scan older files to be harvested based on mod time

CHANGELOG.asciidoc

@@ -132,7 +132,7 @@ https://github.com/elastic/beats/compare/v6.0.0-alpha1...v6.0.0-alpha2[View comm
 - Add `logging.files` `permissions` option. {pull}4295[4295]
 
 *Filebeat*
-
+- Added ability to sort harvested files. {pull}4374[4374]
 - Add experimental Redis slow log prospector type. {pull}4180[4180]
 
 *Metricbeat*

filebeat/docs/reference/configuration/filebeat-options.asciidoc

@@ -302,6 +302,25 @@ If you require log lines to be sent in near real time do not use a very low `sca
 
 The default setting is 10s.
 
+
+[[scan-sort]]
+===== scan.sort
+
+Specifies if files should be harvested in order and how to determine the order. Possible values are modtime, filename and none. To sort by file modification time use modtime otherwise use filename.
+
+If you specify a value other than none for this setting you can determine whether to use ascending or descending order using `scan.order`.
+
+The default setting is none.
+
+
+[[scan-order]]
+===== scan.order
+
+Specifies ascending or descending order if `scan.sort` is set to a value other than none. Possible values are asc or desc.
+
+The default setting is asc.
+
+
 ===== harvester_buffer_size
 
 The size in bytes of the buffer that each harvester uses when fetching a file. The default is 16384.

filebeat/prospector/log/config.go

@@ -29,6 +29,8 @@ var (
 		HarvesterLimit: 0,
 		Symlinks:       false,
 		TailFiles:      false,
+		ScanSort:       "none",
+		ScanOrder:      "asc",
 
 		// Harvester
 		BufferSize: 16 * humanize.KiByte,
@@ -69,6 +71,8 @@ type config struct {
 	// Harvester
 	BufferSize int    `config:"harvester_buffer_size"`
 	Encoding   string `config:"encoding"`
+	ScanOrder  string `config:"scan.order"`
+	ScanSort   string `config:"scan.sort"`
 
 	ExcludeLines []match.Matcher         `config:"exclude_lines"`
 	IncludeLines []match.Matcher         `config:"include_lines"`

filebeat/prospector/log/prospector.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sort"
+	"strings"
 	"time"
 
 	"github.com/elastic/beats/filebeat/channel"
@@ -264,10 +266,116 @@ func (p *Prospector) matchesFile(filePath string) bool {
 	return false
 }
 
+type FileSortInfo struct {
+	info os.FileInfo
+	path string
+}
+
+func getSortInfos(paths map[string]os.FileInfo) []FileSortInfo {
+
+	sortInfos := make([]FileSortInfo, 0, len(paths))
+	for path, info := range paths {
+		sortInfo := FileSortInfo{info: info, path: path}
+		sortInfos = append(sortInfos, sortInfo)
+	}
+
+	return sortInfos
+}
+
+func getSortedFiles(scanOrder string, scanSort string, sortInfos []FileSortInfo) ([]FileSortInfo, error) {
+	var sortFunc func(i, j int) bool
+	switch scanSort {
+	case "modtime":
+		switch scanOrder {
+		case "asc":
+			sortFunc = func(i, j int) bool {
+				return sortInfos[i].info.ModTime().Before(sortInfos[j].info.ModTime())
+			}
+		case "desc":
+			sortFunc = func(i, j int) bool {
+				return sortInfos[i].info.ModTime().After(sortInfos[j].info.ModTime())
+			}
+		default:
+			return nil, fmt.Errorf("Unexpected value for scan.order: %v", scanOrder)
+		}
+	case "filename":
+		switch scanOrder {
+		case "asc":
+			sortFunc = func(i, j int) bool {
+				return strings.Compare(sortInfos[i].info.Name(), sortInfos[j].info.Name()) < 0
+			}
+		case "desc":
+			sortFunc = func(i, j int) bool {
+				return strings.Compare(sortInfos[i].info.Name(), sortInfos[j].info.Name()) > 0
+			}
+		default:
+			return nil, fmt.Errorf("Unexpected value for scan.order: %v", scanOrder)
+		}
+	default:
+		return nil, fmt.Errorf("Unexpected value for scan.sort: %v", scanSort)
+	}
+
+	if sortFunc != nil {
+		sort.Slice(sortInfos, sortFunc)
+	}
+
+	return sortInfos, nil
+}
+
+func getFileState(path string, info os.FileInfo, p *Prospector) (file.State, error) {
+	var err error
+	var absolutePath string
+	absolutePath, err = filepath.Abs(path)
+	if err != nil {
+		return file.State{}, fmt.Errorf("could not fetch abs path for file %s: %s", absolutePath, err)
+	}
+	logp.Debug("prospector", "Check file for harvesting: %s", absolutePath)
+	// Create new state for comparison
+	newState := file.NewState(info, absolutePath, p.config.Type)
+	return newState, nil
+}
+
+func getKeys(paths map[string]os.FileInfo) []string {
+	files := make([]string, 0)
+	for file := range paths {
+		files = append(files, file)
+	}
+	return files
+}
+
 // Scan starts a scanGlob for each provided path/glob
 func (p *Prospector) scan() {
 
-	for path, info := range p.getFiles() {
+	var sortInfos []FileSortInfo
+	var files []string
+
+	paths := p.getFiles()
+
+	var err error
+
+	if p.config.ScanSort != "none" {
+		sortInfos, err = getSortedFiles(p.config.ScanOrder, p.config.ScanSort, getSortInfos(paths))
+		if err != nil {
+			logp.Err("Failed to sort files during scan due to error %s", err)
+		}
+	}
+
+	if sortInfos == nil {
+		files = getKeys(paths)
+	}
+
+	for i := 0; i < len(paths); i++ {
+
+		var path string
+		var info os.FileInfo
+
+		if sortInfos == nil {
+			path = files[i]
+			info = paths[path]
+		} else {
+			path = sortInfos[i].path
+			info = sortInfos[i].info
+		}
 
 		select {
 		case <-p.done:
@@ -276,15 +384,10 @@ func (p *Prospector) scan() {
 		default:
 		}
 
-		var err error
-		path, err = filepath.Abs(path)
+		newState, err := getFileState(path, info, p)
 		if err != nil {
-			logp.Err("could not fetch abs path for file %s: %s", path, err)
+			logp.Err("Skipping file %s due to error %s", path, err)
 		}
-		logp.Debug("prospector", "Check file for harvesting: %s", path)
-
-		// Create new state for comparison
-		newState := file.NewState(info, path, p.config.Type)
 
 		// Load last state
 		lastState := p.states.FindPrevious(newState)