encoding/json/v2: report EOF for top-level values in UnmarshalDecode

The fully streaming UnmarshalJSONFrom method and UnmarshalFromFunc
introduce an edge case where they can encounter EOF in the stream,
where it should be reported upstream as EOF rather than
ErrUnexpectedEOF or be wrapped within a SemanticError.

This is not possible with other unmarshal methods since the
"json" package would read the appropriate JSON value
before calling the custom method or function.

To avoid custom unmarshal methods from encountering EOF,
check whether the stream is already at EOF for top-level values
before calling the custom method.

Also, when wrapping EOF within a SemanticError, convert it
to ErrUnexpectedEOF to better indicate that this is unexpected.

Fixes #75802

Change-Id: I001396734b7e95b5337f77b71326284974ee730a
Reviewed-on: https://go-review.googlesource.com/c/go/+/710877
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

1 files changed, 13 insertions, 3 deletions

diff --git a/src/encoding/json/v2/errors.go b/src/encoding/json/v2/errors.go
index 0f50d608c9..4421f8b821 100644
--- a/src/encoding/json/v2/errors.go
+++ b/src/encoding/json/v2/errors.go
@@ -10,6 +10,7 @@ import (
 	"cmp"
 	"errors"
 	"fmt"
+	"io"
 	"reflect"
 	"strconv"
 	"strings"
@@ -118,7 +119,7 @@ func newInvalidFormatError(c coder, t reflect.Type) error {
 // newMarshalErrorBefore wraps err in a SemanticError assuming that e
 // is positioned right before the next token or value, which causes an error.
 func newMarshalErrorBefore(e *jsontext.Encoder, t reflect.Type, err error) error {
-	return &SemanticError{action: "marshal", GoType: t, Err: err,
+	return &SemanticError{action: "marshal", GoType: t, Err: toUnexpectedEOF(err),
 		ByteOffset:  e.OutputOffset() + int64(export.Encoder(e).CountNextDelimWhitespace()),
 		JSONPointer: jsontext.Pointer(export.Encoder(e).AppendStackPointer(nil, +1))}
 }
@@ -134,7 +135,7 @@ func newUnmarshalErrorBefore(d *jsontext.Decoder, t reflect.Type, err error) err
 	if export.Decoder(d).Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
 		k = d.PeekKind()
 	}
-	return &SemanticError{action: "unmarshal", GoType: t, Err: err,
+	return &SemanticError{action: "unmarshal", GoType: t, Err: toUnexpectedEOF(err),
 		ByteOffset:  d.InputOffset() + int64(export.Decoder(d).CountNextDelimWhitespace()),
 		JSONPointer: jsontext.Pointer(export.Decoder(d).AppendStackPointer(nil, +1)),
 		JSONKind:    k}
@@ -157,7 +158,7 @@ func newUnmarshalErrorBeforeWithSkipping(d *jsontext.Decoder, t reflect.Type, er
 // is positioned right after the previous token or value, which caused an error.
 func newUnmarshalErrorAfter(d *jsontext.Decoder, t reflect.Type, err error) error {
 	tokOrVal := export.Decoder(d).PreviousTokenOrValue()
-	return &SemanticError{action: "unmarshal", GoType: t, Err: err,
+	return &SemanticError{action: "unmarshal", GoType: t, Err: toUnexpectedEOF(err),
 		ByteOffset:  d.InputOffset() - int64(len(tokOrVal)),
 		JSONPointer: jsontext.Pointer(export.Decoder(d).AppendStackPointer(nil, -1)),
 		JSONKind:    jsontext.Value(tokOrVal).Kind()}
@@ -206,6 +207,7 @@ func newSemanticErrorWithPosition(c coder, t reflect.Type, prevDepth int, prevLe
 	if serr == nil {
 		serr = &SemanticError{Err: err}
 	}
+	serr.Err = toUnexpectedEOF(serr.Err)
 	var currDepth int
 	var currLength int64
 	var coderState interface{ AppendStackPointer([]byte, int) []byte }
@@ -432,3 +434,11 @@ func newDuplicateNameError(ptr jsontext.Pointer, quotedName []byte, offset int64
 		Err:         jsontext.ErrDuplicateName,
 	}
 }
+
+// toUnexpectedEOF converts [io.EOF] to [io.ErrUnexpectedEOF].
+func toUnexpectedEOF(err error) error {
+	if err == io.EOF {
+		return io.ErrUnexpectedEOF
+	}
+	return err
+}