util/ringbuffer: rename to ringlog

I need a ringbuffer in the more traditional sense, one that has a notion
of item removal as well as tail loss on overrun. This implementation is
really a clearable log window, and is used as such where it is used.

Updates #cleanup
Updates tailscale/corp#31762

Signed-off-by: James Tucker <james@tailscale.com>

util/ringlog/ringlog_test.go

@@ -0,0 +1,55 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ringlog
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestRingLog(t *testing.T) {
+	const numItems = 10
+	rb := New[int](numItems)
+
+	for i := range numItems - 1 {
+		rb.Add(i)
+	}
+
+	t.Run("NotFull", func(t *testing.T) {
+		if ll := rb.Len(); ll != numItems-1 {
+			t.Fatalf("got len %d; want %d", ll, numItems-1)
+		}
+		all := rb.GetAll()
+		want := []int{0, 1, 2, 3, 4, 5, 6, 7, 8}
+		if !reflect.DeepEqual(all, want) {
+			t.Fatalf("items mismatch\ngot: %v\nwant %v", all, want)
+		}
+	})
+
+	t.Run("Full", func(t *testing.T) {
+		// Append items to evict something
+		rb.Add(98)
+		rb.Add(99)
+
+		if ll := rb.Len(); ll != numItems {
+			t.Fatalf("got len %d; want %d", ll, numItems)
+		}
+		all := rb.GetAll()
+		want := []int{1, 2, 3, 4, 5, 6, 7, 8, 98, 99}
+		if !reflect.DeepEqual(all, want) {
+			t.Fatalf("items mismatch\ngot: %v\nwant %v", all, want)
+		}
+	})
+
+	t.Run("Clear", func(t *testing.T) {
+		rb.Clear()
+		if ll := rb.Len(); ll != 0 {
+			t.Fatalf("got len %d; want 0", ll)
+		}
+		all := rb.GetAll()
+		if len(all) != 0 {
+			t.Fatalf("got non-empty list; want empty")
+		}
+	})
+}