encoding/json: add Valid for checking validity of input bytes

Fixes #18086

Change-Id: Idc501dd37893e04a01c6ed9920147d24c0c1fa18
Reviewed-on: https://go-review.googlesource.com/34202
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/scanner.go

@@ -15,6 +15,11 @@ package json
 
 import "strconv"
 
+// Valid reports whether data is a valid JSON encoding.
+func Valid(data []byte) bool {
+	return checkValid(data, &scanner{}) == nil
+}
+
 // checkValid verifies that data is valid JSON-encoded data.
 // scan is passed in for use by checkValid to avoid an allocation.
 func checkValid(data []byte, scan *scanner) error {

src/encoding/json/scanner_test.go

@@ -12,6 +12,26 @@ import (
 	"testing"
 )
 
+var validTests = []struct {
+	data string
+	ok   bool
+}{
+	{`foo`, false},
+	{`}{`, false},
+	{`{]`, false},
+	{`{}`, true},
+	{`{"foo":"bar"}`, true},
+	{`{"foo":"bar","bar":{"baz":["qux"]}}`, true},
+}
+
+func TestValid(t *testing.T) {
+	for _, tt := range validTests {
+		if ok := Valid([]byte(tt.data)); ok != tt.ok {
+			t.Errorf("Valid(%#q) = %v, want %v", tt.data, ok, tt.ok)
+		}
+	}
+}
+
 // Tests of simple examples.
 
 type example struct {