log/slog: require entire Attr to be empty to elide

Specify that Handlers should ignore zero-valued Attrs.

Implement that policy in the built-in handlers.

Fixes #59282.

Change-Id: I4430686b61f49bdac849ee300daaabfac9895849
Reviewed-on: https://go-review.googlesource.com/c/go/+/484095
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/log/slog/attr.go

@@ -83,6 +83,8 @@ func (a Attr) String() string {
 	return fmt.Sprintf("%s=%s", a.Key, a.Value)
 }
 
+// isEmpty reports whether a has an empty key and a nil value.
+// That can be written as Attr{} or Any("", nil).
 func (a Attr) isEmpty() bool {
 	return a.Key == "" && a.Value.num == 0 && a.Value.any == nil
 }

src/log/slog/example_wrap_test.go

@@ -30,7 +30,7 @@ func Example_wrapping() {
 	replace := func(groups []string, a slog.Attr) slog.Attr {
 		// Remove time.
 		if a.Key == slog.TimeKey && len(groups) == 0 {
-			a.Key = ""
+			return slog.Attr{}
 		}
 		// Remove the directory from the source's filename.
 		if a.Key == slog.SourceKey {

src/log/slog/handler.go

@@ -50,8 +50,8 @@ type Handler interface {
 	// Handle methods that produce output should observe the following rules:
 	//   - If r.Time is the zero time, ignore the time.
 	//   - If r.PC is zero, ignore it.
-	//   - If an Attr's key is the empty string and the value is not a group,
-	//     ignore the Attr.
+	//   - If an Attr's key and value are both the zero value, ignore the Attr.
+	//     This can be tested with attr.Equal(Attr{}).
 	//   - If a group's key is empty, inline the group's Attrs.
 	//   - If a group has no Attrs (even if it has a non-empty key),
 	//     ignore it.
@@ -437,26 +437,22 @@ func (s *handleState) closeGroup(name string) {
 // It handles replacement and checking for an empty key.
 // after replacement).
 func (s *handleState) appendAttr(a Attr) {
-	v := a.Value
-	// Elide a non-group with an empty key.
-	if a.Key == "" && v.Kind() != KindGroup {
-		return
-	}
-	if rep := s.h.opts.ReplaceAttr; rep != nil && v.Kind() != KindGroup {
+	if rep := s.h.opts.ReplaceAttr; rep != nil && a.Value.Kind() != KindGroup {
 		var gs []string
 		if s.groups != nil {
 			gs = *s.groups
 		}
-		a = rep(gs, Attr{a.Key, v})
-		if a.Key == "" {
-			return
-		}
+		a = rep(gs, a)
 		// Although all attributes in the Record are already resolved,
 		// This one came from the user, so it may not have been.
-		v = a.Value.Resolve()
+		a.Value = a.Value.Resolve()
+	}
+	// Elide empty Attrs.
+	if a.isEmpty() {
+		return
 	}
-	if v.Kind() == KindGroup {
-		attrs := v.Group()
+	if a.Value.Kind() == KindGroup {
+		attrs := a.Value.Group()
 		// Output only non-empty groups.
 		if len(attrs) > 0 {
 			// Inline a group with an empty key.
@@ -472,7 +468,7 @@ func (s *handleState) appendAttr(a Attr) {
 		}
 	} else {
 		s.appendKey(a.Key)
-		s.appendValue(v)
+		s.appendValue(a.Value)
 	}
 }
 

src/log/slog/handler_test.go

@@ -108,12 +108,10 @@ func TestDefaultHandle(t *testing.T) {
 func TestJSONAndTextHandlers(t *testing.T) {
 	ctx := context.Background()
 
-	// ReplaceAttr functions
-
 	// remove all Attrs
 	removeAll := func(_ []string, a Attr) Attr { return Attr{} }
 
-	attrs := []Attr{String("a", "one"), Int("b", 2), Any("", "ignore me")}
+	attrs := []Attr{String("a", "one"), Int("b", 2), Any("", nil)}
 	preAttrs := []Attr{Int("pre", 3), String("x", "y")}
 
 	for _, test := range []struct {
@@ -131,6 +129,12 @@ func TestJSONAndTextHandlers(t *testing.T) {
 			wantText: "time=2000-01-02T03:04:05.000Z level=INFO msg=message a=one b=2",
 			wantJSON: `{"time":"2000-01-02T03:04:05Z","level":"INFO","msg":"message","a":"one","b":2}`,
 		},
+		{
+			name:     "empty key",
+			attrs:    append(slices.Clip(attrs), Any("", "v")),
+			wantText: `time=2000-01-02T03:04:05.000Z level=INFO msg=message a=one b=2 ""=v`,
+			wantJSON: `{"time":"2000-01-02T03:04:05Z","level":"INFO","msg":"message","a":"one","b":2,"":"v"}`,
+		},
 		{
 			name:     "cap keys",
 			replace:  upperCaseKey,
@@ -296,7 +300,7 @@ func TestJSONAndTextHandlers(t *testing.T) {
 			wantJSON: `{"msg":"message","a":1,"b":2,"c":3,"d":4}`,
 		},
 	} {
-		r := NewRecord(testTime, LevelInfo, "message", 1)
+		r := NewRecord(testTime, LevelInfo, "message", 0)
 		r.AddAttrs(test.attrs...)
 		var buf bytes.Buffer
 		opts := HandlerOptions{ReplaceAttr: test.replace}

src/log/slog/internal/slogtest/slogtest.go

@@ -12,7 +12,7 @@ import "log/slog"
 // to make example output deterministic.
 func RemoveTime(groups []string, a slog.Attr) slog.Attr {
 	if a.Key == slog.TimeKey && len(groups) == 0 {
-		a.Key = ""
+		return slog.Attr{}
 	}
 	return a
 }

src/log/slog/text_handler.go

@@ -138,6 +138,9 @@ func byteSlice(a any) ([]byte, bool) {
 }
 
 func needsQuoting(s string) bool {
+	if len(s) == 0 {
+		return true
+	}
 	for i := 0; i < len(s); {
 		b := s[i]
 		if b < utf8.RuneSelf {

src/log/slog/text_handler_test.go

@@ -185,7 +185,7 @@ func TestNeedsQuoting(t *testing.T) {
 		in   string
 		want bool
 	}{
-		{"", false},
+		{"", true},
 		{"ab", false},
 		{"a=b", true},
 		{`"ab"`, true},

src/log/slog/value_test.go

@@ -185,7 +185,6 @@ func TestLogValue(t *testing.T) {
 	if !attrsEqual(got2, want2) {
 		t.Errorf("got %v, want %v", got2, want2)
 	}
-
 }
 
 func TestZeroTime(t *testing.T) {