From ee5af46172e64eceddb56018de8ea850fe0a6cae Mon Sep 17 00:00:00 2001 From: Joe Tsai Date: Sat, 11 Oct 2025 11:57:46 -0700 Subject: encoding/json: avoid misleading errors under goexperiment.jsonv2 The jsontext package represents the location of JSON errors using a JSON Pointer (RFC 6901). This uses the JSON type system. Unfortunately the v1 json.UnmarshalTypeError assumes a Go struct-based mechanism for reporting the location of errors (and has historically never been implemented correctly since it was a weird mix of both JSON and Go namespaces; see #43126). Trying to map a JSON Pointer into UnmarshalTypeError.{Struct,Field} is difficult to get right without teaching jsontext about the Go type system. To reduce the probability of misleading errors, check whether the last token looks like a JSON array index and if so, elide the phrase "into Go struct field". Fixes #74801 Change-Id: Id2088ffb9c339a9238ed38c90223d86a89422842 Reviewed-on: https://go-review.googlesource.com/c/go/+/710676 LUCI-TryBot-Result: Go LUCI Reviewed-by: Dmitri Shuralyov Reviewed-by: Damien Neil --- src/encoding/json/v2_decode.go | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) (limited to 'src/encoding/json/v2_decode.go') diff --git a/src/encoding/json/v2_decode.go b/src/encoding/json/v2_decode.go index 1041ec7ee4..f17d7ebcca 100644 --- a/src/encoding/json/v2_decode.go +++ b/src/encoding/json/v2_decode.go @@ -14,6 +14,7 @@ import ( "fmt" "reflect" "strconv" + "strings" "encoding/json/internal/jsonwire" "encoding/json/jsontext" @@ -119,7 +120,20 @@ type UnmarshalTypeError struct { func (e *UnmarshalTypeError) Error() string { var s string if e.Struct != "" || e.Field != "" { - s = "json: cannot unmarshal " + e.Value + " into Go struct field " + e.Struct + "." + e.Field + " of type " + e.Type.String() + // The design of UnmarshalTypeError overly assumes a struct-based + // Go representation for the JSON value. + // The logic in jsontext represents paths using a JSON Pointer, + // which is agnostic to the Go type system. + // Trying to convert a JSON Pointer into a UnmarshalTypeError.Field + // is difficult. As a heuristic, if the last path token looks like + // an index into a JSON array (e.g., ".foo.bar.0"), + // avoid the phrase "Go struct field ". + intoWhat := "Go struct field " + i := strings.LastIndexByte(e.Field, '.') + len(".") + if len(e.Field[i:]) > 0 && strings.TrimRight(e.Field[i:], "0123456789") == "" { + intoWhat = "" // likely a Go slice or array + } + s = "json: cannot unmarshal " + e.Value + " into " + intoWhat + e.Struct + "." + e.Field + " of type " + e.Type.String() } else { s = "json: cannot unmarshal " + e.Value + " into Go value of type " + e.Type.String() } -- cgit v1.3