Add implementation of FSWatcher and FSScanner for filestream (elastic…

…#21444)

## What does this PR do?

This PR adds the implementation for `FSWatcher` and `FSScanner` for the `filestream` input.

The implementation of `FSScanner` is called `fileScanner`. It is responsible for
* resolves recursive globs on creation
* normalizes glob patterns on creation
* finds files which match the configured paths and returns `FileInfo` for those

This is the refactored version of the `log` input's scanner, globber functions.

The implementation of `FSWatcher` is called `fileWatcher`. It checks the file list returned by `fileScanner` and creates events based on the result.

## Why is it important?

It is required for the `filestream` input.

## Related issues

Related elastic#20243

filebeat/input/filestream/fswatch.go

@@ -0,0 +1,375 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package filestream
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/elastic/beats/v7/filebeat/input/file"
+	loginp "github.com/elastic/beats/v7/filebeat/input/filestream/internal/input-logfile"
+	"github.com/elastic/beats/v7/libbeat/common"
+	"github.com/elastic/beats/v7/libbeat/common/match"
+	"github.com/elastic/beats/v7/libbeat/logp"
+	"github.com/elastic/go-concert/unison"
+)
+
+const (
+	recursiveGlobDepth = 8
+	scannerName        = "scanner"
+	watcherDebugKey    = "file_watcher"
+)
+
+var (
+	watcherFactories = map[string]watcherFactory{
+		scannerName: newScannerWatcher,
+	}
+)
+
+type watcherFactory func(paths []string, cfg *common.Config) (loginp.FSWatcher, error)
+
+// fileScanner looks for files which match the patterns in paths.
+// It is able to exclude files and symlinks.
+type fileScanner struct {
+	paths         []string
+	excludedFiles []match.Matcher
+	symlinks      bool
+
+	log *logp.Logger
+}
+
+type fileWatcherConfig struct {
+	// Interval is the time between two scans.
+	Interval time.Duration
+	// Scanner is the configuration of the scanner.
+	Scanner fileScannerConfig
+}
+
+// fileWatcher gets the list of files from a FSWatcher and creates events by
+// comparing the files between its last two runs.
+type fileWatcher struct {
+	interval time.Duration
+	prev     map[string]os.FileInfo
+	scanner  loginp.FSScanner
+	log      *logp.Logger
+	events   chan loginp.FSEvent
+}
+
+func newFileWatcher(paths []string, ns *common.ConfigNamespace) (loginp.FSWatcher, error) {
+	if ns == nil {
+		return newScannerWatcher(paths, nil)
+	}
+
+	watcherType := ns.Name()
+	f, ok := watcherFactories[watcherType]
+	if !ok {
+		return nil, fmt.Errorf("no such file watcher: %s", watcherType)
+	}
+
+	return f(paths, ns.Config())
+}
+
+func newScannerWatcher(paths []string, c *common.Config) (loginp.FSWatcher, error) {
+	config := defaultFileWatcherConfig()
+	err := c.Unpack(&config)
+	if err != nil {
+		return nil, err
+	}
+	scanner, err := newFileScanner(paths, config.Scanner)
+	if err != nil {
+		return nil, err
+	}
+	return &fileWatcher{
+		log:      logp.NewLogger(watcherDebugKey),
+		interval: config.Interval,
+		prev:     make(map[string]os.FileInfo, 0),
+		scanner:  scanner,
+		events:   make(chan loginp.FSEvent),
+	}, nil
+}
+
+func defaultFileWatcherConfig() fileWatcherConfig {
+	return fileWatcherConfig{
+		Interval: 10 * time.Second,
+		Scanner:  defaultFileScannerConfig(),
+	}
+}
+
+func (w *fileWatcher) Run(ctx unison.Canceler) {
+	defer close(w.events)
+
+	ticker := time.NewTicker(w.interval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			w.watch(ctx)
+		}
+	}
+}
+
+func (w *fileWatcher) watch(ctx unison.Canceler) {
+	w.log.Info("Start next scan")
+
+	paths := w.scanner.GetFiles()
+
+	newFiles := make(map[string]os.FileInfo)
+
+	for path, info := range paths {
+
+		prevInfo, ok := w.prev[path]
+		if !ok {
+			newFiles[path] = paths[path]
+			continue
+		}
+
+		if prevInfo.ModTime() != info.ModTime() {
+			select {
+			case <-ctx.Done():
+				return
+			case w.events <- writeEvent(path, info):
+			}
+		}
+
+		// delete from previous state, as we have more up to date info
+		delete(w.prev, path)
+	}
+
+	// remaining files are in the prev map are the ones that are missing
+	// either because they have been deleted or renamed
+	for removedPath, removedInfo := range w.prev {
+		for newPath, newInfo := range newFiles {
+			if os.SameFile(removedInfo, newInfo) {
+				select {
+				case <-ctx.Done():
+					return
+				case w.events <- renamedEvent(removedPath, newPath, newInfo):
+					delete(newFiles, newPath)
+					goto CHECK_NEXT_REMOVED
+				}
+			}
+		}
+
+		select {
+		case <-ctx.Done():
+			return
+		case w.events <- deleteEvent(removedPath, removedInfo):
+		}
+	CHECK_NEXT_REMOVED:
+	}
+
+	// remaining files in newFiles are new
+	for path, info := range newFiles {
+		select {
+		case <-ctx.Done():
+			return
+		case w.events <- createEvent(path, info):
