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_test.go

@@ -0,0 +1,105 @@
+//go:build cgo && (darwin || dragonfly || freebsd || openbsd)
+
+package netxlite
+
+import (
+	"errors"
+	"syscall"
+	"testing"
+)
+
+func TestGetaddrinfoAIFlags(t *testing.T) {
+	var wrong bool
+	wrong = getaddrinfoAIFlags != (aiCanonname|aiV4Mapped|aiAll)&aiMask
+	if wrong {
+		t.Fatal("wrong flags for platform")
+	}
+}
+
+func TestGetaddrinfoStateToError(t *testing.T) {
+	type args struct {
+		code int64
+		err  error
+		goos string
+	}
+	type expects struct {
+		message string // message obtained using .Error
+		code    int64
+		err     error
+	}
+	var inputs = []struct {
+		name    string
+		args    args
+		expects expects
+	}{{
+		name: "with C.EAI_SYSTEM and non-nil error",
+		args: args{
+			code: eaiSystem,
+			err:  syscall.EAGAIN,
+			goos: "darwin",
+		},
+		expects: expects{
+			message: syscall.EAGAIN.Error(),
+			code:    eaiSystem,
+			err:     syscall.EAGAIN,
+		},
+	}, {
+		name: "with C.EAI_SYSTEM and nil error",
+		args: args{
+			code: eaiSystem,
+			err:  nil,
+			goos: "darwin",
+		},
+		expects: expects{
+			message: syscall.EMFILE.Error(),
+			code:    eaiSystem,
+			err:     syscall.EMFILE,
+		},
+	}, {
+		name: "with C.EAI_NONAME",
+		args: args{
+			code: eaiNoName,
+			err:  nil,
+			goos: "darwin",
+		},
+		expects: expects{
+			message: ErrOODNSNoSuchHost.Error(),
+			code:    eaiNoName,
+			err:     ErrOODNSNoSuchHost,
+		},
+	}, {
+		name: "with an unhandled error",
+		args: args{
+			code: eaiBadFlags,
+			err:  nil,
+			goos: "darwin",
+		},
+		expects: expects{
+			message: ErrOODNSMisbehaving.Error(),
+			code:    eaiBadFlags,
+			err:     ErrOODNSMisbehaving,
+		},
+	}}
+	for _, input := range inputs {
+		t.Run(input.name, func(t *testing.T) {
+			state := newGetaddrinfoState(getaddrinfoNumSlots)
+			err := state.toError(input.args.code, input.args.err, input.args.goos)
+			if err == nil {
+				t.Fatal("expected non-nil error here")
+			}
+			if err.Error() != input.expects.message {
+				t.Fatal("unexpected error message")
+			}
+			var gaierr *ErrGetaddrinfo
+			if !errors.As(err, &gaierr) {
+				t.Fatal("cannot convert error to ErrGetaddrinfo")
+			}
+			if gaierr.Code != input.expects.code {
+				t.Fatal("unexpected code")
+			}
+			if !errors.Is(gaierr.Underlying, input.expects.err) {
+				t.Fatal("unexpected underlying error")
+			}
+		})
+	}
+}