add MmapError type

db.go

@@ -39,6 +39,11 @@ const (
 // default page size for db is set to the OS page size.
 var defaultPageSize = os.Getpagesize()
 
+// MmapError represents an error resulting from a failed mmap call. Typically,
+// this error means that no further database writes will be possible. The most
+// common cause is insufficient disk space.
+type MmapError error
+
 // DB represents a collection of buckets persisted to a file on disk.
 // All data access is performed through transactions which can be obtained through the DB.
 // All the functions on DB will return a ErrDatabaseNotOpen if accessed before Open() is called.
@@ -280,7 +285,7 @@ func (db *DB) mmap(minsz int) error {
 		if err2 := mmap(db, db.datasz); err2 != nil {
 			panic(fmt.Sprintf("failed to revert db size after failed mmap: %v", err2))
 		}
-		return err
+		return MmapError(err)
 	}
 
 	// Save references to the meta pages.