+		}
+
+	}
+
+	w.log.Debugf("Found %d paths", len(paths))
+	w.prev = paths
+}
+
+func createEvent(path string, fi os.FileInfo) loginp.FSEvent {
+	return loginp.FSEvent{Op: loginp.OpCreate, OldPath: "", NewPath: path, Info: fi}
+}
+
+func writeEvent(path string, fi os.FileInfo) loginp.FSEvent {
+	return loginp.FSEvent{Op: loginp.OpWrite, OldPath: path, NewPath: path, Info: fi}
+}
+
+func renamedEvent(oldPath, path string, fi os.FileInfo) loginp.FSEvent {
+	return loginp.FSEvent{Op: loginp.OpRename, OldPath: oldPath, NewPath: path, Info: fi}
+}
+
+func deleteEvent(path string, fi os.FileInfo) loginp.FSEvent {
+	return loginp.FSEvent{Op: loginp.OpDelete, OldPath: path, NewPath: "", Info: fi}
+}
+
+func (w *fileWatcher) Event() loginp.FSEvent {
+	return <-w.events
+}
+
+type fileScannerConfig struct {
+	Paths         []string
+	ExcludedFiles []match.Matcher
+	Symlinks      bool
+	RecursiveGlob bool
+}
+
+func defaultFileScannerConfig() fileScannerConfig {
+	return fileScannerConfig{
+		Symlinks:      false,
+		RecursiveGlob: true,
+	}
+}
+
+func newFileScanner(paths []string, cfg fileScannerConfig) (loginp.FSScanner, error) {
+	fs := fileScanner{
+		paths:         paths,
+		excludedFiles: cfg.ExcludedFiles,
+		symlinks:      cfg.Symlinks,
+		log:           logp.NewLogger(scannerName),
+	}
+	err := fs.resolveRecursiveGlobs(cfg)
+	if err != nil {
+		return nil, err
+	}
+	err = fs.normalizeGlobPatterns()
+	if err != nil {
+		return nil, err
+	}
+
+	return &fs, nil
+}
+
+// resolveRecursiveGlobs expands `**` from the globs in multiple patterns
+func (s *fileScanner) resolveRecursiveGlobs(c fileScannerConfig) error {
+	if !c.RecursiveGlob {
+		s.log.Debug("recursive glob disabled")
+		return nil
+	}
+
+	s.log.Debug("recursive glob enabled")
+	var paths []string
+	for _, path := range s.paths {
+		patterns, err := file.GlobPatterns(path, recursiveGlobDepth)
+		if err != nil {
+			return err
+		}
+		if len(patterns) > 1 {
+			s.log.Debugf("%q expanded to %#v", path, patterns)
+		}
+		paths = append(paths, patterns...)
+	}
+	s.paths = paths
+	return nil
+}
+
+// normalizeGlobPatterns calls `filepath.Abs` on all the globs from config
+func (s *fileScanner) normalizeGlobPatterns() error {
+	var paths []string
+	for _, path := range s.paths {
+		pathAbs, err := filepath.Abs(path)
+		if err != nil {
+			return fmt.Errorf("failed to get the absolute path for %s: %v", path, err)
+		}
+		paths = append(paths, pathAbs)
+	}
+	s.paths = paths
+	return nil
+}
+
+// GetFiles returns a map of files and fileinfos which
+// match the configured paths.
+func (s *fileScanner) GetFiles() map[string]os.FileInfo {
+	pathInfo := map[string]os.FileInfo{}
+
+	for _, path := range s.paths {
+		matches, err := filepath.Glob(path)
+		if err != nil {
+			s.log.Errorf("glob(%s) failed: %v", path, err)
+			continue
+		}
+
+		for _, file := range matches {
+			if s.shouldSkipFile(file) {
+				continue
+			}
+
+			// If symlink is enabled, it is checked that original is not part of same input
+			// If original is harvested by other input, states will potentially overwrite each other
+			if s.isOriginalAndSymlinkConfigured(file, pathInfo) {
+				continue
+			}
+
+			fileInfo, err := os.Stat(file)
+			if err != nil {
+				s.log.Debug("stat(%s) failed: %s", file, err)
+				continue
+			}
+			pathInfo[file] = fileInfo
+		}
+	}
+
+	return pathInfo
+}
+
+func (s *fileScanner) shouldSkipFile(file string) bool {
+	if s.isFileExcluded(file) {
+		s.log.Debugf("Exclude file: %s", file)
+		return true
+	}
+
+	fileInfo, err := os.Lstat(file)
+	if err != nil {
+		s.log.Debugf("lstat(%s) failed: %s", file, err)
+		return true
+	}
+
+	if fileInfo.IsDir() {
+		s.log.Debugf("Skipping directory: %s", file)
+		return true
+	}
+
+	isSymlink := fileInfo.Mode()&os.ModeSymlink > 0
+	if isSymlink && !s.symlinks {
+		s.log.Debugf("File %s skipped as it is a symlink", file)
+		return true
+	}
+
+	return false
+}
+
+func (s *fileScanner) isOriginalAndSymlinkConfigured(file string, paths map[string]os.FileInfo) bool {
+	if s.symlinks {
+		fileInfo, err := os.Stat(file)
+		if err != nil {
+			s.log.Debugf("stat(%s) failed: %s", file, err)
+			return false
+		}
+
+		for _, finfo := range paths {
+			if os.SameFile(finfo, fileInfo) {
+				s.log.Info("Same file found as symlink and original. Skipping file: %s (as it same as %s)", file, finfo.Name())
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func (s *fileScanner) isFileExcluded(file string) bool {
+	return len(s.excludedFiles) > 0 && s.matchAny(s.excludedFiles, file)
+}
+
+// matchAny checks if the text matches any of the regular expressions
+func (s *fileScanner) matchAny(matchers []match.Matcher, text string) bool {
+	for _, m := range matchers {
+		if m.MatchString(text) {
+			return true
+		}
+	}
+	return false
+}