From 42bc65abad3d7d0633fe525e1a0013bdc2c99780 Mon Sep 17 00:00:00 2001
From: Brandur <brandur@brandur.org>
Date: Sun, 7 Sep 2025 14:25:56 +0200
Subject: [PATCH] Add lazy slice formatters to `slogutil`

Adds a couple slice helpers to `slogutil` that implement `slog.LogValue`
to achieve the effect of formatting slices that are used as values in
slog attributes, but only when we actually need to do so because a slog
log line is being printed. Otherwise, `LogValue` is never called and we
avoid the associated work and allocations.
---
 rivershared/util/slogutil/slog_util.go      | 25 ++++++++
 rivershared/util/slogutil/slog_util_test.go | 70 +++++++++++++++++++++
 2 files changed, 95 insertions(+)
 create mode 100644 rivershared/util/slogutil/slog_util_test.go

diff --git a/rivershared/util/slogutil/slog_util.go b/rivershared/util/slogutil/slog_util.go
index b452b698..bc341ee4 100644
--- a/rivershared/util/slogutil/slog_util.go
+++ b/rivershared/util/slogutil/slog_util.go
@@ -6,8 +6,33 @@ import (
 	"io"
 	"log/slog"
 	"os"
+	"strconv"
+	"strings"
+
+	"github.com/riverqueue/river/rivershared/util/sliceutil"
 )
 
+// SliceInt64 is a type that implements slog.LogValue and which will format a
+// slice for inclusion in logging, but lazily so that no work is done unless a
+// log line is actually emitted.
+type SliceInt64 []int64
+
+func (s SliceInt64) LogValue() slog.Value {
+	return slog.StringValue(strings.Join(
+		sliceutil.Map(s, func(i int64) string { return strconv.FormatInt(i, 10) }),
+		",",
+	))
+}
+
+// SliceString is a type that implements slog.LogValue and which will format a
+// slice for inclusion in logging, but lazily so that no work is done unless a
+// log line is actually emitted.
+type SliceString []string
+
+func (s SliceString) LogValue() slog.Value {
+	return slog.StringValue(strings.Join(s, ","))
+}
+
 // SlogMessageOnlyHandler is a trivial slog handler that prints only messages.
 // All attributes and groups are ignored. It's useful in example tests where it
 // produces output that's normalized so we match against it (normally, all log
diff --git a/rivershared/util/slogutil/slog_util_test.go b/rivershared/util/slogutil/slog_util_test.go
new file mode 100644
index 00000000..e6af4601
--- /dev/null
+++ b/rivershared/util/slogutil/slog_util_test.go
@@ -0,0 +1,70 @@
+package slogutil
+
+import (
+	"bytes"
+	"log/slog"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSliceInt64(t *testing.T) {
+	t.Parallel()
+
+	t.Run("Empty", func(t *testing.T) {
+		t.Parallel()
+
+		logger, buf := plainLoggerAndBuffer()
+		logger.Info("log_entry", slog.Any("values", SliceInt64(nil)))
+
+		require.Equal(t, `msg=log_entry values=""`+"\n", buf.String())
+	})
+
+	t.Run("Values", func(t *testing.T) {
+		t.Parallel()
+
+		logger, buf := plainLoggerAndBuffer()
+		logger.Info("log_entry", slog.Any("values", SliceInt64([]int64{1, 2, 3})))
+
+		require.Equal(t, "msg=log_entry values=1,2,3\n", buf.String())
+	})
+}
+
+func TestSliceString(t *testing.T) {
+	t.Parallel()
+
+	t.Run("Empty", func(t *testing.T) {
+		t.Parallel()
+
+		logger, buf := plainLoggerAndBuffer()
+		logger.Info("log_entry", slog.Any("values", SliceString(nil)))
+
+		require.Equal(t, `msg=log_entry values=""`+"\n", buf.String())
+	})
+
+	t.Run("Values", func(t *testing.T) {
+		t.Parallel()
+
+		logger, buf := plainLoggerAndBuffer()
+		logger.Info("log_entry", slog.Any("values", SliceString([]string{"foo", "bar"})))
+
+		require.Equal(t, "msg=log_entry values=foo,bar\n", buf.String())
+	})
+}
+
+func plainLoggerAndBuffer() (*slog.Logger, *bytes.Buffer) {
+	var buf bytes.Buffer
+	return slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		// Removes the `level` and `time` keys so that we have clean and stable
+		// output to match against in assertions.
+		ReplaceAttr: func(groups []string, attr slog.Attr) slog.Attr {
+			if len(groups) < 1 {
+				switch attr.Key {
+				case slog.LevelKey, slog.TimeKey:
+					return slog.Attr{}
+				}
+			}
+			return attr
+		},
+	})), &buf
+}