Promtail: Fix file descriptor leak (#8987)

This reduces the amount of open file descriptors per followed journal
file from 3 to 2. It also releases file descriptors for removed files
(journal vacuuming).

The `defaultJournalEntryFunc` is only called once upon startup. It
creates a `Journal` instance to only read a single entry to determine
the age of the entry referenced in the positions file. Right after, this
instance is not required anymore.

The actually used `Journal` instance used to follow the journal is
created inside `journalTargetWithReader`.

CHANGELOG.md

@@ -77,6 +77,7 @@
 
 * [8988](https://github.com/grafana/loki/pull/8988) **darxriggs**: Promtail: Prevent logging errors on normal shutdown.
 * [9155](https://github.com/grafana/loki/pull/9155) **farodin91**: Promtail: Break on iterate journal failure.
+* [8987](https://github.com/grafana/loki/pull/8987) **darxriggs**: Promtail: Fix file descriptor leak.
 
 #### LogCLI
 

clients/pkg/promtail/targets/journal/journaltarget.go

@@ -56,21 +56,25 @@ var defaultJournalReaderFunc = func(c sdjournal.JournalReaderConfig) (journalRea
 	return sdjournal.NewJournalReader(c)
 }
 
-var defaultJournalEntryFunc = func(c sdjournal.JournalReaderConfig, cursor string) (*sdjournal.JournalEntry, error) {
-	var (
-		journal *sdjournal.Journal
-		err     error
-	)
+var defaultJournalEntryFunc = func(c sdjournal.JournalReaderConfig, cursor string) (entry *sdjournal.JournalEntry, err error) {
+	var journal *sdjournal.Journal
 
 	if c.Path != "" {
 		journal, err = sdjournal.NewJournalFromDir(c.Path)
 	} else {
 		journal, err = sdjournal.NewJournal()
 	}
-
 	if err != nil {
 		return nil, err
-	} else if err := journal.SeekCursor(cursor); err != nil {
+	}
+	defer func() {
+		if errClose := journal.Close(); err == nil {
+			err = errClose
+		}
+	}()
+
+	err = journal.SeekCursor(cursor)
+	if err != nil {
 		return nil, err
 	}