testing/slogtest: tests for slog handlers

Add a package for testing that a slog.Handler implementation
satisfies that interface's documented requirements.

Code copied from x/exp/slog/slogtest.

Updates #56345.

Change-Id: I89e94d93bfbe58e3c524758f7ac3c3fba2a2ea96
Reviewed-on: https://go-review.googlesource.com/c/go/+/487895
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

2 files changed, 352 insertions, 0 deletions

diff --git a/src/testing/slogtest/example_test.go b/src/testing/slogtest/example_test.go
new file mode 100644
index 0000000000..61e4b46e12
--- /dev/null
+++ b/src/testing/slogtest/example_test.go
@@ -0,0 +1,44 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slogtest_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"log"
+	"log/slog"
+	"testing/slogtest"
+)
+
+// This example demonstrates one technique for testing a handler with this
+// package. The handler is given a [bytes.Buffer] to write to, and each line
+// of the resulting output is parsed.
+// For JSON output, [encoding/json.Unmarshal] produces a result in the desired
+// format when given a pointer to a map[string]any.
+func Example_parsing() {
+	var buf bytes.Buffer
+	h := slog.NewJSONHandler(&buf)
+
+	results := func() []map[string]any {
+		var ms []map[string]any
+		for _, line := range bytes.Split(buf.Bytes(), []byte{'\n'}) {
+			if len(line) == 0 {
+				continue
+			}
+			var m map[string]any
+			if err := json.Unmarshal(line, &m); err != nil {
+				panic(err) // In a real test, use t.Fatal.
+			}
+			ms = append(ms, m)
+		}
+		return ms
+	}
+	err := slogtest.TestHandler(h, results)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+}
diff --git a/src/testing/slogtest/slogtest.go b/src/testing/slogtest/slogtest.go
new file mode 100644
index 0000000000..71076e52f4
--- /dev/null
+++ b/src/testing/slogtest/slogtest.go
@@ -0,0 +1,308 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package slogtest implements support for testing implementations of log/slog.Handler.
+package slogtest
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"reflect"
+	"runtime"
+	"time"
+)
+
+type testCase struct {
+	// If non-empty, explanation explains the violated constraint.
+	explanation string
+	// f executes a single log event using its argument logger.
+	// So that mkdescs.sh can generate the right description,
+	// the body of f must appear on a single line whose first
+	// non-whitespace characters are "l.".
+	f func(*slog.Logger)
+	// If mod is not nil, it is called to modify the Record
+	// generated by the Logger before it is passed to the Handler.
+	mod func(*slog.Record)
+	// checks is a list of checks to run on the result.
+	checks []check
+}
+
+// TestHandler tests a [slog.Handler].
+// If TestHandler finds any misbehaviors, it returns an error for each,
+// combined into a single error with errors.Join.
+//
+// TestHandler installs the given Handler in a [slog.Logger] and
+// makes several calls to the Logger's output methods.
+//
+// The results function is invoked after all such calls.
+// It should return a slice of map[string]any, one for each call to a Logger output method.
+// The keys and values of the map should correspond to the keys and values of the Handler's
+// output. Each group in the output should be represented as its own nested map[string]any.
+// The standard keys slog.TimeKey, slog.LevelKey and slog.MessageKey should be used.
+//
