support a slog.Logger pointer in a context

Conversion is done on demand. A program which uses either slog or logr for
logging remains as efficient as before. What is less efficient is storing a
logger of one type and then retrieving it as the other.

Ideally, reusable packages should avoid using a slog.Logger and prefer
logr.Logger instead when retrieving a logger from a context. There is a
significant amount of packages which already work that way (Kubernetes and
related packages), while storing and retrieving a logr.Logger is new and not
done anywhere yet. Emulating existing behavior ensures better performance.

README.md

@@ -91,6 +91,7 @@ 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
@@ -208,24 +209,36 @@ with logr implementations without slog support is not important, then
 ### 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 NewSlogHandler(logger)
-        }
-        return slog.Default().Handler()
-    }
-
-    func ContextWithHandler(ctx context.Context, handler slog.Handler) context.Context {
-        return logr.NewContext(ctx, 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 NewFromSlogHandler(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(NewSlogHandler(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)
+}

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 {