encoding/json: take new decoder code off Decode path completely

The new Token API is meant to sit on the side of the Decoder,
so that you only get the new code (and any latent bugs in it)
if you are actively using the Token API.

The unconditional use of dec.peek in dec.tokenPrepareForDecode
violates that intention.

Change tokenPrepareForDecode not to call dec.peek unless needed
(because the Token API has advanced the state).
This restores the old code path behavior, no peeking allowed.

I checked by patching in the new tests from CL 12726 that
this change suffices to "fix" the error handling bug in dec.peek.
Obviously that bug should be fixed too, but the point is that
with this CL, bugs in dec.peek do not affect plain use of Decode
or Unmarshal.

I also checked by putting a panic in dec.peek that the only
tests that now invoke peek are:

	TestDecodeInStream
	ExampleDecoder_Token
	ExampleDecoder_Decode_stream

and those tests all invoke dec.Token directly.

Change-Id: I0b242d0cb54a9c830548644670dc5ab5ccef69f2
Reviewed-on: https://go-review.googlesource.com/12740Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Peter Waldschmidt <peter@waldschmidt.com>

src/encoding/json/stream.go

@@ -252,18 +252,25 @@ const (
 
 // advance tokenstate from a separator state to a value state
 func (dec *Decoder) tokenPrepareForDecode() error {
-	c, err := dec.peek()
-	if err != nil {
-		return err
-	}
+	// Note: Not calling peek before switch, to avoid
+	// putting peek into the standard Decode path.
+	// peek is only called when using the Token API.
 	switch dec.tokenState {
 	case tokenArrayComma:
+		c, err := dec.peek()
+		if err != nil {
+			return err
+		}
 		if c != ',' {
 			return &SyntaxError{"expected comma after array element", 0}
 		}
 		dec.scanp++
 		dec.tokenState = tokenArrayValue
 	case tokenObjectColon:
+		c, err := dec.peek()
+		if err != nil {
+			return err
+		}
 		if c != ':' {
 			return &SyntaxError{"expected colon after object key", 0}
 		}