Implement database abstraction, error checking (ref #5907) (#6107)

This PR does two things, because one lead to the other:

- Move the leveldb specific stuff into a small "backend" package that
defines a backend interface and the leveldb implementation. This allows,
potentially, in the future, switching the db implementation so another
KV store should we wish to do so.

- Add proper error handling all along the way. The db and backend
packages are now errcheck clean. However, I drew the line at modifying
the FileSet API in order to keep this manageable and not continue
refactoring all of the rest of Syncthing. As such, the FileSet methods
still panic on database errors, except for the "database is closed"
error which is instead handled by silently returning as quickly as
possible, with the assumption that we're anyway "on the way out".

cmd/stindex/dump.go

@@ -13,11 +13,15 @@ import (
 	"time"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
-func dump(ldb *db.Lowlevel) {
-	it := ldb.NewIterator(nil, nil)
+func dump(ldb backend.Backend) {
+	it, err := ldb.NewPrefixIterator(nil)
+	if err != nil {
+		log.Fatal(err)
+	}
 	for it.Next() {
 		key := it.Key()
 		switch key[0] {

cmd/stindex/dumpsize.go

@@ -10,8 +10,10 @@ import (
 	"container/heap"
 	"encoding/binary"
 	"fmt"
+	"log"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 type SizedElement struct {
@@ -37,11 +39,14 @@ func (h *ElementHeap) Pop() interface{} {
 	return x
 }
 
-func dumpsize(ldb *db.Lowlevel) {
+func dumpsize(ldb backend.Backend) {
 	h := &ElementHeap{}
 	heap.Init(h)
 
-	it := ldb.NewIterator(nil, nil)
+	it, err := ldb.NewPrefixIterator(nil)
+	if err != nil {
+		log.Fatal(err)
+	}
 	var ele SizedElement
 	for it.Next() {
 		key := it.Key()

cmd/stindex/idxck.go

@@ -10,8 +10,10 @@ import (
 	"bytes"
 	"encoding/binary"
 	"fmt"
+	"log"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
@@ -31,7 +33,7 @@ type sequenceKey struct {
 	sequence uint64
 }
 
-func idxck(ldb *db.Lowlevel) (success bool) {
+func idxck(ldb backend.Backend) (success bool) {
 	folders := make(map[uint32]string)
 	devices := make(map[uint32]string)
 	deviceToIDs := make(map[string]uint32)
@@ -42,7 +44,10 @@ func idxck(ldb *db.Lowlevel) (success bool) {
 	var localDeviceKey uint32
 	success = true
 
-	it := ldb.NewIterator(nil, nil)
+	it, err := ldb.NewPrefixIterator(nil)
+	if err != nil {
+		log.Fatal(err)
+	}
 	for it.Next() {
 		key := it.Key()
 		switch key[0] {

cmd/stindex/main.go

@@ -13,7 +13,7 @@ import (
 	"os"
 	"path/filepath"
 
-	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 func main() {
@@ -30,7 +30,7 @@ func main() {
 		path = filepath.Join(defaultConfigDir(), "index-v0.14.0.db")
 	}
 
-	ldb, err := db.OpenRO(path)
+	ldb, err := backend.OpenLevelDBRO(path)
 	if err != nil {
 		log.Fatal(err)
 	}