log/slog: function argument to Record.Attrs returns bool

Record.Attrs stops as soon as its argument function returns false.

Fixes golang#59060.

Change-Id: I578d64635e0e52b0fcdbc57f6d5a27a6efac8c70
Reviewed-on: https://go-review.googlesource.com/c/go/+/484096
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>

api/next/59060.txt

@@ -0,0 +1,2 @@
+pkg log/slog, method (Record) Attrs(func(Attr) bool) #59060
+

src/log/slog/handler.go

@@ -330,8 +330,9 @@ func (s *handleState) appendNonBuiltIns(r Record) {
 	defer s.prefix.Free()
 	s.prefix.WriteString(s.h.groupPrefix)
 	s.openGroups()
-	r.Attrs(func(a Attr) {
+	r.Attrs(func(a Attr) bool {
 		s.appendAttr(a)
+		return true
 	})
 	if s.h.json {
 		// Close all open groups.

src/log/slog/internal/benchmarks/handlers.go

@@ -47,11 +47,12 @@ func (h *fastTextHandler) Handle(_ context.Context, r slog.Record) error {
 	buf.WriteByte(' ')
 	buf.WriteString("msg=")
 	buf.WriteString(r.Message)
-	r.Attrs(func(a slog.Attr) {
+	r.Attrs(func(a slog.Attr) bool {
 		buf.WriteByte(' ')
 		buf.WriteString(a.Key)
 		buf.WriteByte('=')
 		h.appendValue(buf, a.Value)
+		return true
 	})
 	buf.WriteByte('\n')
 	_, err := h.w.Write(*buf)

src/log/slog/internal/benchmarks/handlers_test.go

@@ -37,6 +37,6 @@ func TestHandlers(t *testing.T) {
 
 func attrSlice(r slog.Record) []slog.Attr {
 	var as []slog.Attr
-	r.Attrs(func(a slog.Attr) { as = append(as, a) })
+	r.Attrs(func(a slog.Attr) bool { as = append(as, a); return true })
 	return as
 }

src/log/slog/record.go

@@ -86,13 +86,18 @@ func (r Record) NumAttrs() int {
 }
 
 // Attrs calls f on each Attr in the Record.
+// Iteration stops if f returns false.
 // The Attrs are already resolved.
-func (r Record) Attrs(f func(Attr)) {
+func (r Record) Attrs(f func(Attr) bool) {
 	for i := 0; i < r.nFront; i++ {
-		f(r.front[i])
+		if !f(r.front[i]) {
+			return
+		}
 	}
 	for _, a := range r.back {
-		f(a)
+		if !f(a) {
+			return
+		}
 	}
 }
 

src/log/slog/record_test.go

@@ -23,6 +23,17 @@ func TestRecordAttrs(t *testing.T) {
 	if got := attrsSlice(r); !attrsEqual(got, as) {
 		t.Errorf("got %v, want %v", got, as)
 	}
+
+	// Early return.
+	var got []Attr
+	r.Attrs(func(a Attr) bool {
+		got = append(got, a)
+		return len(got) < 2
+	})
+	want := as[:2]
+	if !attrsEqual(got, want) {
+		t.Errorf("got %v, want %v", got, want)
+	}
 }
 
 func TestRecordSourceLine(t *testing.T) {
@@ -102,7 +113,7 @@ func newRecordWithAttrs(as []Attr) Record {
 
 func attrsSlice(r Record) []Attr {
 	s := make([]Attr, 0, r.NumAttrs())
-	r.Attrs(func(a Attr) { s = append(s, a) })
+	r.Attrs(func(a Attr) bool { s = append(s, a); return true })
 	return s
 }
 
@@ -157,7 +168,7 @@ func BenchmarkRecord(b *testing.B) {
 		for j := 0; j < nAttrs; j++ {
 			r.AddAttrs(Int("k", j))
 		}
-		r.Attrs(func(b Attr) { a = b })
+		r.Attrs(func(b Attr) bool { a = b; return true })
 	}
 	_ = a
 }