Add experimental symlink support

This implementation of symlinks takes a symlink and opens the original file. The only difference is that as Source the symlink path is reported. The advantage of this implementation is that everything related to file handling is identical with non symlink files as the original file is harvested. All close_* options work as expected. State information is stored for the original file, not the symlink itself. In case a symlink and original file are harvested, only one will be harvested as they share the same inode information.

* Add test to verify that symlinks are disabled by default
* Add test for symlink handling
* Improve error handling in harvester. Return proper error messages instead of logging it. Prevents too many log messages.
* Remove IsRegular file call as not needed anymore and leads to additional Stat calls
* Add documentation

See #2277 and #1767

CHANGELOG.asciidoc

@@ -86,6 +86,7 @@ https://github.com/elastic/beats/compare/v5.0.0-alpha5...master[Check the HEAD d
 - Update to Go 1.7. {pull}2306[2306]
 - Log total non-zero internal metrics on shutdown. {pull}2349[2349]
 - Add support for encrypted private key files by introducing `ssl.key_passphrase` setting. {pull}2330[2330]
+- Add experimental symlink support with `symlinks` config {pull}2478[2478]
 
 *Metricbeat*
 

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

@@ -391,6 +391,18 @@ This option applies to files that Filebeat has not already processed. If you ran
 
 NOTE: You can use this setting to avoid indexing old log lines when you run Filebeat on a set of log files for the first time. After the first run, we recommend disabling this option, or you risk losing lines during file rotation.
 
+===== symlinks
+
+experimental[]
+
+The symlinks options allows to also harvest symlinks in addition to regular files. In case of symlinks, the file that is opened and read in the end is the original file even though the path of the symlink is reported.
+
+It must be made sure, that if a symlink is defined for harvesting, the original is excluded. In case the symlink and original file are harvested by the same prospector, this will be detected and only one of the two files will be harvested which is the first one found by filebeat. In case the symlink and original are defined in two different prospectors, it can happen that both will be harvested at the same time, send duplicated data and will overwrite each other state.
+
+This option can be useful if symlinks to the log files have additional meta data in the file name which can be processed in Logstash. This is for example the case for kubernetes log files.
+
+As this option may lead to data loss, it is disabled by default.
+
 ===== backoff
 
 The backoff options specify how aggressively Filebeat crawls new files for updates.

filebeat/etc/beat.full.yml

@@ -143,6 +143,10 @@ filebeat.prospectors:
   # this can mean that the first entries of a new file are skipped.
   #tail_files: false
 
+  # Experimental: If symlinks is enabled, symlinks are opened and harvested. The harvester is openening the
+  # original for harvesting but will report the symlink name as source.
+  #symlinks: false
+
   # Backoff values define how aggressively filebeat crawls new files for updates
   # The default values can be used in most cases. Backoff defines how long it is waited
   # to check a file again after EOF is reached. Default is 1s which means the file

filebeat/filebeat.full.yml

@@ -143,6 +143,10 @@ filebeat.prospectors:
   # this can mean that the first entries of a new file are skipped.
   #tail_files: false
 
+  # Experimental: If symlinks is enabled, symlinks are opened and harvested. The harvester is openening the
+  # original for harvesting but will report the symlink name as source.
+  #symlinks: false
+
   # Backoff values define how aggressively filebeat crawls new files for updates
   # The default values can be used in most cases. Backoff defines how long it is waited
   # to check a file again after EOF is reached. Default is 1s which means the file

filebeat/harvester/log.go

@@ -10,6 +10,8 @@ import (
 
 	"golang.org/x/text/transform"
 
+	"fmt"
+
 	"github.com/elastic/beats/filebeat/config"
 	"github.com/elastic/beats/filebeat/harvester/reader"
 	"github.com/elastic/beats/filebeat/harvester/source"
@@ -163,8 +165,7 @@ func (h *Harvester) openFile() error {
 
 	f, err := file.ReadOpen(h.state.Source)
 	if err != nil {
-		logp.Err("Failed opening %s: %s", h.state.Source, err)
-		return err
+		return fmt.Errorf("Failed opening %s: %s", h.state.Source, err)
 	}
 
 	harvesterOpenFiles.Add(1)
@@ -182,16 +183,16 @@ func (h *Harvester) openFile() error {
 }
 
 func (h *Harvester) validateFile(f *os.File) error {
-	// Check we are not following a rabbit hole (symlinks, etc.)
-	if !file.IsRegular(f) {
-		return errors.New("Given file is not a regular file.")
-	}
 
 	info, err := f.Stat()
 	if err != nil {
-		logp.Err("Failed getting stats for file %s: %s", h.state.Source, err)
-		return err
+		return fmt.Errorf("Failed getting stats for file %s: %s", h.state.Source, err)
 	}
+
+	if !info.Mode().IsRegular() {
+		return fmt.Errorf("Tried to open non regular file: %q %s", info.Mode(), info.Name())
+	}
+
 	// Compares the stat of the opened file to the state given by the prospector. Abort if not match.
 	if !os.SameFile(h.state.Fileinfo, info) {
 		return errors.New("File info is not identical with opened file. Aborting harvesting and retrying file later again.")

