log/slog: ensure ReplaceAttr does not see a group

The ReplaceAttr function should not see groups, only leaf attributes.

Previously, we checked an Value for being a group, then resolved it,
then called ReplaceAttr. We neglected to see if it was a group
after resolving it.

Now we resolve first, then check.

Fixes #62731.

Change-Id: I2fc40758e77c445f82deb2c9de8cae7a3b0e22cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/530478
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: jba

src/log/slog/handler.go

@@ -438,16 +438,17 @@ func (s *handleState) closeGroup(name string) {
 // It handles replacement and checking for an empty key.
 // after replacement).
 func (s *handleState) appendAttr(a Attr) {
+	a.Value = a.Value.Resolve()
 	if rep := s.h.opts.ReplaceAttr; rep != nil && a.Value.Kind() != KindGroup {
 		var gs []string
 		if s.groups != nil {
 			gs = *s.groups
 		}
-		// Resolve before calling ReplaceAttr, so the user doesn't have to.
-		a.Value = a.Value.Resolve()
+		// a.Value is resolved before calling ReplaceAttr, so the user doesn't have to.
 		a = rep(gs, a)
+		// The ReplaceAttr function may return an unresolved Attr.
+		a.Value = a.Value.Resolve()
 	}
-	a.Value = a.Value.Resolve()
 	// Elide empty Attrs.
 	if a.isEmpty() {
 		return

src/log/slog/handler_test.go

@@ -491,6 +491,18 @@ func TestJSONAndTextHandlers(t *testing.T) {
 			wantText: "g.x=0 g.n=4 g.h.b=2",
 			wantJSON: `{"g":{"x":0,"n":4,"h":{"b":2,"i":{}}}}`,
 		},
+		{
+			name: "replace resolved group",
+			replace: func(groups []string, a Attr) Attr {
+				if a.Value.Kind() == KindGroup {
+					return Attr{"bad", IntValue(1)}
+				}
+				return removeKeys(TimeKey, LevelKey, MessageKey)(groups, a)
+			},
+			attrs:    []Attr{Any("name", logValueName{"Perry", "Platypus"})},
+			wantText: "name.first=Perry name.last=Platypus",
+			wantJSON: `{"name":{"first":"Perry","last":"Platypus"}}`,
+		},
 	} {
 		r := NewRecord(testTime, LevelInfo, "message", callerPC(2))
 		line := strconv.Itoa(r.source().Line)