log/slog: generate valid JSON string with empty attributes in Groups

Fixes #62152

Change-Id: I1f76ab26eae3a44292ebe6003006258d5e31d50f
Reviewed-on: https://go-review.googlesource.com/c/go/+/521135
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/log/slog/handler.go

@@ -233,8 +233,11 @@ func (h *commonHandler) withAttrs(as []Attr) *commonHandler {
 	state := h2.newHandleState((*buffer.Buffer)(&h2.preformattedAttrs), false, "")
 	defer state.free()
 	state.prefix.WriteString(h.groupPrefix)
-	if len(h2.preformattedAttrs) > 0 {
+	if pfa := h2.preformattedAttrs; len(pfa) > 0 {
 		state.sep = h.attrSep()
+		if h2.json && pfa[len(pfa)-1] == '{' {
+			state.sep = ""
+		}
 	}
 	state.openGroups()
 	for _, a := range as {
@@ -310,10 +313,13 @@ func (h *commonHandler) handle(r Record) error {
 
 func (s *handleState) appendNonBuiltIns(r Record) {
 	// preformatted Attrs
-	if len(s.h.preformattedAttrs) > 0 {
+	if pfa := s.h.preformattedAttrs; len(pfa) > 0 {
 		s.buf.WriteString(s.sep)
-		s.buf.Write(s.h.preformattedAttrs)
+		s.buf.Write(pfa)
 		s.sep = s.h.attrSep()
+		if s.h.json && pfa[len(pfa)-1] == '{' {
+			s.sep = ""
+		}
 	}
 	// Attrs in Record -- unlike the built-in ones, they are in groups started
 	// from WithGroup.

src/log/slog/handler_test.go

@@ -435,6 +435,62 @@ func TestJSONAndTextHandlers(t *testing.T) {
 			wantText: `time.mins=3 time.secs=2 msg=message`,
 			wantJSON: `{"time":{"mins":3,"secs":2},"msg":"message"}`,
 		},
+		{
+			name: "replace empty 1",
+			with: func(h Handler) Handler {
+				return h.WithGroup("g").WithAttrs([]Attr{Int("a", 1)})
+			},
+			replace:  func([]string, Attr) Attr { return Attr{} },
+			attrs:    []Attr{Group("h", Int("b", 2))},
+			wantText: "",
+			wantJSON: `{"g":{"h":{}}}`,
+		},
+		{
+			name: "replace empty 2",
+			with: func(h Handler) Handler {
+				return h.WithGroup("g").WithAttrs([]Attr{Int("a", 1)}).WithGroup("h").WithAttrs([]Attr{Int("b", 2)})
+			},
+			replace:  func([]string, Attr) Attr { return Attr{} },
+			attrs:    []Attr{Group("i", Int("c", 3))},
+			wantText: "",
+			wantJSON: `{"g":{"h":{"i":{}}}}`,
+		},
+		{
+			name: "replace partial empty attrs 1",
+			with: func(h Handler) Handler {
+				return h.WithGroup("g").WithAttrs([]Attr{Int("a", 1)}).WithGroup("h").WithAttrs([]Attr{Int("b", 2)})
+			},
+			replace: func(groups []string, attr Attr) Attr {
+				return removeKeys(TimeKey, LevelKey, MessageKey, "a")(groups, attr)
+			},
+			attrs:    []Attr{Group("i", Int("c", 3))},
+			wantText: "g.h.b=2 g.h.i.c=3",
+			wantJSON: `{"g":{"h":{"b":2,"i":{"c":3}}}}`,
+		},
+		{
+			name: "replace partial empty attrs 2",
+			with: func(h Handler) Handler {
+				return h.WithGroup("g").WithAttrs([]Attr{Int("a", 1)}).WithAttrs([]Attr{Int("n", 4)}).WithGroup("h").WithAttrs([]Attr{Int("b", 2)})
+			},
+			replace: func(groups []string, attr Attr) Attr {
+				return removeKeys(TimeKey, LevelKey, MessageKey, "a", "b")(groups, attr)
+			},
+			attrs:    []Attr{Group("i", Int("c", 3))},
+			wantText: "g.n=4 g.h.i.c=3",
+			wantJSON: `{"g":{"n":4,"h":{"i":{"c":3}}}}`,
+		},
+		{
+			name: "replace partial empty attrs 3",
+			with: func(h Handler) Handler {
+				return h.WithGroup("g").WithAttrs([]Attr{Int("x", 0)}).WithAttrs([]Attr{Int("a", 1)}).WithAttrs([]Attr{Int("n", 4)}).WithGroup("h").WithAttrs([]Attr{Int("b", 2)})
+			},
+			replace: func(groups []string, attr Attr) Attr {
+				return removeKeys(TimeKey, LevelKey, MessageKey, "a", "c")(groups, attr)
+			},
+			attrs:    []Attr{Group("i", Int("c", 3))},
+			wantText: "g.x=0 g.n=4 g.h.b=2",
+			wantJSON: `{"g":{"x":0,"n":4,"h":{"b":2,"i":{}}}}`,
+		},
 	} {
 		r := NewRecord(testTime, LevelInfo, "message", callerPC(2))
 		line := strconv.Itoa(r.source().Line)