funcr: Handle panic in Stringer, Marshaler, error

Produce a maybe-useful error rather than panic when we are in the middle
of logging a value (similar to fmt.Printf).

funcr/funcr.go

@@ -351,15 +351,15 @@ func (f Formatter) prettyWithFlags(value interface{}, flags uint32, depth int) s
 	if v, ok := value.(logr.Marshaler); ok {
 		// Replace the value with what the type wants to get logged.
 		// That then gets handled below via reflection.
-		value = v.MarshalLog()
+		value = invokeMarshaler(v)
 	}
 
 	// Handle types that want to format themselves.
 	switch v := value.(type) {
 	case fmt.Stringer:
-		value = v.String()
+		value = invokeStringer(v)
 	case error:
-		value = v.Error()
+		value = invokeError(v)
 	}
 
 	// Handling the most common types without reflect is a small perf win.
@@ -597,6 +597,33 @@ func isEmpty(v reflect.Value) bool {
 	return false
 }
 
+func invokeMarshaler(m logr.Marshaler) (ret interface{}) {
+	defer func() {
+		if r := recover(); r != nil {
+			ret = fmt.Sprintf("<panic: %s>", r)
+		}
+	}()
+	return m.MarshalLog()
+}
+
+func invokeStringer(s fmt.Stringer) (ret string) {
+	defer func() {
+		if r := recover(); r != nil {
+			ret = fmt.Sprintf("<panic: %s>", r)
+		}
+	}()
+	return s.String()
+}
+
+func invokeError(e error) (ret string) {
+	defer func() {
+		if r := recover(); r != nil {
+			ret = fmt.Sprintf("<panic: %s>", r)
+		}
+	}()
+	return e.Error()
+}
+
 // Caller represents the original call site for a log line, after considering
 // logr.Logger.WithCallDepth and logr.Logger.WithCallStackHelper.  The File and
 // Line fields will always be provided, while the Func field is optional.

funcr/funcr_test.go

@@ -52,7 +52,7 @@ func (p pointErr) MarshalText() ([]byte, error) {
 }
 
 // Logging this should result in the MarshalLog() value.
-type Tmarshaler string
+type Tmarshaler struct{ val string }
 
 func (t Tmarshaler) MarshalLog() interface{} {
 	return struct{ Inner string }{"I am a logr.Marshaler"}
@@ -66,8 +66,15 @@ func (t Tmarshaler) Error() string {
 	return "Error(): you should not see this"
 }
 
+// Logging this should result in a panic.
+type Tmarshalerpanic struct{ val string }
+
+func (t Tmarshalerpanic) MarshalLog() interface{} {
+	panic("Tmarshalerpanic")
+}
+
 // Logging this should result in the String() value.
-type Tstringer string
+type Tstringer struct{ val string }
 
 func (t Tstringer) String() string {
 	return "I am a fmt.Stringer"
@@ -77,6 +84,27 @@ func (t Tstringer) Error() string {
 	return "Error(): you should not see this"
 }
 
+// Logging this should result in a panic.
+type Tstringerpanic struct{ val string }
+
+func (t Tstringerpanic) String() string {
+	panic("Tstringerpanic")
+}
+
+// Logging this should result in the Error() value.
+type Terror struct{ val string }
+
+func (t Terror) Error() string {
+	return "I am an error"
+}
+
+// Logging this should result in a panic.
+type Terrorpanic struct{ val string }
+
+func (t Terrorpanic) Error() string {
+	panic("Terrorpanic")
+}
+
 type TjsontagsString struct {
 	String1 string `json:"string1"`           // renamed
 	String2 string `json:"-"`                 // ignored
@@ -351,16 +379,52 @@ func TestPretty(t *testing.T) {
 			},
 		},
 		{
-			val: Tmarshaler("foobar"),
+			val: Tmarshaler{"foobar"},
 			exp: `{"Inner":"I am a logr.Marshaler"}`,
 		},
 		{
-			val: Tstringer("foobar"),
+			val: &Tmarshaler{"foobar"},
+			exp: `{"Inner":"I am a logr.Marshaler"}`,
+		},
+		{
+			val: (*Tmarshaler)(nil),
+			exp: `"<panic: value method github.com/go-logr/logr/funcr.Tmarshaler.MarshalLog called using nil *Tmarshaler pointer>"`,
+		},
+		{
+			val: Tmarshalerpanic{"foobar"},
+			exp: `"<panic: Tmarshalerpanic>"`,
+		},
+		{
+			val: Tstringer{"foobar"},
+			exp: `"I am a fmt.Stringer"`,
+		},
+		{
+			val: &Tstringer{"foobar"},
 			exp: `"I am a fmt.Stringer"`,
 		},
 		{
-			val: fmt.Errorf("error"),
-			exp: `"error"`,
+			val: (*Tstringer)(nil),
+			exp: `"<panic: value method github.com/go-logr/logr/funcr.Tstringer.String called using nil *Tstringer pointer>"`,
+		},
+		{
+			val: Tstringerpanic{"foobar"},
+			exp: `"<panic: Tstringerpanic>"`,
+		},
+		{
+			val: Terror{"foobar"},
+			exp: `"I am an error"`,
+		},
+		{
+			val: &Terror{"foobar"},
+			exp: `"I am an error"`,
+		},
+		{
+			val: (*Terror)(nil),
+			exp: `"<panic: value method github.com/go-logr/logr/funcr.Terror.Error called using nil *Terror pointer>"`,
+		},
+		{
+			val: Terrorpanic{"foobar"},
+			exp: `"<panic: Terrorpanic>"`,
 		},
 		{
 			val: TjsontagsString{