slog context support

README.md

@@ -91,11 +91,12 @@ logr design but also left out some parts and changed others:
 | Adding a name to a logger | `WithName` | no API |
 | Modify verbosity of log entries in a call chain | `V` | no API |
 | Grouping of key/value pairs | not supported | `WithGroup`, `GroupValue` |
+| Pass context for extracting additional values | no API | API variants like `InfoCtx` |
 
 The high-level slog API is explicitly meant to be one of many different APIs
 that can be layered on top of a shared `slog.Handler`. logr is one such
-alternative API, with [interoperability](#slog-interoperability) provided by the [`slogr`](slogr)
-package.
+alternative API, with [interoperability](#slog-interoperability) provided by
+some conversion functions.
 
 ### Inspiration
 
@@ -145,24 +146,24 @@ There are implementations for the following logging libraries:
 ## slog interoperability
 
 Interoperability goes both ways, using the `logr.Logger` API with a `slog.Handler`
-and using the `slog.Logger` API with a `logr.LogSink`. [slogr](./slogr) provides `NewLogr` and
-`NewSlogHandler` API calls to convert between a `logr.Logger` and a `slog.Handler`.
+and using the `slog.Logger` API with a `logr.LogSink`. `FromSlogHandler` and
+`ToSlogHandler` convert between a `logr.Logger` and a `slog.Handler`.
 As usual, `slog.New` can be used to wrap such a `slog.Handler` in the high-level
-slog API. `slogr` itself leaves that to the caller.
+slog API.
 
-## Using a `logr.Sink` as backend for slog
+### Using a `logr.LogSink` as backend for slog
 
 Ideally, a logr sink implementation should support both logr and slog by
-implementing both the normal logr interface(s) and `slogr.SlogSink`.  Because
+implementing both the normal logr interface(s) and `SlogSink`.  Because
 of a conflict in the parameters of the common `Enabled` method, it is [not
 possible to implement both slog.Handler and logr.Sink in the same
 type](https://github.com/golang/go/issues/59110).
 
 If both are supported, log calls can go from the high-level APIs to the backend
-without the need to convert parameters. `NewLogr` and `NewSlogHandler` can
+without the need to convert parameters. `FromSlogHandler` and `ToSlogHandler` can
 convert back and forth without adding additional wrappers, with one exception:
 when `Logger.V` was used to adjust the verbosity for a `slog.Handler`, then
-`NewSlogHandler` has to use a wrapper which adjusts the verbosity for future
+`ToSlogHandler` has to use a wrapper which adjusts the verbosity for future
 log calls.
 
 Such an implementation should also support values that implement specific
@@ -187,13 +188,13 @@ Not supporting slog has several drawbacks:
 These drawbacks are severe enough that applications using a mixture of slog and
 logr should switch to a different backend.
 
-## Using a `slog.Handler` as backend for logr
+### Using a `slog.Handler` as backend for logr
 
 Using a plain `slog.Handler` without support for logr works better than the
 other direction:
 - All logr verbosity levels can be mapped 1:1 to their corresponding slog level
   by negating them.
-- Stack unwinding is done by the `slogr.SlogSink` and the resulting program
+- Stack unwinding is done by the `SlogSink` and the resulting program
   counter is passed to the `slog.Handler`.
 - Names added via `Logger.WithName` are gathered and recorded in an additional
   attribute with `logger` as key and the names separated by slash as value.
@@ -205,27 +206,39 @@ ideally support both `logr.Marshaler` and `slog.Valuer`. If compatibility
 with logr implementations without slog support is not important, then
 `slog.Valuer` is sufficient.
 
