lib/model: Fix accounting error in rescan with multiple subs (fixes #3028)

When doing prefix scans in the database, "foo" should not be considered
a prefix of "foo2". Instead, it should match "foo" exactly and also
strings with the prefix "foo/". This is more restrictive than what the
standard leveldb prefix scan does so we add some code to enforce it.

Also exposes the initialScanCompleted on the rwfolder for testing, and
change it to be a channel (so we can wait for it from another
goroutine). Otherwise we can't be sure when the initial scan has
completed, and we need to wait for that or it might pick up changes
we're doing at an unexpected time.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3067

lib/db/leveldb_dbinstance.go

@@ -262,7 +262,17 @@ func (db *Instance) withHave(folder, device, prefix []byte, truncate bool, fn It
 	dbi := t.NewIterator(util.BytesPrefix(db.deviceKey(folder, device, prefix)[:keyPrefixLen+keyFolderLen+keyDeviceLen+len(prefix)]), nil)
 	defer dbi.Release()
 
+	slashedPrefix := prefix
+	if !bytes.HasSuffix(prefix, []byte{'/'}) {
+		slashedPrefix = append(slashedPrefix, '/')
+	}
+
 	for dbi.Next() {
+		name := db.deviceKeyName(dbi.Key())
+		if len(prefix) > 0 && !bytes.Equal(name, prefix) && !bytes.HasPrefix(name, slashedPrefix) {
+			return
+		}
+
 		// The iterator function may keep a reference to the unmarshalled
 		// struct, which in turn references the buffer it was unmarshalled
 		// from. dbi.Value() just returns an internal slice that it reuses, so
@@ -359,6 +369,11 @@ func (db *Instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator
 	dbi := t.NewIterator(util.BytesPrefix(db.globalKey(folder, prefix)), nil)
 	defer dbi.Release()
 
+	slashedPrefix := prefix
+	if !bytes.HasSuffix(prefix, []byte{'/'}) {
+		slashedPrefix = append(slashedPrefix, '/')
+	}
+
 	var fk []byte
 	for dbi.Next() {
 		var vl versionList
@@ -370,7 +385,12 @@ func (db *Instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator
 			l.Debugln(dbi.Key())
 			panic("no versions?")
 		}
+
 		name := db.globalKeyName(dbi.Key())
+		if len(prefix) > 0 && !bytes.Equal(name, prefix) && !bytes.HasPrefix(name, slashedPrefix) {
+			return
+		}
+
 		fk = db.deviceKeyInto(fk[:cap(fk)], folder, vl.versions[0].device, name)
 		bs, err := t.Get(fk, nil)
 		if err != nil {