From 598ec463bcaac6f8d9a9f2e34d0f6fcf1582b1e8 Mon Sep 17 00:00:00 2001
From: Will Norris <will@tailscale.com>
Date: Tue, 31 Jan 2023 16:01:02 -0800
Subject: [PATCH] fixup! util/vizerror: add new package for visible errors

Signed-off-by: Will Norris <will@tailscale.com>
---
 tsweb/tsweb.go            | 12 +++++++++++-
 tsweb/tsweb_test.go       | 18 ++++++++++++++++++
 util/vizerror/vizerror.go | 31 ++++++++++++++++++++++++-------
 3 files changed, 53 insertions(+), 8 deletions(-)

diff --git a/tsweb/tsweb.go b/tsweb/tsweb.go
index 21ea23cb6..4f80f915d 100644
--- a/tsweb/tsweb.go
+++ b/tsweb/tsweb.go
@@ -32,6 +32,7 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/strs"
+	"tailscale.com/util/vizerror"
 	"tailscale.com/version"
 )
 
@@ -263,7 +264,16 @@ func (h retHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	lw := &loggingResponseWriter{ResponseWriter: w, logf: h.opts.Logf}
 	err := h.rh.ServeHTTPReturn(lw, r)
-	hErr, hErrOK := err.(HTTPError)
+
+	var hErr HTTPError
+	var vizErr vizerror.Error
+	var hErrOK bool
+	if errors.As(err, &hErr) {
+		hErrOK = true
+	} else if errors.As(err, &vizErr) {
+		hErrOK = true
+		hErr = HTTPError{Msg: vizErr.Error()}
+	}
 
 	if lw.code == 0 && err == nil && !lw.hijacked {
 		// If the handler didn't write and didn't send a header, that still means 200.
diff --git a/tsweb/tsweb_test.go b/tsweb/tsweb_test.go
index 132e35052..c19c2c5bf 100644
--- a/tsweb/tsweb_test.go
+++ b/tsweb/tsweb_test.go
@@ -21,6 +21,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"tailscale.com/metrics"
 	"tailscale.com/tstest"
+	"tailscale.com/util/vizerror"
 	"tailscale.com/version"
 )
 
@@ -151,6 +152,23 @@ func TestStdHandler(t *testing.T) {
 			},
 		},
 
+		{
+			name:     "handler returns user visible error",
+			rh:       handlerErr(0, vizerror.New("visible error")),
+			r:        req(bgCtx, "http://example.com/foo"),
+			wantCode: 500,
+			wantLog: AccessLogRecord{
+				When:       clock.Start,
+				Seconds:    1.0,
+				Proto:      "HTTP/1.1",
+				Host:       "example.com",
+				Method:     "GET",
+				RequestURI: "/foo",
+				Err:        "visible error",
+				Code:       500,
+			},
+		},
+
 		{
 			name:     "handler returns generic error",
 			rh:       handlerErr(0, testErr),
diff --git a/util/vizerror/vizerror.go b/util/vizerror/vizerror.go
index 2e0713847..bc3f8e90c 100644
--- a/util/vizerror/vizerror.go
+++ b/util/vizerror/vizerror.go
@@ -11,14 +11,31 @@ import (
 )
 
 // Error is an error that is safe to display to end users.
-type Error error
+type Error struct {
+	err error
+}
+
+// Error implements the error interface.
+func (e Error) Error() string {
+	return e.err.Error()
+}
+
+// New returns an Error that formats as the given text.
+func New(text string) Error {
+	return Error{errors.New(text)}
+}
+
+// Errorf returns an Error with the specified format and values.
+func Errorf(format string, a ...any) Error {
+	return Error{fmt.Errorf(format, a...)}
+}
 
-// New returns an error that formats as the given text.
-func New(text string) error {
-	return Error(errors.New(text))
+// Unwrap returns the underlying error.
+func (e Error) Unwrap() error {
+	return e.err
 }
 
-// Errorf returns an error with the specified format and values.
-func Errorf(format string, a ...any) error {
-	return Error(fmt.Errorf(format, a...))
+// Wrap err with a vizerror.Error.
+func Wrap(err error) Error {
+	return Error{err}
 }