Add cron schedules and sync docs

agent/internal/config/config.go

@@ -11,6 +11,7 @@ import (
 	"unicode/utf8"
 
 	"github.com/BurntSushi/toml"
+	"github.com/robfig/cron/v3"
 )
 
 var idPattern = regexp.MustCompile(`^[A-Za-z0-9._:-]+$`)
@@ -58,6 +59,7 @@ type CheckConfig struct {
 	Name                 string         `toml:"name"`
 	Command              string         `toml:"command"`
 	Interval             Duration       `toml:"interval"`
+	Cron                 string         `toml:"cron"`
 	Timeout              Duration       `toml:"timeout"`
 	NotificationsEnabled *bool          `toml:"notifications_enabled"`
 	DedupeKey            string         `toml:"dedupe_key"`
@@ -211,14 +213,24 @@ func (c *Config) Validate() error {
 		if strings.TrimSpace(ch.Command) == "" {
 			return fmt.Errorf("check %q: command is required", ch.ID)
 		}
-		if ch.Interval.Duration <= 0 {
-			return fmt.Errorf("check %q: interval must be > 0", ch.ID)
-		}
 		if ch.Timeout.Duration <= 0 {
 			return fmt.Errorf("check %q: timeout must be > 0", ch.ID)
 		}
-		if ch.Timeout.Duration > ch.Interval.Duration {
+		if ch.Interval.Duration < 0 {
+			return fmt.Errorf("check %q: interval must be > 0", ch.ID)
+		}
+		ch.Cron = strings.TrimSpace(ch.Cron)
+		hasInterval := ch.Interval.Duration > 0
+		hasCron := ch.Cron != ""
+		switch {
+		case hasInterval == hasCron:
+			return fmt.Errorf("check %q: exactly one of interval or cron is required", ch.ID)
+		case hasInterval && ch.Timeout.Duration > ch.Interval.Duration:
 			return fmt.Errorf("check %q: timeout must be <= interval", ch.ID)
+		case hasCron:
+			if _, err := ParseCronSchedule(ch.Cron); err != nil {
+				return fmt.Errorf("check %q: invalid cron: %w", ch.ID, err)
+			}
 		}
 		if len(ch.DedupeKey) > 256 {
 			return fmt.Errorf("check %q: dedupe_key too long", ch.ID)
@@ -253,6 +265,42 @@ func (c CheckConfig) NotificationsOn() bool {
 	return c.NotificationsEnabled == nil || *c.NotificationsEnabled
 }
 
+func (c CheckConfig) UsesInterval() bool {
+	return c.Interval.Duration > 0
+}
+
+func (c CheckConfig) UsesCron() bool {
+	return strings.TrimSpace(c.Cron) != ""
+}
+
+func ParseCronSchedule(spec string) (cron.Schedule, error) {
+	spec = strings.TrimSpace(spec)
+	if spec == "" {
+		return nil, fmt.Errorf("cron is empty")
+	}
+	withoutTZ := specWithoutTZ(spec)
+	lower := strings.ToLower(withoutTZ)
+	if strings.HasPrefix(lower, "@every") {
+		return nil, fmt.Errorf("@every is not supported; use interval")
+	}
+	if lower == "@reboot" {
+		return nil, fmt.Errorf("@reboot is not supported")
+	}
+	return cron.ParseStandard(spec)
+}
+
+func specWithoutTZ(spec string) string {
+	fields := strings.Fields(spec)
+	if len(fields) == 0 {
+		return ""
+	}
+	first := fields[0]
+	if strings.HasPrefix(first, "CRON_TZ=") || strings.HasPrefix(first, "TZ=") {
+		return strings.ToLower(strings.Join(fields[1:], " "))
+	}
+	return strings.ToLower(strings.Join(fields, " "))
+}
+
 func validateResourceLimits(checkID string, limits ResourceLimits) error {
 	if limits.CPUTime.Duration < 0 {
 		return fmt.Errorf("check %q: resource_limits.cpu_time must be > 0", checkID)