encoding/json: fix EOF bug decoding HTTP stream

Fixes bug referenced in this thread on golang-dev:
https://groups.google.com/d/topic/golang-dev/U4LSpMzL82c/discussion

Change-Id: If01a2644863f9e5625dd2f95f9d344bda772e12c
Reviewed-on: https://go-review.googlesource.com/12726
Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/json/stream.go

@@ -437,6 +437,7 @@ func (dec *Decoder) More() bool {
 }
 
 func (dec *Decoder) peek() (byte, error) {
+	var err error
 	for {
 		for i := dec.scanp; i < len(dec.buf); i++ {
 			c := dec.buf[i]
@@ -446,9 +447,11 @@ func (dec *Decoder) peek() (byte, error) {
 			dec.scanp = i
 			return c, nil
 		}
-		if err := dec.refill(); err != nil {
+		// buffer has been scanned, now report any error
+		if err != nil {
 			return 0, err
 		}
+		err = dec.refill()
 	}
 }
 

src/encoding/json/stream_test.go

@@ -8,7 +8,10 @@ import (
 	"bytes"
 	"io"
 	"io/ioutil"
+	"log"
 	"net"
+	"net/http"
+	"net/http/httptest"
 	"reflect"
 	"strings"
 	"testing"
@@ -315,3 +318,44 @@ func TestDecodeInStream(t *testing.T) {
 	}
 
 }
+
+const raw = `{ "foo": "bar" }`
+
+func makeHTTP() io.ReadCloser {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte(raw))
+	})
+	ts := httptest.NewServer(mux)
+	defer ts.Close()
+	res, err := http.Get(ts.URL)
+	if err != nil {
+		log.Fatalf("GET failed: %v", err)
+	}
+	return res.Body
+}
+
+func TestHttpDecoding(t *testing.T) {
+
+	foo := struct {
+		Foo string
+	}{}
+
+	rc := makeHTTP()
+	defer rc.Close()
+
+	d := NewDecoder(rc)
+	err := d.Decode(&foo)
+	if err != nil {
+		t.Errorf("Unexpected error %v", err)
+	}
+	if foo.Foo != "bar" {
+		t.Errorf("Expected \"bar\", was %v", foo.Foo)
+	}
+
+	// make sure we get the EOF the second time
+	err = d.Decode(&foo)
+	if err != io.EOF {
+		t.Errorf("Expected io.EOF, was %v", err)
+	}
+}