filebeat/input/file/file.go

@@ -13,26 +13,6 @@ type File struct {
 	State    *State
 }
 
-// Check that the file isn't a symlink, mode is regular or file is nil
-func (f *File) IsRegular() bool {
-	if f.File == nil {
-		logp.Critical("Harvester: BUG: f arg is nil")
-		return false
-	}
-
-	info, e := f.File.Stat()
-	if e != nil {
-		logp.Err("File check fault: stat error: %s", e.Error())
-		return false
-	}
-
-	if !info.Mode().IsRegular() {
-		logp.Warn("Harvester: not a regular file: %q %s", info.Mode(), info.Name())
-		return false
-	}
-	return true
-}
-
 // Checks if the two files are the same.
 func (f *File) IsSameFile(f2 *File) bool {
 	return os.SameFile(f.FileInfo, f2.FileInfo)
@@ -49,8 +29,3 @@ func IsSameFile(path string, info os.FileInfo) bool {
 
 	return os.SameFile(fileInfo, info)
 }
-
-func IsRegular(file *os.File) bool {
-	f := &File{File: file}
-	return f.IsRegular()
-}

filebeat/prospector/config.go

@@ -16,6 +16,7 @@ var (
 		CleanInactive:  0,
 		CleanRemoved:   false,
 		HarvesterLimit: 0,
+		Symlinks:       false,
 	}
 )
 
@@ -28,6 +29,7 @@ type prospectorConfig struct {
 	CleanInactive  time.Duration    `config:"clean_inactive" validate:"min=0"`
 	CleanRemoved   bool             `config:"clean_removed"`
 	HarvesterLimit uint64           `config:"harvester_limit" validate:"min=0"`
+	Symlinks       bool             `config:"symlinks"`
 }
 
 func (config *prospectorConfig) Validate() error {

filebeat/prospector/prospector_log.go

@@ -99,6 +99,7 @@ func (p *ProspectorLog) getFiles() map[string]os.FileInfo {
 			continue
 		}
 
+	OUTER:
 		// Check any matched files to see if we need to start a harvester
 		for _, file := range matches {
 
@@ -108,24 +109,39 @@ func (p *ProspectorLog) getFiles() map[string]os.FileInfo {
 				continue
 			}
 
-			fileinfo, err := os.Lstat(file)
+			// Fetch Lstat File info to detected also symlinks
+			fileInfo, err := os.Lstat(file)
 			if err != nil {
 				logp.Debug("prospector", "stat(%s) failed: %s", file, err)
 				continue
 			}
 
-			// Check if file is symlink
-			if fileinfo.Mode()&os.ModeSymlink != 0 {
-				logp.Debug("prospector", "File %s skipped as it is a symlink.", file)
+			if fileInfo.IsDir() {
+				logp.Debug("prospector", "Skipping directory: %s", file)
 				continue
 			}
 
-			if fileinfo.IsDir() {
-				logp.Debug("prospector", "Skipping directory: %s", file)
+			isSymlink := fileInfo.Mode()&os.ModeSymlink > 0
+			if isSymlink && !p.config.Symlinks {
+				logp.Debug("prospector", "File %s skipped as it is a symlink.", file)
 				continue
 			}
 
-			paths[file] = fileinfo
+			// Fetch Stat file info which fetches the inode from the original and is used for comparison
+			fileInfo, err = os.Stat(file)
+
+			// If symlink is enabled, it is checked that original is not part of same prospector
+			// It original is harvested by other prospector, states will potentially overwrite each other
+			if p.config.Symlinks {
+				for _, finfo := range paths {
+					if os.SameFile(finfo, fileInfo) {
+						logp.Info("Same file found as symlink and original. Skipping file: %s", file)
+						continue OUTER
+					}
+				}
+			}
+
+			paths[file] = fileInfo
 		}
 	}
 

filebeat/tests/system/config/filebeat.yml.j2

@@ -27,6 +27,7 @@ filebeat.prospectors:
   clean_inactive: {{clean_inactive}}
   clean_removed: {{clean_removed}}
   harvester_limit: {{harvester_limit | default(0) }}
+  symlinks: {{symlinks}}
 
   {% if fields %}
   fields: