refactor snapper & support cron-based snapshotting

fixes https://github.com/zrepl/zrepl/issues/554
refs https://github.com/zrepl/zrepl/discussions/547#discussioncomment-1936126

daemon/snapper/cron_test.go

@@ -0,0 +1,68 @@
+package snapper
+
+import (
+	"fmt"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/zrepl/yaml-config"
+
+	"github.com/zrepl/zrepl/config"
+)
+
+func TestCronLibraryWorks(t *testing.T) {
+
+	type testCase struct {
+		spec   string
+		in     time.Time
+		expect time.Time
+	}
+	dhm := func(day, hour, minutes int) time.Time {
+		return time.Date(2022, 7, day, hour, minutes, 0, 0, time.UTC)
+	}
+	hm := func(hour, minutes int) time.Time {
+		return dhm(23, hour, minutes)
+	}
+
+	tcs := []testCase{
+		{"0-10 * * * *", dhm(17, 1, 10), dhm(17, 2, 0)},
+		{"0-10 * * * *", dhm(17, 23, 10), dhm(18, 0, 0)},
+		{"0-10 * * * *", hm(1, 9), hm(1, 10)},
+		{"0-10 * * * *", hm(1, 9), hm(1, 10)},
+
+		{"1,3,5 * * * *", hm(1, 1), hm(1, 3)},
+		{"1,3,5 * * * *", hm(1, 2), hm(1, 3)},
+		{"1,3,5 * * * *", hm(1, 3), hm(1, 5)},
+		{"1,3,5 * * * *", hm(1, 5), hm(2, 1)},
+
+		{"* 0-5,8,12 * * *", hm(0, 0), hm(0, 1)},
+		{"* 0-5,8,12 * * *", hm(4, 59), hm(5, 0)},
+		{"* 0-5,8,12 * * *", hm(5, 0), hm(5, 1)},
+		{"* 0-5,8,12 * * *", hm(5, 59), hm(8, 0)},
+		{"* 0-5,8,12 * * *", hm(8, 59), hm(12, 0)},
+
+		// https://github.com/zrepl/zrepl/pull/614#issuecomment-1188358989
+		{"53 17,18,19 * * *", dhm(23, 17, 52), dhm(23, 17, 53)},
+		{"53 17,18,19 * * *", dhm(23, 17, 53), dhm(23, 18, 53)},
+		{"53 17,18,19 * * *", dhm(23, 18, 53), dhm(23, 19, 53)},
+		{"53 17,18,19 * * *", dhm(23, 19, 53), dhm(24 /* ! */, 17, 53)},
+	}
+
+	for i, tc := range tcs {
+		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
+			var s struct {
+				Cron config.CronSpec `yaml:"cron"`
+			}
+			inp := fmt.Sprintf("cron: %q", tc.spec)
+			fmt.Println("spec is ", inp)
+			err := yaml.UnmarshalStrict([]byte(inp), &s)
+			require.NoError(t, err)
+
+			actual := s.Cron.Schedule.Next(tc.in)
+			assert.Equal(t, tc.expect, actual)
+		})
+	}
+
+}