Update Stringer panic check to look like stdlib (#857)

There's no behaviour changes, but there are a couple of refactorings: * Name the named return error `retErr`, and use explicit return values. The only purpose of the named return is for the panic handling. * Make the panic handling look more similar to the standard library and add a reference to the stdlib code in fmt that does the same checks.

Author: prashantv

zapcore/field.go

@@ -205,18 +205,23 @@ func addFields(enc ObjectEncoder, fields []Field) {
 }
 }
 
-func encodeStringer(key string, stringer interface{}, enc ObjectEncoder) (err error) {
+func encodeStringer(key string, stringer interface{}, enc ObjectEncoder) (retErr error) {
+// Try to capture panics (from nil references or otherwise) when calling
+// the String() method, similar to https://golang.org/src/fmt/print.go#L540
 defer func() {
-if v := recover(); v != nil {
-val := reflect.ValueOf(stringer)
-if val.Kind() == reflect.Ptr && val.IsNil() {
+if err := recover(); err != nil {
+// If it's a nil pointer, just say "<nil>". The likeliest causes are a
+// Stringer that fails to guard against nil or a nil pointer for a
+// value receiver, and in either case, "<nil>" is a nice result.
+if v := reflect.ValueOf(stringer); v.Kind() == reflect.Ptr && v.IsNil() {
 enc.AddString(key, "<nil>")
-} else {
-err = fmt.Errorf("PANIC=%v", v)
+return
 }
+
+retErr = fmt.Errorf("PANIC=%v", err)
 }
 }()
 
 enc.AddString(key, stringer.(fmt.Stringer).String())
-return
+return nil
 }