+// If the Handler outputs JSON, then calling [encoding/json.Unmarshal] with a `map[string]any`
+// will create the right data structure.
+//
+// If a Handler intentionally drops an attribute that is checked by a test,
+// then the results function should check for its absence and add it to the map it returns.
+func TestHandler(h slog.Handler, results func() []map[string]any) error {
+	cases := []testCase{
+		{
+			explanation: withSource("this test expects slog.TimeKey, slog.LevelKey and slog.MessageKey"),
+			f: func(l *slog.Logger) {
+				l.Info("message")
+			},
+			checks: []check{
+				hasKey(slog.TimeKey),
+				hasKey(slog.LevelKey),
+				hasAttr(slog.MessageKey, "message"),
+			},
+		},
+		{
+			explanation: withSource("a Handler should output attributes passed to the logging function"),
+			f: func(l *slog.Logger) {
+				l.Info("message", "k", "v")
+			},
+			checks: []check{
+				hasAttr("k", "v"),
+			},
+		},
+		{
+			explanation: withSource("a Handler should ignore an empty Attr"),
+			f: func(l *slog.Logger) {
+				l.Info("msg", "a", "b", "", nil, "c", "d")
+			},
+			checks: []check{
+				hasAttr("a", "b"),
+				missingKey(""),
+				hasAttr("c", "d"),
+			},
+		},
+		{
+			explanation: withSource("a Handler should ignore a zero Record.Time"),
+			f: func(l *slog.Logger) {
+				l.Info("msg", "k", "v")
+			},
+			mod: func(r *slog.Record) { r.Time = time.Time{} },
+			checks: []check{
+				missingKey(slog.TimeKey),
+			},
+		},
+		{
+			explanation: withSource("a Handler should include the attributes from the WithAttrs method"),
+			f: func(l *slog.Logger) {
+				l.With("a", "b").Info("msg", "k", "v")
+			},
+			checks: []check{
+				hasAttr("a", "b"),
+				hasAttr("k", "v"),
+			},
+		},
+		{
+			explanation: withSource("a Handler should handle Group attributes"),
+			f: func(l *slog.Logger) {
+				l.Info("msg", "a", "b", slog.Group("G", slog.String("c", "d")), "e", "f")
+			},
+			checks: []check{
+				hasAttr("a", "b"),
+				inGroup("G", hasAttr("c", "d")),
+				hasAttr("e", "f"),
+			},
+		},
+		{
+			explanation: withSource("a Handler should ignore an empty group"),
+			f: func(l *slog.Logger) {
+				l.Info("msg", "a", "b", slog.Group("G"), "e", "f")
+			},
+			checks: []check{
+				hasAttr("a", "b"),
+				missingKey("G"),
+				hasAttr("e", "f"),
+			},
+		},
+		{
+			explanation: withSource("a Handler should inline the Attrs of a group with an empty key"),
+			f: func(l *slog.Logger) {
+				l.Info("msg", "a", "b", slog.Group("", slog.String("c", "d")), "e", "f")
+
+			},
+			checks: []check{
+				hasAttr("a", "b"),
+				hasAttr("c", "d"),
+				hasAttr("e", "f"),
+			},
+		},
+		{
+			explanation: withSource("a Handler should handle the WithGroup method"),
+			f: func(l *slog.Logger) {
+				l.WithGroup("G").Info("msg", "a", "b")
+			},
+			checks: []check{
+				hasKey(slog.TimeKey),
+				hasKey(slog.LevelKey),
+				hasAttr(slog.MessageKey, "msg"),
+				missingKey("a"),
+				inGroup("G", hasAttr("a", "b")),
+			},
+		},
+		{
+			explanation: withSource("a Handler should handle multiple WithGroup and WithAttr calls"),
+			f: func(l *slog.Logger) {
+				l.With("a", "b").WithGroup("G").With("c", "d").WithGroup("H").Info("msg", "e", "f")
+			},
+			checks: []check{
+				hasKey(slog.TimeKey),
+				hasKey(slog.LevelKey),
+				hasAttr(slog.MessageKey, "msg"),
+				hasAttr("a", "b"),
+				inGroup("G", hasAttr("c", "d")),
+				inGroup("G", inGroup("H", hasAttr("e", "f"))),
