[Enhancement] Add RotateOnStartup feature flag for file output

CHANGELOG.next.asciidoc

@@ -521,6 +521,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Add keystore support for autodiscover static configurations. {pull]16306[16306]
 - Add TLS support to Kerberos authentication in Elasticsearch. {pull}18607[18607]
 - Add support for multiple sets of hints on autodiscover {pull}18883[18883]
+- Add config option `rotate_on_startup` to file output {issue}19150[19150] {pull}19347[19347]
 - Add a configurable delay between retries when an app metadata cannot be retrieved by `add_cloudfoundry_metadata`. {pull}19181[19181]
 - Added the `max_cached_sessions` option to the script processor. {pull}19562[19562]
 - Add support for DNS over TLS for the dns_processor. {pull}19321[19321]

libbeat/outputs/fileout/config.go

@@ -25,19 +25,21 @@ import (
 )
 
 type config struct {
-	Path          string       `config:"path"`
-	Filename      string       `config:"filename"`
-	RotateEveryKb uint         `config:"rotate_every_kb" validate:"min=1"`
-	NumberOfFiles uint         `config:"number_of_files"`
-	Codec         codec.Config `config:"codec"`
-	Permissions   uint32       `config:"permissions"`
+	Path            string       `config:"path"`
+	Filename        string       `config:"filename"`
+	RotateEveryKb   uint         `config:"rotate_every_kb" validate:"min=1"`
+	NumberOfFiles   uint         `config:"number_of_files"`
+	Codec           codec.Config `config:"codec"`
+	Permissions     uint32       `config:"permissions"`
+	RotateOnStartup bool         `config:"rotate_on_startup"`
 }
 
 var (
 	defaultConfig = config{
-		NumberOfFiles: 7,
-		RotateEveryKb: 10 * 1024,
-		Permissions:   0600,
+		NumberOfFiles:   7,
+		RotateEveryKb:   10 * 1024,
+		Permissions:     0600,
+		RotateOnStartup: true,
 	}
 )
 

libbeat/outputs/fileout/docs/fileout.asciidoc

@@ -22,6 +22,7 @@ output.file:
   #rotate_every_kb: 10000
   #number_of_files: 7
   #permissions: 0600
+  #rotate_on_startup: true
 ------------------------------------------------------------------------------
 
 ==== Configuration options
@@ -61,6 +62,10 @@ The number of files must be between 2 and 1024. The default is 7.
 
 Permissions to use for file creation. The default is 0600.
 
+===== `rotate_on_startup`
+
+If the output file already exists on startup, immediately rotate it and start writing to a new file instead of appending to the existing one. Defaults to true.
+
 ===== `codec`
 
 Output codec configuration. If the `codec` section is missing, events will be json encoded.

libbeat/outputs/fileout/file.go

@@ -87,6 +87,7 @@ func (out *fileOutput) init(beat beat.Info, c config) error {
 		file.MaxSizeBytes(c.RotateEveryKb*1024),
 		file.MaxBackups(c.NumberOfFiles),
 		file.Permissions(os.FileMode(c.Permissions)),
+		file.RotateOnStartup(c.RotateOnStartup),
 		file.WithLogger(logp.NewLogger("rotator").With(logp.Namespace("rotator"))),
 	)
 	if err != nil {