When a node was registered with a tags-only PreAuthKey (no user
associated), the node had User=nil and UserID=nil. When attempting to
re-register this node to a different user via HandleNodeFromAuthPath,
two issues occurred:
1. The code called oldUser.Name() without checking if oldUser was valid,
causing a nil pointer dereference panic.
2. The existing node lookup logic didn't find the tagged node because it
searched by (machineKey, userID), but tagged nodes have no userID.
This caused a new node to be created instead of updating the existing
tagged node.
Fix this by restructuring HandleNodeFromAuthPath to:
1. First check if a node exists for the same user (existing behavior)
2. If not found, check if an existing TAGGED node exists with the same
machine key (regardless of userID)
3. If a tagged node exists, UPDATE it to convert from tagged to
user-owned (preserving the node ID)
4. Only create a new node if the existing node is user-owned by a
different user
This ensures consistent behavior between:
- personal → tagged → personal (same node, same owner)
- tagged (no user) → personal (same node, new owner)
Add a test that reproduces the panic and conversion scenario by:
1. Creating a tags-only PreAuthKey (no user)
2. Registering a node with that key
3. Re-registering the same machine to a different user
4. Verifying the node ID stays the same (conversion, not creation)
Fixes#3038
306aabbbce