-## Context support for slog
+### Context support for slog
 
 Storing a logger in a `context.Context` is not supported by
-slog. `logr.NewContext` and `logr.FromContext` can be used with slog like this
-to fill this gap:
-
-    func HandlerFromContext(ctx context.Context) slog.Handler {
-        logger, err := logr.FromContext(ctx)
-        if err == nil {
-            return slogr.NewSlogHandler(logger)
-        }
-        return slog.Default().Handler()
-    }
-
-    func ContextWithHandler(ctx context.Context, handler slog.Handler) context.Context {
-        return logr.NewContext(ctx, slogr.NewLogr(handler))
-    }
-
-The downside is that storing and retrieving a `slog.Handler` needs more
-allocations compared to using a `logr.Logger`. Therefore the recommendation is
-to use the `logr.Logger` API in code which uses contextual logging.
+slog. `NewContextWithSlogLogger` and `FromContextAsSlogLogger` can be
+used to fill this gap. They store and retrieve a `slog.Logger` pointer
+under the same context key that is also used by `NewContext` and
+`FromContext` for `logr.Logger` value.
+
+When `NewContextWithSlogLogger` is followed by `FromContext`, the latter will
+automatically convert the `slog.Logger` to a
+`logr.Logger`. `FromContextAsSlogLogger` does the same for the other direction.
+
+With this approach, binaries which use either slog or logr are as efficient as
+possible with no unnecessary allocations. This is also why the API stores a
+`slog.Logger` pointer: when storing a `slog.Handler`, creating a `slog.Logger`
+on retrieval would need to allocate one.
+
+The downside is that switching back and forth needs more allocations. Because
+logr is the API that is already in use by different packages, in particular
+Kubernetes, the recommendation is to use the `logr.Logger` API in code which
+uses contextual logging.
+
+An alternative to adding values to a logger and storing that logger in the
+context is to store the values in the context and to configure a logging
+backend to extract those values when emitting log entries. This only works when
+log calls are passed the context, which is not supported by the logr API.
+
+With the slog API, it is possible, but not
+required. https://github.com/veqryn/slog-context is a package for slog which
+provides additional support code for this approach. It also contains wrappers
+for the context functions in logr, so developers who prefer to not use the logr
+APIs directly can use those instead and the resulting code will still be
+interoperable with logr.
 
 ## FAQ
 

context.go

@@ -0,0 +1,33 @@
+/*
+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
+
+// contextKey is how we find Loggers in a context.Context. With Go < 1.21,
+// the value is always a Logger value. With Go >= 1.21, the value can be a
+// Logger value or a slog.Logger pointer.
+type contextKey struct{}
+
+// notFoundError exists to carry an IsNotFound method.
+type notFoundError struct{}
+
+func (notFoundError) Error() string {
+	return "no logr.Logger was present"
+}
+
+func (notFoundError) IsNotFound() bool {
+	return true
+}

context_noslog.go

@@ -0,0 +1,49 @@
+//go:build !go1.21
+// +build !go1.21
+
+/*
+Copyright 2019 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 (
+	"context"
+)
+
+// FromContext returns a Logger from ctx or an error if no Logger is found.
+func FromContext(ctx context.Context) (Logger, error) {
+	if v, ok := ctx.Value(contextKey{}).(Logger); ok {
+		return v, nil
+	}
+
+	return Logger{}, notFoundError{}
+}
+
+// FromContextOrDiscard returns a Logger from ctx.  If no Logger is found, this
+// returns a Logger that discards all log messages.
+func FromContextOrDiscard(ctx context.Context) Logger {
+	if v, ok := ctx.Value(contextKey{}).(Logger); ok {
+		return v
+	}
+
+	return Discard()
+}
+
+// NewContext returns a new Context, derived from ctx, which carries the
+// provided Logger.
+func NewContext(ctx context.Context, logger Logger) context.Context {
+	return context.WithValue(ctx, contextKey{}, logger)
+}

context_slog.go

@@ -0,0 +1,83 @@
+//go:build go1.21
+// +build go1.21
+
+/*
+Copyright 2019 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 (
+	"context"
+	"fmt"
+	"log/slog"
+)
+
+// FromContext returns a Logger from ctx or an error if no Logger is found.
+func FromContext(ctx context.Context) (Logger, error) {
+	v := ctx.Value(contextKey{})
+	if v == nil {
+		return Logger{}, notFoundError{}
+	}
+
+	switch v := v.(type) {
+	case Logger:
+		return v, nil
+	case *slog.Logger:
+		return FromSlogHandler(v.Handler()), nil
+	default:
+		// Not reached.
+		panic(fmt.Sprintf("unexpected value type for logr context key: %T", v))
+	}
+}
+
+// FromContextAsSlogLogger returns a slog.Logger from ctx or nil if no such Logger is found.
+func FromContextAsSlogLogger(ctx context.Context) *slog.Logger {
+	v := ctx.Value(contextKey{})
+	if v == nil {
+		return nil
+	}
+
+	switch v := v.(type) {
+	case Logger:
+		return slog.New(ToSlogHandler(v))
+	case *slog.Logger:
+		return v
+	default:
+		// Not reached.
+		panic(fmt.Sprintf("unexpected value type for logr context key: %T", v))
+	}
+}
+
+// FromContextOrDiscard returns a Logger from ctx.  If no Logger is found, this
+// returns a Logger that discards all log messages.
+func FromContextOrDiscard(ctx context.Context) Logger {
+	if logger, err := FromContext(ctx); err == nil {
+		return logger
+	}
+	return Discard()
+}
+
+// NewContext returns a new Context, derived from ctx, which carries the
+// provided Logger.
+func NewContext(ctx context.Context, logger Logger) context.Context {
+	return context.WithValue(ctx, contextKey{}, logger)
+}
+
+// NewContextWithSlogLogger returns a new Context, derived from ctx, which carries the
+// provided slog.Logger.
+func NewContextWithSlogLogger(ctx context.Context, logger *slog.Logger) context.Context {
+	return context.WithValue(ctx, contextKey{}, logger)
+}

