GODRIVER-700: fix issue with EOF handling in the json scanner when io.Reader returns n > 0 and io.EOF.

Change-Id: Ibad8c2dcada1f5ea0d88129681cdf1d57f51f9a3

Author: craiggwilson

bson/bsonrw/json_scanner.go

@@ -42,9 +42,10 @@ type jsonToken struct {
 }
 
 type jsonScanner struct {
-	r   io.Reader
-	buf []byte
-	pos int
+	r           io.Reader
+	buf         []byte
+	pos         int
+	lastReadErr error
 }
 
 // nextToken returns the next JSON token if one exists. A token is a character
@@ -126,11 +127,23 @@ func (js *jsonScanner) readNNextBytes(dst []byte, n, offset int) error {
 
 // readIntoBuf reads up to 512 bytes from the scanner's io.Reader into the buffer
 func (js *jsonScanner) readIntoBuf() error {
+	if js.lastReadErr != nil {
+		js.buf = js.buf[:0]
+		js.pos = 0
+		return js.lastReadErr
+	}
+
 	if cap(js.buf) == 0 {
 		js.buf = make([]byte, 0, 512)
 	}
 
 	n, err := js.r.Read(js.buf[:cap(js.buf)])
+	if err != nil {
+		js.lastReadErr = err
+		if n > 0 {
+			err = nil
+		}
+	}
 	js.buf = js.buf[:n]
 	js.pos = 0
 

bson/bsonrw/json_scanner_test.go

@@ -9,6 +9,7 @@ package bsonrw
 import (
 	"strings"
 	"testing"
+	"testing/iotest"
 
 	"github.com/google/go-cmp/cmp"
 )
@@ -282,6 +283,20 @@ func TestJsonScannerValidInputs(t *testing.T) {
 		c, err := js.nextToken()
 		jttDiff(t, jttEOF, c.t, tc.desc)
 		noerr(t, err)
+
+		// testing early EOF reading
+		js = &jsonScanner{r: iotest.DataErrReader(strings.NewReader(tc.input))}
+
+		for _, token := range tc.tokens {
+			c, err := js.nextToken()
+			jttDiff(t, token.t, c.t, tc.desc)
+			jtvDiff(t, token.v, c.v, tc.desc)
+			expectNoError(t, err, tc.desc)
+		}
+
+		c, err = js.nextToken()
+		jttDiff(t, jttEOF, c.t, tc.desc)
+		noerr(t, err)
 	}
 }