util/vizerror: add As function to get wrapped Error

Signed-off-by: Will Norris <will@tailscale.com>

tsweb/tsweb.go

@@ -266,11 +266,10 @@ func (h retHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	err := h.rh.ServeHTTPReturn(lw, r)
 
 	var hErr HTTPError
-	var vizErr vizerror.Error
 	var hErrOK bool
 	if errors.As(err, &hErr) {
 		hErrOK = true
-	} else if errors.As(err, &vizErr) {
+	} else if vizErr, ok := vizerror.As(err); ok {
 		hErrOK = true
 		hErr = HTTPError{Msg: vizErr.Error()}
 	}

util/vizerror/vizerror.go

@@ -42,3 +42,9 @@ func Wrap(err error) error {
 	}
 	return Error{err}
 }
+
+// As returns the first vizerror.Error in err's chain.
+func As(err error) (e Error, ok bool) {
+	ok = errors.As(err, &e)
+	return
+}

util/vizerror/vizerror_test.go

@@ -5,6 +5,7 @@ package vizerror
 
 import (
 	"errors"
+	"fmt"
 	"io/fs"
 	"testing"
 )
@@ -28,3 +29,16 @@ func TestErrorf(t *testing.T) {
 		t.Errorf("error chain does not contain fs.ErrNotExist")
 	}
 }
+
+func TestAs(t *testing.T) {
+	verr := New("visible error")
+	err := fmt.Errorf("wrap: %w", verr)
+
+	got, ok := As(err)
+	if !ok {
+		t.Errorf("As() return false, want true")
+	}
+	if got != verr {
+		t.Errorf("As() returned error %v, want %v", got, verr)
+	}
+}