log/slog: make TextHandler discard empty Source

Fixes #73808

Change-Id: Ica4b7a63eebbf0fff41d68f4de928f9da90c8ada
Reviewed-on: https://go-review.googlesource.com/c/go/+/674875
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/log/slog/handler.go

@@ -492,6 +492,9 @@ func (s *handleState) appendAttr(a Attr) bool {
 	// Special case: Source.
 	if v := a.Value; v.Kind() == KindAny {
 		if src, ok := v.Any().(*Source); ok {
+			if src.isEmpty() {
+				return false
+			}
 			if s.h.json {
 				a.Value = src.group()
 			} else {

src/log/slog/handler_test.go

@@ -652,7 +652,7 @@ func TestJSONAndTextHandlersWithUnavailableSource(t *testing.T) {
 		h    Handler
 		want string
 	}{
-		{"text", NewTextHandler(&buf, opts), "source=:0 msg=message"},
+		{"text", NewTextHandler(&buf, opts), "msg=message"},
 		{"json", NewJSONHandler(&buf, opts), `{"msg":"message"}`},
 	} {
 		t.Run(test.name, func(t *testing.T) {

src/log/slog/record.go

@@ -211,6 +211,9 @@ func (s *Source) group() Value {
 	return GroupValue(as...)
 }
 
+// isEmpty returns whether the Source struct is nil or only contains zero fields.
+func (s *Source) isEmpty() bool { return s == nil || *s == Source{} }
+
 // Source returns a new Source for the log event using r's PC.
 // If the PC field is zero, meaning the Record was created without the necessary information
 // or the location is unavailable, then nil is returned.