Cherry-pick elastic#10486 to 6.x: Migrate registry from previous inco…

…rrect path (elastic#10597)

In 6.x Journalbeat placed its registry file under the wrong path ignoring the data.path settings.
This patch lets users migrate from registries under such paths when upgrading from 6.x to 7.x.
(cherry picked from commit 9499811)

checkpoint/checkpoint.go

@@ -22,6 +22,7 @@ package checkpoint
 
 import (
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -32,6 +33,7 @@ import (
 	"gopkg.in/yaml.v2"
 
 	"github.com/elastic/beats/libbeat/logp"
+	"github.com/elastic/beats/libbeat/paths"
 )
 
 // Checkpoint persists event log state information to disk.
@@ -87,6 +89,11 @@ func NewCheckpoint(file string, maxUpdates int, interval time.Duration) (*Checkp
 		save:          make(chan JournalState, 1),
 	}
 
+	err := c.findRegistryFile()
+	if err != nil {
+		return nil, fmt.Errorf("error locating the proper registry file: %+v", err)
+	}
+
 	// Minimum batch size.
 	if c.maxUpdates < 1 {
 		c.maxUpdates = 1
@@ -120,6 +127,53 @@ func NewCheckpoint(file string, maxUpdates int, interval time.Duration) (*Checkp
 	return c, nil
 }
 
+// Previously the registry file was written to the root folder. It was fixed on
+// 7.x but not on 6.x. Thus, migration is needed, so users avoid losing state info.
+func (c *Checkpoint) findRegistryFile() error {
+	migratedPath := paths.Resolve(paths.Data, c.file)
+
+	fs, err := os.Stat(c.file)
+	if os.IsNotExist(err) {
+		c.file = migratedPath
+		return nil
+	} else if err != nil {
+		return fmt.Errorf("error accessing previous registry file: %+v", err)
+	}
+
+	f, err := os.Open(c.file)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	target, err := os.OpenFile(migratedPath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, fs.Mode())
+	if err != nil {
+		return err
+	}
+	defer target.Close()
+
+	if _, err := io.Copy(target, f); err != nil {
+		return err
+	}
+
+	err = target.Sync()
+	if err != nil {
+		return fmt.Errorf("error while syncing new registry file to disk: %+v", err)
+	}
+
+	c.file = migratedPath
+
+	p := filepath.Dir(migratedPath)
+	pf, err := os.Open(p)
+	if err != nil {
+		return nil
+	}
+	defer pf.Close()
+	pf.Sync()
+
+	return nil
+}
+
 // run is worker loop that reads incoming state information from the save
 // channel and persists it when the number of changes reaches maxEvents or
 // the amount of time since the last disk write reaches flushInterval.