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_cgo_test.go

@@ -0,0 +1,89 @@
+//go:build: cgo
+
+package netxlite
+
+import (
+	"context"
+	"errors"
+	"net"
+	"testing"
+	"time"
+)
+
+func TestGetaddrinfoStateAddrinfoToStringWithInvalidFamily(t *testing.T) {
+	aip := staticAddrinfoWithInvalidFamily()
+	state := newGetaddrinfoState(getaddrinfoNumSlots)
+	addr, err := state.addrinfoToString(aip)
+	if !errors.Is(err, errGetaddrinfoUnknownFamily) {
+		t.Fatal("unexpected err", err)
+	}
+	if addr != "" {
+		t.Fatal("expected empty addr here")
+	}
+}
+
+func TestGetaddrinfoStateIfnametoindex(t *testing.T) {
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		t.Fatal(err)
+	}
+	state := newGetaddrinfoState(getaddrinfoNumSlots)
+	for _, iface := range ifaces {
+		name := state.ifnametoindex(iface.Index)
+		if name != iface.Name {
+			t.Fatal("unexpected name")
+		}
+	}
+}
+
+func TestGetaddrinfoStateLookupANYWithNoSlots(t *testing.T) {
+	const (
+		noslots = 0
+		timeout = 10 * time.Millisecond
+	)
+	state := newGetaddrinfoState(noslots)
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
+	defer cancel()
+	addresses, cname, err := state.LookupANY(ctx, "dns.google")
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatal("unexpected err", err)
+	}
+	if len(addresses) > 0 {
+		t.Fatal("expected no addresses", addresses)
+	}
+	if cname != "" {
+		t.Fatal("expected empty cname", cname)
+	}
+}
+
+func TestGetaddrinfoStateToAddressList(t *testing.T) {
+	t.Run("with invalid sockety type", func(t *testing.T) {
+		state := newGetaddrinfoState(0) // number of slots not relevant
+		aip := staticAddrinfoWithInvalidSocketType()
+		addresses, cname, err := state.toAddressList(aip)
+		if !errors.Is(err, ErrOODNSNoAnswer) {
+			t.Fatal("unexpected err", err)
+		}
+		if len(addresses) > 0 {
+			t.Fatal("expected no addresses", addresses)
+		}
+		if cname != "" {
+			t.Fatal("expected empty cname", cname)
+		}
+	})
+
+	t.Run("with invalid family", func(t *testing.T) {
+		state := newGetaddrinfoState(0) // number of slots not relevant
+		aip := staticAddrinfoWithInvalidFamily()
+		addresses, cname, err := state.toAddressList(aip)
+		if !errors.Is(err, ErrOODNSNoAnswer) {
+			t.Fatal("unexpected err", err)
+		}
+		if len(addresses) > 0 {
+			t.Fatal("expected no addresses", addresses)
+		}
+		if cname != "" {
+			t.Fatal("expected empty cname", cname)
+		}
+	})
+}