lib/db: Add Badger backend (fixes #5910) (#6250)

lib/db/backend/backend.go

@@ -7,7 +7,12 @@
 package backend
 
 import (
+	"os"
+	"strings"
 	"sync"
+	"time"
+
+	"github.com/syncthing/syncthing/lib/locations"
 )
 
 // The Reader interface specifies the read-only operations available on the
@@ -118,10 +123,24 @@ const (
 )
 
 func Open(path string, tuning Tuning) (Backend, error) {
+	if os.Getenv("USE_BADGER") != "" {
+		l.Warnln("Using experimental badger db")
+		if err := maybeCopyDatabase(path, strings.Replace(path, locations.BadgerDir, locations.LevelDBDir, 1), OpenBadger, OpenLevelDBRO); err != nil {
+			return nil, err
+		}
+		return OpenBadger(path)
+	}
+
+	if err := maybeCopyDatabase(path, strings.Replace(path, locations.LevelDBDir, locations.BadgerDir, 1), OpenLevelDBAuto, OpenBadger); err != nil {
+		return nil, err
+	}
 	return OpenLevelDB(path, tuning)
 }
 
 func OpenMemory() Backend {
+	if os.Getenv("USE_BADGER") != "" {
+		return OpenBadgerMemory()
+	}
 	return OpenLevelDBMemory()
 }
 
@@ -202,3 +221,70 @@ func (cg *closeWaitGroup) CloseWait() {
 	cg.closeMut.Unlock()
 	cg.WaitGroup.Wait()
 }
+
+type opener func(path string) (Backend, error)
+
+// maybeCopyDatabase copies the database if the destination doesn't exist
+// but the source does.
+func maybeCopyDatabase(toPath, fromPath string, toOpen, fromOpen opener) error {
+	if _, err := os.Lstat(toPath); !os.IsNotExist(err) {
+		// Destination database exists (or is otherwise unavailable), do not
+		// attempt to overwrite it.
+		return nil
+	}
+
+	if _, err := os.Lstat(fromPath); err != nil {
+		// Source database is not available, so nothing to copy
+		return nil
+	}
+
+	fromDB, err := fromOpen(fromPath)
+	if err != nil {
+		return err
+	}
+	defer fromDB.Close()
+
+	toDB, err := toOpen(toPath)
+	if err != nil {
+		// That's odd, but it will be handled & reported in the usual path
+		// so we can ignore it here.
+		return err
+	}
+	defer toDB.Close()
+
+	l.Infoln("Copying database for format conversion...")
+	if err := copyBackend(toDB, fromDB); err != nil {
+		return err
+	}
+
+	// Move the old database out of the way to mark it as migrated.
+	fromDB.Close()
+	_ = os.Rename(fromPath, fromPath+".migrated."+time.Now().Format("20060102150405"))
+	return nil
+}
+
+func copyBackend(to, from Backend) error {
+	srcIt, err := from.NewPrefixIterator(nil)
+	if err != nil {
+		return err
+	}
+	defer srcIt.Release()
+
+	dstTx, err := to.NewWriteTransaction()
+	if err != nil {
+		return err
+	}
+	defer dstTx.Release()
+
+	for srcIt.Next() {
+		if err := dstTx.Put(srcIt.Key(), srcIt.Value()); err != nil {
+			return err
+		}
+	}
+	if srcIt.Error() != nil {
+		return err
+	}
+	srcIt.Release()
+
+	return dstTx.Commit()
+}