👞 clog

Context-aware slog

slog was added in Go 1.21, so using this requires Go 1.21 or later.

Usage

Context Logger

The context Logger can be used to use Loggers from the context. This is sometimes preferred over the Context Handler, since this can make it easier to use different loggers in different contexts (e.g. testing).

This approach is heavily inspired by knative.dev/pkg/logging, but with zero dependencies outside the standard library (compare with pkg/logging's deps).

package main

import (
	"context"
	"log/slog"

	"github.com/chainguard-dev/clog"
)

func main() {
	// One-time setup
	log := clog.New(slog.Default().Handler()).With("a", "b")
	ctx := clog.WithLogger(context.Background(), log)

	f(ctx)
}

func f(ctx context.Context) {
	// Grab logger from context and use.
	log := clog.FromContext(ctx)
	log.Info("in f")

	// Add logging context and pass on.
	ctx = clog.WithLogger(ctx, log.With("f", "hello"))
	g(ctx)
}

func g(ctx context.Context) {
	// Grab logger from context and use.
	log := clog.FromContext(ctx)
	log.Info("in g")

	// Package level context loggers are also aware
	clog.ErrorContext(ctx, "asdf")
}

$ go run .
2009/11/10 23:00:00 INFO in f a=b
2009/11/10 23:00:00 INFO in g a=b f=hello
2009/11/10 23:00:00 ERROR asdf a=b f=hello

Testing

The slogtest package provides utilities to make it easy to create loggers that will use the native testing logging.

func TestFoo(t *testing.T) {
	ctx := slogtest.TestContextWithLogger(t)

	for _, tc := range []string{"a", "b"} {
		t.Run(tc, func(t *testing.T) {
			clog.FromContext(ctx).Infof("hello world")
		})
	}
}

$ go test -v ./examples/logger
=== RUN   TestLog
=== RUN   TestLog/a
=== NAME  TestLog
    slogtest.go:20: time=2023-12-12T18:42:53.020-05:00 level=INFO msg="hello world"

=== RUN   TestLog/b
=== NAME  TestLog
    slogtest.go:20: time=2023-12-12T18:42:53.020-05:00 level=INFO msg="hello world"

--- PASS: TestLog (0.00s)
    --- PASS: TestLog/a (0.00s)
    --- PASS: TestLog/b (0.00s)
PASS
ok      github.com/chainguard-dev/clog/examples/logger

Context Handler

The context Handler can be used to insert values from the context.

func init() {
	slog.SetDefault(slog.New(clog.NewHandler(slog.NewTextHandler(os.Stdout, nil))))
}

func main() {
	ctx := context.Background()
	ctx = clog.WithValues(ctx, "foo", "bar")

	// Use slog package directly
	slog.InfoContext(ctx, "hello world", slog.Bool("baz", true))

	// glog / zap style (note: can't pass additional attributes)
	clog.ErrorContextf(ctx, "hello %s", "world")
}

$ go run .
time=2009-11-10T23:00:00.000Z level=INFO msg="hello world" baz=true foo=bar
time=2009-11-10T23:00:00.000Z level=ERROR msg="hello world" foo=bar

Google Cloud Platform support

This package also provides a GCP-optimized JSON handler for structured logging and trace attribution.

See ./gcp/README.md for details.