unit tests with full SlogSink mock implementation

This turns testLogSink into an implementation of SlogSink if slog is supported,
then runs through some scenarios where values get added using different APIs.
The end result gets compared by formatting with the slog JSON handler because
that is easier than writing custom code for a deep-equal comparison of slog
attributes.

The "with-group-and-logr-values" is the case which was broken.

logr_noslog_test.go

@@ -0,0 +1,23 @@
+//go:build !go1.21
+// +build !go1.21
+
+/*
+Copyright 2023 The logr Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package logr
+
+//nolint:unused
+type testSlogSink struct{}

logr_slog_test.go

@@ -0,0 +1,218 @@
+//go:build go1.21
+// +build go1.21
+
+/*
+Copyright 2023 The logr Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package logr
+
+import (
+	"bytes"
+	"context"
+	"log/slog"
+	"testing"
+	"time"
+)
+
+var _ SlogSink = &testLogSink{}
+
+// testSlogSink gets embedded in testLogSink to add slog-specific fields
+// which are only available when slog is supported by Go.
+type testSlogSink struct {
+	attrs  []slog.Attr
+	groups []string
+
+	fnHandle    func(l *testLogSink, ctx context.Context, record slog.Record)
+	fnWithAttrs func(l *testLogSink, attrs []slog.Attr)
+	fnWithGroup func(l *testLogSink, name string)
+}
+
+func (l *testLogSink) Handle(ctx context.Context, record slog.Record) error {
+	if l.fnHandle != nil {
+		l.fnHandle(l, ctx, record)
+	}
+	return nil
+}
+
+func (l *testLogSink) WithAttrs(attrs []slog.Attr) SlogSink {
+	if l.fnWithAttrs != nil {
+		l.fnWithAttrs(l, attrs)
+	}
+	out := *l
+	n := len(out.attrs)
+	out.attrs = append(out.attrs[:n:n], attrs...)
+	return &out
+}
+
+func (l *testLogSink) WithGroup(name string) SlogSink {
+	if l.fnWithGroup != nil {
+		l.fnWithGroup(l, name)
+	}
+	out := *l
+	n := len(out.groups)
+	out.groups = append(out.groups[:n:n], name)
+	return &out
+}
+
+func withAttrs(record slog.Record, attrs ...slog.Attr) slog.Record {
+	record = record.Clone()
+	record.AddAttrs(attrs...)
+	return record
+}
+
+func toJSON(record slog.Record) string {
+	var buffer bytes.Buffer
+	record.Time = time.Time{}
+	handler := slog.NewJSONHandler(&buffer, nil)
+	if err := handler.Handle(context.Background(), record); err != nil {
+		return err.Error()
+	}
+	return buffer.String()
+}
+
+func TestToSlogHandler(t *testing.T) {
+	lvlThreshold := 0
+	actualCalledHandle := 0
+	var actualRecord slog.Record
+
+	sink := &testLogSink{}
+	logger := New(sink)
+
+	sink.fnEnabled = func(lvl int) bool {
+		return lvl <= lvlThreshold
+	}
+
+	sink.fnHandle = func(l *testLogSink, ctx context.Context, record slog.Record) {
+		actualCalledHandle++
+
+		// Combine attributes from sink and call. Ordering of WithValues and WithAttrs
+		// is wrong, but good enough for test cases.
+		var values slog.Record
+		values.Add(l.withValues...)
+		var attrs []any
+		add := func(attr slog.Attr) bool {
+			attrs = append(attrs, attr)
+			return true
+		}
+		values.Attrs(add)
+		record.Attrs(add)
+		for _, attr := range l.attrs {
+			attrs = append(attrs, attr)
+		}
+
+		// Wrap them in groups - not quite correct for WithValues that
+		// follows WithGroup, but good enough for test cases.
+		for i := len(l.groups) - 1; i >= 0; i-- {
+			attrs = []any{slog.Group(l.groups[i], attrs...)}
+		}
+
+		actualRecord = slog.Record{
+			Level:   record.Level,
+			Message: record.Message,
+		}
+		actualRecord.Add(attrs...)
