Use absolute expiration in peristed db file

This commit ensures that the absolute unix expiration timestamp
is stored for applicable records in the persisted db file.

Previously the relative seconds was stored.

fixes #85

buntdb.go

@@ -61,6 +61,8 @@ var (
 	ErrTxIterating = errors.New("tx is iterating")
 )
 
+const useAbsEx = true
+
 // DB represents a collection of key-value pairs that persist on disk.
 // Transactions are used for all forms of data access to the DB.
 type DB struct {
@@ -895,22 +897,29 @@ func (db *DB) readLoad(rd io.Reader, modTime time.Time) (n int64, err error) {
 				return totalSize, ErrInvalid
 			}
 			if len(parts) == 5 {
-				if strings.ToLower(parts[3]) != "ex" {
+				arg := strings.ToLower(parts[3])
+				if arg != "ex" && arg != "ae" {
 					return totalSize, ErrInvalid
 				}
-				ex, err := strconv.ParseUint(parts[4], 10, 64)
+				ex, err := strconv.ParseInt(parts[4], 10, 64)
 				if err != nil {
 					return totalSize, err
 				}
+				var exat time.Time
 				now := time.Now()
-				dur := (time.Duration(ex) * time.Second) - now.Sub(modTime)
-				if dur > 0 {
+				if arg == "ex" {
+					dur := (time.Duration(ex) * time.Second) - now.Sub(modTime)
+					exat = now.Add(dur)
+				} else {
+					exat = time.Unix(ex, 0)
+				}
+				if exat.After(now) {
 					db.insertIntoDatabase(&dbItem{
 						key: parts[1],
 						val: parts[2],
 						opts: &dbItemOpts{
 							ex:   true,
-							exat: now.Add(dur),
+							exat: exat,
 						},
 					})
 				}
@@ -1330,13 +1339,19 @@ func appendBulkString(buf []byte, s string) []byte {
 // writeSetTo writes an item as a single SET record to the a bufio Writer.
 func (dbi *dbItem) writeSetTo(buf []byte, now time.Time) []byte {
 	if dbi.opts != nil && dbi.opts.ex {
-		ex := dbi.opts.exat.Sub(now) / time.Second
 		buf = appendArray(buf, 5)
 		buf = appendBulkString(buf, "set")
 		buf = appendBulkString(buf, dbi.key)
 		buf = appendBulkString(buf, dbi.val)
-		buf = appendBulkString(buf, "ex")
-		buf = appendBulkString(buf, strconv.FormatUint(uint64(ex), 10))
+		if useAbsEx {
+			ex := dbi.opts.exat.Unix()
+			buf = appendBulkString(buf, "ae")
+			buf = appendBulkString(buf, strconv.FormatUint(uint64(ex), 10))
+		} else {
+			ex := dbi.opts.exat.Sub(now) / time.Second
+			buf = appendBulkString(buf, "ex")
+			buf = appendBulkString(buf, strconv.FormatUint(uint64(ex), 10))
+		}
 	} else {
 		buf = appendArray(buf, 3)
 		buf = appendBulkString(buf, "set")