handle nils safely in Printf.

add some tests for erroneous formats.

R=rsc
CC=golang-dev
https://golang.org/cl/201058

src/pkg/fmt/fmt_test.go

@@ -229,6 +229,11 @@ var fmttests = []fmtTest{
 	fmtTest{"%#v", make(chan int), "(chan int)(PTR)"},
 	fmtTest{"%#v", uint64(1<<64 - 1), "0xffffffffffffffff"},
 	fmtTest{"%#v", 1000000000, "1000000000"},
+
+	// erroneous things
+	fmtTest{"%d", "hello", "%d(string=hello)"},
+	fmtTest{"no args", "hello", "no args?(extra string=hello)"},
+	fmtTest{"%s", nil, "%s(<nil>)"},
 }
 
 func TestSprintf(t *testing.T) {

src/pkg/fmt/print.go

@@ -378,6 +378,9 @@ func getInt(a interface{}) (val int64, signed, ok bool) {
 }
 
 func getString(a interface{}) (val string, ok bool) {
+	if a == nil {
+		return "<nil>", ok
+	}
 	// Is it a regular string or []byte type?
 	switch s := a.(type) {
 	case string:
@@ -941,8 +944,10 @@ func (p *pp) doprintf(format string, a []interface{}) {
 			p.buf.WriteByte('%')
 			p.add(c)
 			p.buf.WriteByte('(')
-			p.buf.WriteString(reflect.Typeof(field).String())
-			p.buf.WriteByte('=')
+			if field != nil {
+				p.buf.WriteString(reflect.Typeof(field).String())
+				p.buf.WriteByte('=')
+			}
 			p.printField(field, false, false, 0)
 			p.buf.WriteByte(')')
 		}