encoding/json: disallow unknown fields in Decoder

Add a DisallowUnknownFields flag to Decoder.

DisallowUnknownFields causes the Decoder to return an error when
the the decoding destination is a struct and the input contains
object keys which do not match any non-ignored, public field the
destination, including keys whose value is set to null.

Note: this fix has already been worked on in 27231, which seems
to be abandoned. This version is a slightly simpler implementation
and is up to date with the master branch.

Fixes #15314

Change-Id: I987a5857c52018df334f4d1a2360649c44a7175d
Reviewed-on: https://go-review.googlesource.com/74830
Reviewed-by: Joe Tsai <joetsai@google.com>
Run-TryBot: Joe Tsai <joetsai@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/encoding/json/stream.go b/src/encoding/json/stream.go
index 76788f5fe7..f6b62c4cf6 100644
--- a/src/encoding/json/stream.go
+++ b/src/encoding/json/stream.go
@@ -35,6 +35,11 @@ func NewDecoder(r io.Reader) *Decoder {
 // Number instead of as a float64.
 func (dec *Decoder) UseNumber() { dec.d.useNumber = true }
 
+// DisallowUnknownFields causes the Decoder to return an error when the destination
+// is a struct and the input contains object keys which do not match any
+// non-ignored, exported fields in the destination.
+func (dec *Decoder) DisallowUnknownFields() { dec.d.disallowUnknownFields = true }
+
 // Decode reads the next JSON-encoded value from its
 // input and stores it in the value pointed to by v.
 //