+			},
+		},
+		{
+			explanation: withSource("a Handler should call Resolve on attribute values"),
+			f: func(l *slog.Logger) {
+				l.Info("msg", "k", &replace{"replaced"})
+			},
+			checks: []check{hasAttr("k", "replaced")},
+		},
+		{
+			explanation: withSource("a Handler should call Resolve on attribute values in groups"),
+			f: func(l *slog.Logger) {
+				l.Info("msg",
+					slog.Group("G",
+						slog.String("a", "v1"),
+						slog.Any("b", &replace{"v2"})))
+			},
+			checks: []check{
+				inGroup("G", hasAttr("a", "v1")),
+				inGroup("G", hasAttr("b", "v2")),
+			},
+		},
+		{
+			explanation: withSource("a Handler should call Resolve on attribute values from WithAttrs"),
+			f: func(l *slog.Logger) {
+				l = l.With("k", &replace{"replaced"})
+				l.Info("msg")
+			},
+			checks: []check{hasAttr("k", "replaced")},
+		},
+		{
+			explanation: withSource("a Handler should call Resolve on attribute values in groups from WithAttrs"),
+			f: func(l *slog.Logger) {
+				l = l.With(slog.Group("G",
+					slog.String("a", "v1"),
+					slog.Any("b", &replace{"v2"})))
+				l.Info("msg")
+			},
+			checks: []check{
+				inGroup("G", hasAttr("a", "v1")),
+				inGroup("G", hasAttr("b", "v2")),
+			},
+		},
+	}
+
+	// Run the handler on the test cases.
+	for _, c := range cases {
+		ht := h
+		if c.mod != nil {
+			ht = &wrapper{h, c.mod}
+		}
+		l := slog.New(ht)
+		c.f(l)
+	}
+
+	// Collect and check the results.
+	var errs []error
+	res := results()
+	if g, w := len(res), len(cases); g != w {
+		return fmt.Errorf("got %d results, want %d", g, w)
+	}
+	for i, got := range results() {
+		c := cases[i]
+		for _, check := range c.checks {
+			if p := check(got); p != "" {
+				errs = append(errs, fmt.Errorf("%s: %s", p, c.explanation))
+			}
+		}
+	}
+	return errors.Join(errs...)
+}
+
+type check func(map[string]any) string
+
+func hasKey(key string) check {
+	return func(m map[string]any) string {
+		if _, ok := m[key]; !ok {
+			return fmt.Sprintf("missing key %q", key)
+		}
+		return ""
+	}
+}
+
+func missingKey(key string) check {
+	return func(m map[string]any) string {
+		if _, ok := m[key]; ok {
+			return fmt.Sprintf("unexpected key %q", key)
+		}
+		return ""
+	}
+}
+
+func hasAttr(key string, wantVal any) check {
+	return func(m map[string]any) string {
+		if s := hasKey(key)(m); s != "" {
+			return s
+		}
+		gotVal := m[key]
+		if !reflect.DeepEqual(gotVal, wantVal) {
+			return fmt.Sprintf("%q: got %#v, want %#v", key, gotVal, wantVal)
+		}
+		return ""
+	}
+}
+
+func inGroup(name string, c check) check {
+	return func(m map[string]any) string {
+		v, ok := m[name]
+		if !ok {
+			return fmt.Sprintf("missing group %q", name)
+		}
+		g, ok := v.(map[string]any)
+		if !ok {
+			return fmt.Sprintf("value for group %q is not map[string]any", name)
+		}
+		return c(g)
+	}
+}
+
+type wrapper struct {
+	slog.Handler
+	mod func(*slog.Record)
+}
+
+func (h *wrapper) Handle(ctx context.Context, r slog.Record) error {
+	h.mod(&r)
+	return h.Handler.Handle(ctx, r)
+}
+
+func withSource(s string) string {
+	_, file, line, ok := runtime.Caller(1)
+	if !ok {
+		panic("runtime.Caller failed")
+	}
+	return fmt.Sprintf("%s (%s:%d)", s, file, line)
+}
+
+type replace struct {
+	v any
+}
+
+func (r *replace) LogValue() slog.Value { return slog.AnyValue(r.v) }
+
+func (r *replace) String() string {
+	return fmt.Sprintf("<replace(%v)>", r.v)
+}