crond: add support for crontab file only (on any OS)

Allows to use an external scheduler that supports crontab format
the existing crond support using crontab binary remains unchanged for linux

constants/global.go

@@ -8,10 +8,12 @@ import (
 
 // Scheduler type
 const (
-	SchedulerLaunchd = "launchd"
-	SchedulerWindows = "taskscheduler"
-	SchedulerSystemd = "systemd"
-	SchedulerCrond   = "crond"
+	SchedulerLaunchd   = "launchd"
+	SchedulerWindows   = "taskscheduler"
+	SchedulerSystemd   = "systemd"
+	SchedulerCrond     = "crond"
+	SchedulerCrontab   = "crontab"
+	SchedulerOSDefault = ""
 )
 
 var (

crond/crontab.go

@@ -1,13 +1,8 @@
-//+build !darwin,!windows
-
 package crond
 
 import (
-	"bytes"
 	"fmt"
 	"io"
-	"os"
-	"os/exec"
 	"regexp"
 	"strings"
 )
@@ -18,24 +13,26 @@ const (
 )
 
 type Crontab struct {
-	entries []Entry
+	file, charset string
+	entries       []Entry
 }
 
-var (
-	crontabBinary = "crontab"
-)
-
 func NewCrontab(entries []Entry) *Crontab {
 	return &Crontab{
 		entries: entries,
 	}
 }
 
+// SetFile toggles whether to read & write a crontab file instead of using CrontabBinary
+func (c *Crontab) SetFile(file string) {
+	c.file = file
+}
+
 // Update crontab entries:
 //
-// If addEntries is set to true, it will delete and add all new entries
+// - If addEntries is set to true, it will delete and add all new entries
 //
-// If addEntries is set to false, it will only delete the matching entries
+// - If addEntries is set to false, it will only delete the matching entries
 //
 // Return values are the number of entries deleted, and an error if any
 func (c *Crontab) Update(source string, addEntries bool, w io.StringWriter) (int, error) {
@@ -116,61 +113,41 @@ func (c *Crontab) Generate(w io.StringWriter) error {
 	return nil
 }
 
-func (c *Crontab) LoadCurrent() (string, error) {
-	buffer := &strings.Builder{}
-	cmd := exec.Command(crontabBinary, "-l")
-	cmd.Stdout = buffer
-	cmd.Stderr = buffer
-	err := cmd.Run()
-	if err != nil && strings.HasPrefix(buffer.String(), "no crontab for ") {
-		// it's ok to be empty
-		return "", nil
-	} else if err != nil {
-		return "", fmt.Errorf("%w: %s", err, buffer.String())
-	}
-	return cleanupCrontab(buffer.String()), nil
+func (c *Crontab) LoadCurrent() (content string, err error) {
+	content, c.charset, err = loadCrontab(c.file)
+	if err == nil {
+		content = cleanupCrontab(content)
+	}
+	return
 }
 
 func (c *Crontab) Rewrite() error {
 	crontab, err := c.LoadCurrent()
 	if err != nil {
 		return err
 	}
-	input := &bytes.Buffer{}
-	_, err = c.Update(crontab, true, input)
-	if err != nil {
-		return err
-	}
 
-	cmd := exec.Command(crontabBinary, "-")
-	cmd.Stdin = input
-	cmd.Stderr = os.Stderr
-	err = cmd.Run()
+	buffer := new(strings.Builder)
+	_, err = c.Update(crontab, true, buffer)
 	if err != nil {
 		return err
 	}
-	return nil
+
+	return saveCrontab(c.file, buffer.String(), c.charset)
 }
 
 func (c *Crontab) Remove() (int, error) {
 	crontab, err := c.LoadCurrent()
 	if err != nil {
 		return 0, err
 	}
-	buffer := &bytes.Buffer{}
-	num, err := c.Update(crontab, false, buffer)
-	if err != nil {
-		return num, err
-	}
 
-	cmd := exec.Command(crontabBinary, "-")
-	cmd.Stdin = buffer
-	cmd.Stderr = os.Stderr
-	err = cmd.Run()
-	if err != nil {
-		return num, err
+	buffer := new(strings.Builder)
+	num, err := c.Update(crontab, false, buffer)
+	if err == nil {
+		err = saveCrontab(c.file, buffer.String(), c.charset)
 	}
-	return num, nil
+	return num, err
 }
 
 func cleanupCrontab(crontab string) string {
@@ -181,7 +158,9 @@ func cleanupCrontab(crontab string) string {
 }
 
 // extractOwnSection returns before our section, inside, and after if found.
+//
 // - It is not returning both start and end markers.
+//
 // - If not found, it returns the file content in the first string
 func extractOwnSection(crontab string) (string, string, string, bool) {
 	start := strings.Index(crontab, startMarker)

crond/crontab_test.go

@@ -1,10 +1,9 @@
-//+build !darwin,!windows
-
 package crond
 
 import (
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -153,14 +152,41 @@ func TestRemoveCrontab(t *testing.T) {
 	assert.Equal(t, "something\n"+startMarker+endMarker, buffer.String())
 }
 
