bufio/Scan: fix error handling at EOF

Fixes #5268.

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://golang.org/cl/8646045

src/pkg/bufio/scan.go

@@ -103,7 +103,8 @@ func (s *Scanner) Text() string {
 
 // Scan advances the Scanner to the next token, which will then be
 // available through the Bytes or Text method. It returns false when the
-// scan stops, either by reaching the end of the input or an error.
+// scan stops, either by reaching the end of the input, a zero-length
+// read from the input, or an error.
 // After Scan returns false, the Err method will return any error that
 // occurred during scanning, except that if it was io.EOF, Err
 // will return nil.
@@ -164,7 +165,7 @@ func (s *Scanner) Scan() bool {
 			s.setErr(err)
 		}
 		if n == 0 { // Don't loop forever if Reader doesn't deliver EOF.
-			s.err = io.EOF
+			s.setErr(io.EOF)
 		}
 		s.end += n
 	}

src/pkg/bufio/scan_test.go

@@ -368,3 +368,21 @@ func TestErrAtEOF(t *testing.T) {
 		t.Fatal("wrong error:", s.Err())
 	}
 }
+
+// Test for issue 5268.
+type alwaysError struct{}
+
+func (alwaysError) Read(p []byte) (int, error) {
+	return 0, io.ErrUnexpectedEOF
+}
+
+func TestNonEOFWithEmptyRead(t *testing.T) {
+	scanner := NewScanner(alwaysError{})
+	for scanner.Scan() {
+		t.Fatal("read should fail")
+	}
+	err := scanner.Err()
+	if err != io.ErrUnexpectedEOF {
+		t.Errorf("unexpected error: %v", err)
+	}
+}