log/slog: update Source method to return nil for unavailable locations

Following proposal https://github.com/golang/go/issues/70280

Signed-off-by: Ernesto Alejandro Santana Hidalgo <ernesto.alejandrosantana@gmail.com>

api/next/70280.txt

@@ -0,0 +1 @@
+pkg log/slog, method (Record) Source() *Source #70280

doc/next/6-stdlib/99-minor/log/slog/70280.md

@@ -0,0 +1 @@
+[Record] now has a Source() method, returning its source location or nil if unavailable.

src/log/slog/handler.go

@@ -299,7 +299,11 @@ func (h *commonHandler) handle(r Record) error {
 	}
 	// source
 	if h.opts.AddSource {
-		state.appendAttr(Any(SourceKey, r.Source()))
+		src := r.Source()
+		if src == nil {
+			src = &Source{}
+		}
+		state.appendAttr(Any(SourceKey, src))
 	}
 	key = MessageKey
 	msg := r.Message

src/log/slog/handler_test.go

@@ -547,7 +547,11 @@ func TestJSONAndTextHandlers(t *testing.T) {
 		},
 	} {
 		r := NewRecord(testTime, LevelInfo, "message", callerPC(2))
-		line := strconv.Itoa(r.Source().Line)
+		source := r.Source()
+		if source == nil {
+			t.Fatal("source is nil")
+		}
+		line := strconv.Itoa(source.Line)
 		r.AddAttrs(test.attrs...)
 		var buf bytes.Buffer
 		opts := HandlerOptions{ReplaceAttr: test.replace, AddSource: test.addSource}
@@ -634,6 +638,40 @@ func TestHandlerEnabled(t *testing.T) {
 	}
 }
 
+func TestJSONAndTextHandlersWithUnavailableSource(t *testing.T) {
+	// Verify that a nil source does not cause a panic.
+	// and that the source is empty.
+	var buf bytes.Buffer
+	opts := &HandlerOptions{
+		ReplaceAttr: removeKeys(LevelKey),
+		AddSource:   true,
+	}
+
+	for _, test := range []struct {
+		name string
+		h    Handler
+		want string
+	}{
+		{"text", NewTextHandler(&buf, opts), "source=:0 msg=message"},
+		{"json", NewJSONHandler(&buf, opts), `{"msg":"message"}`},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			buf.Reset()
+			r := NewRecord(time.Time{}, LevelInfo, "message", 0)
+			err := test.h.Handle(t.Context(), r)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			want := strings.TrimSpace(test.want)
+			got := strings.TrimSpace(buf.String())
+			if got != want {
+				t.Errorf("\ngot  %s\nwant %s", got, want)
+			}
+		})
+	}
+}
+
 func TestSecondWith(t *testing.T) {
 	// Verify that a second call to Logger.With does not corrupt
 	// the original.

src/log/slog/logger_test.go

@@ -191,6 +191,9 @@ func TestCallDepth(t *testing.T) {
 		const wantFile = "logger_test.go"
 		wantLine := startLine + count*2
 		got := h.r.Source()
+		if got == nil {
+			t.Fatal("got nil source")
+		}
 		gotFile := filepath.Base(got.File)
 		if got.Function != wantFunc || gotFile != wantFile || got.Line != wantLine {
 			t.Errorf("got (%s, %s, %d), want (%s, %s, %d)",

src/log/slog/record.go

@@ -211,11 +211,14 @@ func (s *Source) group() Value {
 	return GroupValue(as...)
 }
 
-// Source returns a Source for the log event.
-// If the Record was created without the necessary information,
-// or if the location is unavailable, it returns a non-nil *Source
-// with zero fields.
+// 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.
 func (r Record) Source() *Source {
+	if r.PC == 0 {
+		return nil
+	}
+
 	fs := runtime.CallersFrames([]uintptr{r.PC})
 	f, _ := fs.Next()
 	return &Source{

src/log/slog/record_test.go

@@ -39,17 +39,18 @@ func TestRecordAttrs(t *testing.T) {
 }
 
 func TestRecordSource(t *testing.T) {
-	// Zero call depth => empty *Source.
+	// Zero call depth => nil *Source.
 	for _, test := range []struct {
 		depth            int
 		wantFunction     string
 		wantFile         string
 		wantLinePositive bool
+		wantNil          bool
 	}{
-		{0, "", "", false},
-		{-16, "", "", false},
-		{1, "log/slog.TestRecordSource", "record_test.go", true}, // 1: caller of NewRecord
-		{2, "testing.tRunner", "testing.go", true},
+		{0, "", "", false, true},
+		{-16, "", "", false, true},
+		{1, "log/slog.TestRecordSource", "record_test.go", true, false}, // 1: caller of NewRecord
+		{2, "testing.tRunner", "testing.go", true, false},
 	} {
 		var pc uintptr
 		if test.depth > 0 {
@@ -57,6 +58,16 @@ func TestRecordSource(t *testing.T) {
 		}
 		r := NewRecord(time.Time{}, 0, "", pc)
 		got := r.Source()
+		if test.wantNil {
+			if got != nil {
+				t.Errorf("depth %d: got non-nil Source, want nil", test.depth)
+			}
+			continue
+		}
+		if got == nil {
+			t.Errorf("depth %d: got nil Source, want non-nil", test.depth)
+			continue
+		}
 		if i := strings.LastIndexByte(got.File, '/'); i >= 0 {
 			got.File = got.File[i+1:]
 		}