	Txp model.DNSTransport
+}
+
+// UnwrappedParallelResolver creates a new ParallelResolver instance. This instance is
+// not wrapped and you should wrap if before using it.
+func NewUnwrappedParallelResolver(t model.DNSTransport) *ParallelResolver {
+	return &ParallelResolver{
+		Encoder:     &DNSEncoderMiekg{},
+		Decoder:     &DNSDecoderMiekg{},
+		NumTimeouts: &atomicx.Int64{},
+		Txp:         t,
+	}
+}
+
+// Transport returns the transport being used.
+func (r *ParallelResolver) Transport() model.DNSTransport {
+	return r.Txp
+}
+
+// Network returns the "network" of the underlying transport.
+func (r *ParallelResolver) Network() string {
+	return r.Txp.Network()
+}
+
+// Address returns the "address" of the underlying transport.
+func (r *ParallelResolver) Address() string {
+	return r.Txp.Address()
+}
+
+// CloseIdleConnections closes idle connections, if any.
+func (r *ParallelResolver) CloseIdleConnections() {
+	r.Txp.CloseIdleConnections()
+}
+
+// LookupHost performs an A lookup in parallel with an AAAA lookup.
+func (r *ParallelResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	ach := make(chan *parallelResolverResult)
+	go r.lookupHost(ctx, hostname, dns.TypeA, ach)
+	aaaach := make(chan *parallelResolverResult)
+	go r.lookupHost(ctx, hostname, dns.TypeAAAA, aaaach)
+	ares := <-ach
+	aaaares := <-aaaach
+	if ares.err != nil && aaaares.err != nil {
+		// Note: we choose to return the A error because we assume that
+		// it's the more meaningful one: the AAAA error may just be telling
+		// us that there is no AAAA record for the website.
+		return nil, ares.err
+	}
+	var addrs []string
+	addrs = append(addrs, ares.addrs...)
+	addrs = append(addrs, aaaares.addrs...)
+	return addrs, nil
+}
+
+// LookupHTTPS implements Resolver.LookupHTTPS.
+func (r *ParallelResolver) LookupHTTPS(
+	ctx context.Context, hostname string) (*model.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)
+}
+
+// parallelResolverResult is the internal representation of a
+// lookup using either the A or the AAAA query type.
+type parallelResolverResult struct {
+	addrs []string
+	err   error
+}
+
+// lookupHost issues a lookup host query for the specified qtype (e.g., dns.A).
+func (r *ParallelResolver) lookupHost(ctx context.Context, hostname string,
+	qtype uint16, out chan<- *parallelResolverResult) {
+	querydata, err := r.Encoder.Encode(hostname, qtype, r.Txp.RequiresPadding())
+	if err != nil {
+		out <- &parallelResolverResult{
+			addrs: []string{},
+			err:   err,
+		}
+		return
+	}
+	replydata, err := r.Txp.RoundTrip(ctx, querydata)
+	if err != nil {
+		out <- &parallelResolverResult{
+			addrs: []string{},
+			err:   err,
+		}
+		return
+	}
+	addrs, err := r.Decoder.DecodeLookupHost(qtype, replydata)
+	out <- &parallelResolverResult{
+		addrs: addrs,
+		err:   err,
+	}
+}