+	}
+
+	verify := func(t *testing.T, expectedRecord slog.Record) {
+		actual := toJSON(actualRecord)
+		expected := toJSON(expectedRecord)
+		if expected != actual {
+			t.Errorf("JSON dump did not match, expected:\n%s\nGot:\n%s\n", expected, actual)
+		}
+	}
+
+	reset := func() {
+		lvlThreshold = 0
+		actualCalledHandle = 0
+		actualRecord = slog.Record{}
+	}
+
+	testcases := map[string]struct {
+		run            func()
+		expectedRecord slog.Record
+	}{
+		"simple": {
+			func() { slog.New(ToSlogHandler(logger)).Info("simple") },
+			slog.Record{Message: "simple"},
+		},
+
+		"disabled": {
+			func() { slog.New(ToSlogHandler(logger.V(1))).Info("") },
+			slog.Record{},
+		},
+
+		"enabled": {
+			func() {
+				lvlThreshold = 1
+				slog.New(ToSlogHandler(logger.V(1))).Info("enabled")
+			},
+			slog.Record{Level: -1, Message: "enabled"},
+		},
+
+		"error": {
+			func() { slog.New(ToSlogHandler(logger.V(100))).Error("error") },
+			slog.Record{Level: slog.LevelError, Message: "error"},
+		},
+
+		"with-parameters": {
+			func() { slog.New(ToSlogHandler(logger)).Info("", "answer", 42, "foo", "bar") },
+			withAttrs(slog.Record{}, slog.Int("answer", 42), slog.String("foo", "bar")),
+		},
+
+		"with-values": {
+			func() { slog.New(ToSlogHandler(logger.WithValues("answer", 42, "foo", "bar"))).Info("") },
+			withAttrs(slog.Record{}, slog.Int("answer", 42), slog.String("foo", "bar")),
+		},
+
+		"with-group": {
+			func() { slog.New(ToSlogHandler(logger)).WithGroup("group").Info("", "answer", 42, "foo", "bar") },
+			withAttrs(slog.Record{}, slog.Group("group", slog.Int("answer", 42), slog.String("foo", "bar"))),
+		},
+
+		"with-values-and-group": {
+			func() {
+				slog.New(ToSlogHandler(logger.WithValues("answer", 42, "foo", "bar"))).WithGroup("group").Info("")
+			},
+			// Behavior of testLogSink is not quite correct here.
+			withAttrs(slog.Record{}, slog.Group("group", slog.Int("answer", 42), slog.String("foo", "bar"))),
+		},
+
+		"with-group-and-values": {
+			func() {
+				slog.New(ToSlogHandler(logger)).WithGroup("group").With("answer", 42, "foo", "bar").Info("")
+			},
+			withAttrs(slog.Record{}, slog.Group("group", slog.Int("answer", 42), slog.String("foo", "bar"))),
+		},
+
+		"with-group-and-logr-values": {
+			func() {
+				slogLogger := slog.New(ToSlogHandler(logger)).WithGroup("group")
+				logrLogger := FromSlogHandler(slogLogger.Handler()).WithValues("answer", 42, "foo", "bar")
+				slogLogger = slog.New(ToSlogHandler(logrLogger))
+				slogLogger.Info("")
+			},
+			withAttrs(slog.Record{}, slog.Group("group", slog.Int("answer", 42), slog.String("foo", "bar"))),
+		},
+	}
+
+	for name, tc := range testcases {
+		t.Run(name, func(t *testing.T) {
+			tc.run()
+			verify(t, tc.expectedRecord)
+			reset()
+		})
+	}
+}

logr_test.go

@@ -33,6 +33,13 @@ type testLogSink struct {
 	fnError      func(err error, msg string, kv ...any)
 	fnWithValues func(kv ...any)
 	fnWithName   func(name string)
+
+	withValues []any
+
+	// testSlogSink contains some additional fields if (and only if) slog is supported by Go.
+	// See logr_slog_test.go.
+	//nolint:unused // Only unused with Go < 1.21.
+	testSlogSink
 }
 
 var _ LogSink = &testLogSink{}
@@ -67,6 +74,8 @@ func (l *testLogSink) WithValues(kv ...any) LogSink {
 		l.fnWithValues(kv...)
 	}
 	out := *l
+	n := len(out.withValues)
+	out.withValues = append(out.withValues[:n:n], kv...)
 	return &out
 }