refactor snapper & support cron-based snapshotting

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

config/config.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/pkg/errors"
+	"github.com/robfig/cron/v3"
 	"github.com/zrepl/yaml-config"
 
 	"github.com/zrepl/zrepl/util/datasizeunit"
@@ -233,6 +234,38 @@ type SnapshottingPeriodic struct {
 	Hooks    HookList      `yaml:"hooks,optional"`
 }
 
+type CronSpec struct {
+	Schedule cron.Schedule
+}
+
+var _ yaml.Unmarshaler = &CronSpec{}
+
+func (s *CronSpec) UnmarshalYAML(unmarshal func(v interface{}, not_strict bool) error) error {
+	var specString string
+	if err := unmarshal(&specString, false); err != nil {
+		return err
+	}
+
+	// Use standard cron format.
+	// Disable the various "descriptors" (@daily, etc)
+	// They are just aliases to "top of hour", "midnight", etc.
+	parser := cron.NewParser(cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.SecondOptional)
+
+	sched, err := parser.Parse(specString)
+	if err != nil {
+		return errors.Wrap(err, "cron syntax invalid")
+	}
+	s.Schedule = sched
+	return nil
+}
+
+type SnapshottingCron struct {
+	Type   string   `yaml:"type"`
+	Prefix string   `yaml:"prefix"`
+	Cron   CronSpec `yaml:"cron"`
+	Hooks  HookList `yaml:"hooks,optional"`
+}
+
 type SnapshottingManual struct {
 	Type string `yaml:"type"`
 }
@@ -556,6 +589,7 @@ func (t *SnapshottingEnum) UnmarshalYAML(u func(interface{}, bool) error) (err e
 	t.Ret, err = enumUnmarshal(u, map[string]interface{}{
 		"periodic": &SnapshottingPeriodic{},
 		"manual":   &SnapshottingManual{},
+		"cron":     &SnapshottingCron{},
 	})
 	return
 }

config/config_test.go

@@ -13,6 +13,7 @@ import (
 	"github.com/kr/pretty"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"github.com/zrepl/yaml-config"
 )
 
 func TestSampleConfigsAreParsedWithoutErrors(t *testing.T) {
@@ -86,3 +87,53 @@ func TestTrimSpaceEachLineAndPad(t *testing.T) {
 	`
 	assert.Equal(t, "  \n  foo\n  bar baz\n  \n", trimSpaceEachLineAndPad(foo, "  "))
 }
+
+func TestCronSpec(t *testing.T) {
+
+	expectAccept := []string{
+		`"* * * * *"`,
+		`"0-10 * * * *"`,
+		`"* 0-5,8,12 * * *"`,
+	}
+
+	expectFail := []string{
+		`* * * *`,
+		``,
+		`23`,
+		`"@reboot"`,
+		`"@every 1h30m"`,
+		`"@daily"`,
+		`* * * * * *`,
+	}
+
+	for _, input := range expectAccept {
+		t.Run(input, func(t *testing.T) {
+			s := fmt.Sprintf("spec: %s\n", input)
+			var v struct {
+				Spec CronSpec
+			}
+			v.Spec.Schedule = nil
+			t.Logf("input:\n%s", s)
+			err := yaml.UnmarshalStrict([]byte(s), &v)
+			t.Logf("error: %T %s", err, err)
+			require.NoError(t, err)
+			require.NotNil(t, v.Spec.Schedule)
+		})
+	}
+
+	for _, input := range expectFail {
+		t.Run(input, func(t *testing.T) {
+			s := fmt.Sprintf("spec: %s\n", input)
+			var v struct {
+				Spec CronSpec
+			}
+			v.Spec.Schedule = nil
+			t.Logf("input: %q", s)
+			err := yaml.UnmarshalStrict([]byte(s), &v)
+			t.Logf("error: %T %s", err, err)
+			require.Error(t, err)
+			require.Nil(t, v.Spec.Schedule)
+		})
+	}
+
+}

config/samples/snap_cron.yml

@@ -0,0 +1,14 @@
+jobs:
+- name: snapjob
+  type: snap
+  filesystems: {
+    "tank<": true,
+  }
+  snapshotting:
+    type: cron
+    prefix: zrepl_snapjob_
+    cron: "*/5 * * * *"
+  pruning:
+    keep:
+      - type: last_n
+        count: 60