lib/db: Refactor: use a Lowlevel type underneath Instance (ref #5198) (#5212)

This adds a thin type that holds the state associated with the
leveldb.DB, leaving the huge Instance type more or less stateless. Also
moves some keying stuff into the DB package so that other packages need
not know the keying specifics.

(This does not, yet, fix the cmd/stindex program, in order to keep the
diff size down. Hence the keying constants are still exported.)

lib/db/smallindex.go

@@ -8,6 +8,7 @@ package db
 
 import (
 	"encoding/binary"
+	"sort"
 
 	"github.com/syncthing/syncthing/lib/sync"
 	"github.com/syndtr/goleveldb/leveldb"
@@ -46,8 +47,11 @@ func (i *smallIndex) load() {
 	for it.Next() {
 		val := string(it.Value())
 		id := binary.BigEndian.Uint32(it.Key()[len(i.prefix):])
-		i.id2val[id] = val
-		i.val2id[val] = id
+		if val != "" {
+			// Empty value means the entry has been deleted.
+			i.id2val[id] = val
+			i.val2id[val] = id
+		}
 		if id >= i.nextID {
 			i.nextID = id + 1
 		}
@@ -96,3 +100,45 @@ func (i *smallIndex) Val(id uint32) ([]byte, bool) {
 
 	return []byte(val), true
 }
+
+func (i *smallIndex) Delete(val []byte) {
+	i.mut.Lock()
+	defer i.mut.Unlock()
+
+	// Check the reverse mapping to get the ID for the value.
+	if id, ok := i.val2id[string(val)]; ok {
+		// Generate the corresponding database key.
+		key := make([]byte, len(i.prefix)+8) // prefix plus uint32 id
+		copy(key, i.prefix)
+		binary.BigEndian.PutUint32(key[len(i.prefix):], id)
+
+		// Put an empty value into the database. This indicates that the
+		// entry does not exist any more and prevents the ID from being
+		// reused in the future.
+		i.db.Put(key, []byte{}, nil)
+
+		// Delete reverse mapping.
+		delete(i.id2val, id)
+	}
+
+	// Delete forward mapping.
+	delete(i.val2id, string(val))
+}
+
+// Values returns the set of values in the index
+func (i *smallIndex) Values() []string {
+	// In principle this method should return [][]byte because all the other
+	// methods deal in []byte keys. However, in practice, where it's used
+	// wants a []string and it's easier to just create that here rather than
+	// having to convert both here and there...
+
+	i.mut.Lock()
+	vals := make([]string, 0, len(i.val2id))
+	for val := range i.val2id {
+		vals = append(vals, val)
+	}
+	i.mut.Unlock()
+
+	sort.Strings(vals)
+	return vals
+}