slogr: add glue code for logging to slog.Handler and with slog.Logger

[pohly]

Interoperability with log/slog from Go 1.21 includes the ability to use a slog.Handler as backend with logr.Logger as frontend and vice versa.

This is only the initial step. In particular writing with slog.Logger to a logr.LogSink is not working well (time stamp and call site from record get ignored). Further work is needed to improve this.

[pohly]

Tests cover 100% of the new code lines. I also tried to consider edge cases.

[thockin]

I am AFK for a few days, will look more when I get back

[thockin]

First commit

[thockin]

I like having runnable examples for quick-testing. Worthing including?

diff --git a/slogr/slog_example/main.go b/slogr/slog_example/main.go
new file mode 100644
index 0000000..2e67539
--- /dev/null
+++ b/slogr/handler_example/main.go
@@ -0,0 +1,65 @@
+/*
+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 main is an example of using funcr.
+package main
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+
+	"github.com/go-logr/logr/funcr"
+	"github.com/go-logr/logr/slogr"
+)
+
+type e struct {
+	str string
+}
+
+func (e e) Error() string {
+	return e.str
+}
+
+func helper(log *slog.Logger, msg string) {
+	helper2(log, msg)
+}
+
+func helper2(log *slog.Logger, msg string) {
+	log.Info(msg)
+}
+
+func main() {
+	logrLogger := funcr.New(
+		func(pfx, args string) { fmt.Println(pfx, args) },
+		funcr.Options{
+			LogCaller:    funcr.All,
+			LogTimestamp: true,
+			Verbosity:    1,
+		})
+	log := slog.New(slogr.NewSlogHandler(logrLogger))
+	example(log)
+}
+
+func example(log *slog.Logger) {
+	log = log.With("saved", "value")
+	log.Info("1) hello", "val1", 1, "val2", map[string]int{"k": 1})
+	log.Log(context.TODO(), slog.Level(-1), "2) you should see this")
+	log.Log(context.TODO(), slog.Level(-2), "you should NOT see this")
+	log.Error("3) uh oh", "trouble", true, "reasons", []float64{0.1, 0.11, 3.14})
+	log.Error("4) goodbye", "code", -1, "err", e{"an error occurred"})
+	helper(log, "5) thru a helper")
+}

In particular, this lets us verify caller information visually. I like your tests, too, and maybe we don't need both?

[pohly]

I like having runnable examples for quick-testing. Worthing including?

Yes. I merged it with the example for the other direction #205 (comment) because some parts are common (error) and this way it's easier to see both approaches without having to open two files side-by-side.

Initially, the slog.Logger calls recorded the wrong logger.go from the slog package as caller. This is inevitable, but at least for the current Go 1.21.0 and simple usage (i.e. slog.Logger directly calls the handler) it can be compensated through WithCallDepth calls - I've added that.

I like your tests, too, and maybe we don't need both?

Both = example and tests? I think we want (example) and need (tests) both.

[thockin]

Did you forget to add the example?

[thockin]

First commit only

[pohly]

Did you forget to add the example?

Yes 😢

Fixed.

[thockin]

Sort of a nit, but the "gets negated" is very precise. Can we write this a bit more loosely, like:

"""
NewLogr returns a logr.Logger which writes to the slog.Handler. The logr verbosity level is mapped to slog levels such that V(0) becomes slog.LevelInfo and V(4) becomes slog.LevelDebug.
"""

[pohly]

Okay.

[thockin]

closer!

[thockin]

rename to levelBias ?

[pohly]

Done, and added documentation, also for groupPrefix.

[thockin]

NewJSON for equivalence?

[pohly]

Done.

[thockin]

I don't understand this line - Warning is 4, but the comment above says slog.LevelError which is 8, right?

[pohly]

It was simply wrong: I mis-remembered and thought that Warning >= Error.

Because Info < Warning < Error, slog.Logger.Warning can be used as example for a level that gets mapped to the same logr.Level 0 - changed.

[thockin]

Just that one comment I don't understand

[thockin]

merged!