state/nodestore: in memory representation of nodes

Initial work on a nodestore which stores all of the nodes
and their relations in memory with relationship for peers
precalculated.

It is a copy-on-write structure, replacing the "snapshot"
when a change to the structure occurs. It is optimised for reads,
and while batches are not fast, they are grouped together
to do less of the expensive peer calculation if there are many
changes rapidly.

Writes will block until commited, while reads are never
blocked.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/db/users.go

@@ -198,19 +198,20 @@ func ListNodesByUser(tx *gorm.DB, uid types.UserID) (types.Nodes, error) {
 }
 
 // AssignNodeToUser assigns a Node to a user.
+// Note: Validation should be done in the state layer before calling this function.
 func AssignNodeToUser(tx *gorm.DB, nodeID types.NodeID, uid types.UserID) error {
-	node, err := GetNodeByID(tx, nodeID)
-	if err != nil {
-		return err
+	// Check if the user exists
+	var userExists bool
+	if err := tx.Model(&types.User{}).Select("count(*) > 0").Where("id = ?", uid).Find(&userExists).Error; err != nil {
+		return fmt.Errorf("failed to check if user exists: %w", err)
 	}
-	user, err := GetUserByID(tx, uid)
-	if err != nil {
-		return err
+
+	if !userExists {
+		return ErrUserNotFound
 	}
-	node.User = *user
-	node.UserID = user.ID
-	if result := tx.Save(&node); result.Error != nil {
-		return result.Error
+
+	if err := tx.Model(&types.Node{}).Where("id = ?", nodeID).Update("user_id", uid).Error; err != nil {
+		return fmt.Errorf("failed to assign node to user: %w", err)
 	}
 
 	return nil