slogr/example/main.go → examples/slog/main.go

@@ -28,7 +28,6 @@ import (
 
 	"github.com/go-logr/logr"
 	"github.com/go-logr/logr/funcr"
-	"github.com/go-logr/logr/slogr"
 )
 
 type e struct {
@@ -62,7 +61,7 @@ func main() {
 		Level:     slog.Level(-1),
 	}
 	handler := slog.NewJSONHandler(os.Stderr, &opts)
-	logrLogger := slogr.NewLogr(handler)
+	logrLogger := logr.FromSlogHandler(handler)
 	logrExample(logrLogger)
 
 	logrLogger = funcr.NewJSON(
@@ -72,7 +71,7 @@ func main() {
 			LogTimestamp: true,
 			Verbosity:    1,
 		})
-	slogLogger := slog.New(slogr.NewSlogHandler(logrLogger))
+	slogLogger := slog.New(logr.ToSlogHandler(logrLogger))
 	slogExample(slogLogger)
 }
 
@@ -91,7 +90,7 @@ func logrExample(log logr.Logger) {
 
 func slogExample(log *slog.Logger) {
 	// There's no guarantee that this logs the right source code location.
-	// It works for Go 1.21.0 by compensating in slogr.NewSlogHandler
+	// It works for Go 1.21.0 by compensating in logr.ToSlogHandler
 	// for the additional callers, but those might change.
 	log = log.With("saved", "value")
 	log.Info("1) hello", "val1", 1, "val2", map[string]int{"k": 1})

logr.go

@@ -207,10 +207,6 @@ limitations under the License.
 // those.
 package logr
 
-import (
-	"context"
-)
-
 // New returns a new Logger instance.  This is primarily used by libraries
 // implementing LogSink, rather than end users.  Passing a nil sink will create
 // a Logger which discards all log lines.
@@ -410,45 +406,6 @@ func (l Logger) IsZero() bool {
 	return l.sink == nil
 }
 
-// contextKey is how we find Loggers in a context.Context.
-type contextKey struct{}
-
-// FromContext returns a Logger from ctx or an error if no Logger is found.
-func FromContext(ctx context.Context) (Logger, error) {
-	if v, ok := ctx.Value(contextKey{}).(Logger); ok {
-		return v, nil
-	}
-
-	return Logger{}, notFoundError{}
-}
-
-// notFoundError exists to carry an IsNotFound method.
-type notFoundError struct{}
-
-func (notFoundError) Error() string {
-	return "no logr.Logger was present"
-}
-
-func (notFoundError) IsNotFound() bool {
-	return true
-}
-
-// FromContextOrDiscard returns a Logger from ctx.  If no Logger is found, this
-// returns a Logger that discards all log messages.
-func FromContextOrDiscard(ctx context.Context) Logger {
-	if v, ok := ctx.Value(contextKey{}).(Logger); ok {
-		return v
-	}
-
-	return Discard()
-}
-
-// NewContext returns a new Context, derived from ctx, which carries the
-// provided Logger.
-func NewContext(ctx context.Context, logger Logger) context.Context {
-	return context.WithValue(ctx, contextKey{}, logger)
-}
-
 // RuntimeInfo holds information that the logr "core" library knows which
 // LogSinks might want to know.
 type RuntimeInfo struct {