+func TestLoadCurrentFromFile(t *testing.T) {
+	file, err := filepath.Abs(filepath.Join(t.TempDir(), "crontab"))
+	defer func() {
+		CrontabBinary = DefaultCrontabBinary
+		_ = os.Remove(file)
+	}()
+
+	crontab := NewCrontab([]Entry{NewEntry(calendar.NewEvent(func(event *calendar.Event) {
+		event.Minute.MustAddValue(1)
+		event.Hour.MustAddValue(1)
+	}), "", "", "", "resticprofile backup", "")})
+
+	CrontabBinary = ""
+	assert.ErrorContains(t, crontab.Rewrite(), "no contrab file was specified")
+
+	crontab.SetFile(file)
+
+	assert.NoFileExists(t, file)
+	assert.NoError(t, crontab.Rewrite())
+	assert.FileExists(t, file)
+
+	result, err := crontab.LoadCurrent()
+	assert.NoError(t, err)
+	assert.Contains(t, result, "01 01 * * *\tresticprofile backup")
+}
+
 func TestLoadCurrent(t *testing.T) {
 	defer func() {
+		CrontabBinary = DefaultCrontabBinary
 		_ = os.Remove("./crontab")
 	}()
 	cmd := exec.Command("go", "build", "-o", "crontab", "./stdin")
 	err := cmd.Run()
 	require.NoError(t, err)
-	crontabBinary = "./crontab"
+	CrontabBinary = "./crontab"
 
 	crontab := NewCrontab(nil)
 	assert.NotNil(t, crontab)

crond/entry.go

@@ -1,5 +1,3 @@
-//+build !darwin,!windows
-
 package crond
 
 import (

crond/entry_test.go

@@ -1,5 +1,3 @@
-//+build !darwin,!windows
-
 package crond
 
 import (

crond/io.go

@@ -0,0 +1,103 @@
+package crond
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+const (
+	maxCrontabFileSize      = 16 * 1024 * 1024
+	defaultCrontabFilePerms = fs.FileMode(0644)
+)
+
+func verifyCrontabFile(file string) error {
+	if file == "" {
+		return fmt.Errorf("no contrab file was specified")
+	}
+	return nil
+}
+
+func loadCrontabFile(file string) (content, charset string, err error) {
+	if err = verifyCrontabFile(file); err != nil {
+		return
+	}
+	var f *os.File
+	if f, err = os.Open(file); err == nil {
+		defer func() { _ = f.Close() }()
+
+		var bytes []byte
+		bytes, err = io.ReadAll(io.LimitReader(f, maxCrontabFileSize))
+		if err == nil && len(bytes) == maxCrontabFileSize {
+			err = fmt.Errorf("max file size of %d bytes exceeded in %q", maxCrontabFileSize, file)
+		}
+		if err == nil {
+			// TODO: handle charsets
+			charset = ""
+			content = string(bytes)
+		}
+	} else if errors.Is(err, os.ErrNotExist) {
+		err = nil
+	}
+	return
+}
+
+func saveCrontabFile(file, content, charset string) (err error) {
+	if err = verifyCrontabFile(file); err != nil {
+		return
+	}
+
+	// TODO: handle charsets
+	bytes := []byte(content)
+
+	if len(bytes) >= maxCrontabFileSize {
+		err = fmt.Errorf("max file size of %d bytes exceeded in new %q", maxCrontabFileSize, file)
+	} else {
+		err = os.WriteFile(file, bytes, defaultCrontabFilePerms)
+	}
+	return
+}
+
+// CrontabBinary is the name of the crontab binary to use when no file is set
+var CrontabBinary = DefaultCrontabBinary
+
+func loadCrontab(file string) (content, charset string, err error) {
+	if file == "" && CrontabBinary != "" {
+		buffer := new(strings.Builder)
+		{
+			cmd := exec.Command(CrontabBinary, "-l")
+			cmd.Stdout = buffer
+			cmd.Stderr = buffer
+			err = cmd.Run()
+		}
+		if err != nil {
+			if strings.HasPrefix(buffer.String(), "no crontab for ") {
+				err = nil // it's ok to be empty
+			} else {
+				err = fmt.Errorf("%w: %s", err, buffer.String())
+			}
+		}
+		if err == nil {
+			content = buffer.String()
+		}
+		return
+	} else {
+		return loadCrontabFile(file)
+	}
+}
+
+func saveCrontab(file, content, charset string) (err error) {
+	if file == "" && CrontabBinary != "" {
+		cmd := exec.Command(CrontabBinary, "-")
+		cmd.Stdin = strings.NewReader(content)
+		cmd.Stderr = os.Stderr
+		err = cmd.Run()
+	} else {
+		err = saveCrontabFile(file, content, charset)
+	}
+	return
+}

crond/io_linux.go

@@ -0,0 +1,4 @@
+package crond
+
+// DefaultCrontabBinary names the default `crontab` executable for this platform
+const DefaultCrontabBinary = "crontab"

crond/io_other.go

@@ -0,0 +1,6 @@
+//go:build !linux
+
+package crond
+
+// DefaultCrontabBinary is empty as this platform has no default `crontab` executable
+const DefaultCrontabBinary = ""

schedule/handler.go

@@ -21,6 +21,25 @@ type Handler interface {
 	DisplayJobStatus(job *config.ScheduleConfig) error
 }
 
+// NewHandler creates a schedule handler depending on the configuration
+func NewHandler(config SchedulerConfig) Handler {
+	for _, provider := range providers {
+		if handler := provider(config); handler != nil {
+			return handler
+		}
+	}
+	panic(fmt.Errorf("scheduler %q is not supported in this environment", config.Type()))
+}
+
+type HandlerProvider func(config SchedulerConfig) Handler
+
+var providers []HandlerProvider
+
+// AddHandlerProvider allows to register a provider for a SchedulerConfig handler
+func AddHandlerProvider(provider HandlerProvider) {
+	providers = append(providers, provider)
+}
+
 func lookupBinary(name, binary string) error {
 	found, err := exec.LookPath(binary)
 	if err != nil || found == "" {