Update cache to prevent adding/updating users without group

This follows the changes to the userinfo returned by IsAuthenticated. We
have validation for the return before getting to the cache, but it's
best to have multiple layers of protection.

internal/cache/getusers.go

@@ -13,7 +13,7 @@ type UserPasswdShadow struct {
 	Name  string
 	UID   int
 	GID   int
-	Gecos string
+	Gecos string // Gecos is an optional field. It can be empty.
 	Dir   string
 	Shell string
 

internal/cache/update.go

@@ -16,15 +16,17 @@ import (
 // UpdateFromUserInfo inserts or updates user and group buckets from the user information.
 func (c *Cache) UpdateFromUserInfo(u users.UserInfo) error {
 	// create bucket contents dynamically
-	gid := -1
-	if len(u.Groups) > 0 && u.Groups[0].GID != nil {
-		gid = *u.Groups[0].GID
+	if len(u.Groups) == 0 {
+		return fmt.Errorf("no group provided for user %s (%v)", u.Name, u.UID)
+	}
+	if u.Groups[0].GID == nil {
+		return fmt.Errorf("no gid provided for default group %q", u.Groups[0].Name)
 	}
 	userDB := userDB{
 		UserPasswdShadow: UserPasswdShadow{
 			Name:           u.Name,
 			UID:            u.UID,
-			GID:            gid,
+			GID:            *u.Groups[0].GID,
 			Gecos:          u.Gecos,
 			Dir:            u.Dir,
 			Shell:          u.Shell,
@@ -66,26 +68,6 @@ func (c *Cache) UpdateFromUserInfo(u users.UserInfo) error {
 			return err
 		}
 
-		// No groups were specified for this request.
-		if userDB.GID == -1 {
-			if len(previousGroupsForCurrentUser.GIDs) == 0 {
-				return fmt.Errorf("no group provided for user %v (%v) and no previous record found", userDB.Name, userDB.UID)
-			}
-
-			for _, gid := range previousGroupsForCurrentUser.GIDs {
-				g, err := getFromBucket[groupDB](buckets[groupByIDBucketName], gid)
-				if err != nil {
-					c.requestClearDatabase()
-					return err
-				}
-				groupContents = append(groupContents, groupDB{
-					Name: g.Name,
-					GID:  g.GID,
-				})
-			}
-			userDB.GID = groupContents[0].GID
-		}
-
 		/* 1. Handle user update */
 		updateUser(buckets, userDB)