text/template: wrap errors returned by template functions instead of stringifying them

Fixes #34201

Change-Id: Ic2e2967e4b01167345cf38bd006cabb206a64377
GitHub-Last-Rev: 5d0c4856550614484a8dbfb68c37aa1abcfcc529
GitHub-Pull-Request: golang/go#42398
Reviewed-on: https://go-review.googlesource.com/c/go/+/267838
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Pontus Leitzler <leitzler@gmail.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>

3 files changed, 27 insertions, 2 deletions

diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 4637b2035f..f1305a29a0 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -727,7 +727,7 @@ func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, a
 	// error to the caller.
 	if err != nil {
 		s.at(node)
-		s.errorf("error calling %s: %v", name, err)
+		s.errorf("error calling %s: %w", name, err)
 	}
 	if v.Type() == reflectValueType {
 		v = v.Interface().(reflect.Value)
diff --git a/src/text/template/exec_test.go b/src/text/template/exec_test.go
index 1a129ed5af..255b111b34 100644
--- a/src/text/template/exec_test.go
+++ b/src/text/template/exec_test.go
@@ -902,6 +902,28 @@ func TestExecError(t *testing.T) {
 	}
 }
 
+type CustomError struct{}
+
+func (*CustomError) Error() string { return "heyo !" }
+
+// Check that a custom error can be returned.
+func TestExecError_CustomError(t *testing.T) {
+	failingFunc := func() (string, error) {
+		return "", &CustomError{}
+	}
+	tmpl := Must(New("top").Funcs(FuncMap{
+		"err": failingFunc,
+	}).Parse("{{ err }}"))
+
+	var b bytes.Buffer
+	err := tmpl.Execute(&b, nil)
+
+	var e *CustomError
+	if !errors.As(err, &e) {
+		t.Fatalf("expected custom error; got %s", err)
+	}
+}
+
 func TestJSEscaping(t *testing.T) {
 	testCases := []struct {
 		in, exp string
diff --git a/src/text/template/funcs.go b/src/text/template/funcs.go
index 1b6940a84a..9dd332c068 100644
--- a/src/text/template/funcs.go
+++ b/src/text/template/funcs.go
@@ -23,6 +23,9 @@ import (
 // return value evaluates to non-nil during execution, execution terminates and
 // Execute returns that error.
 //
+// Errors returned by Execute wrap the underlying error; call errors.As to
+// uncover them.
+//
 // When template execution invokes a function with an argument list, that list
 // must be assignable to the function's parameter types. Functions meant to
 // apply to arguments of arbitrary type can use parameters of type interface{} or
@@ -344,7 +347,7 @@ func call(fn reflect.Value, args ...reflect.Value) (reflect.Value, error) {
 
 		var err error
 		if argv[i], err = prepareArg(arg, argType); err != nil {
-			return reflect.Value{}, fmt.Errorf("arg %d: %s", i, err)
+			return reflect.Value{}, fmt.Errorf("arg %d: %w", i, err)
 		}
 	}
 	return safeCall(fn, argv)