upload: return error if the short IDs collide for whatever reason

Metadata.go

@@ -3,6 +3,7 @@ package contented
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"log"
 	"net/http"
 	"os"
@@ -62,9 +63,17 @@ func (this *Server) AddMetadata(m Metadata) (string, error) {
 
 	err = this.db.Update(func(tx *bolt.Tx) error {
 		b := tx.Bucket(this.metadataBucket)
-		seq, _ := b.NextSequence() // cannot fail
+		seq, err := b.NextSequence()
+		if err != nil {
+			return fmt.Errorf("NextSequence: %w", err)
+		}
 		shortRef = idToString(seq)
-		return tx.Bucket(this.metadataBucket).Put([]byte(shortRef), jb)
+
+		if b.Get([]byte(shortRef)) != nil {
+			return fmt.Errorf("Next bucket sequence %d creates colliding ID %s", seq, shortRef)
+		}
+
+		return b.Put([]byte(shortRef), jb)
 	})
 	if err != nil {
 		return "", err