bring back last_seen in database (#2579)

* db: add back last_seen to the database

Fixes #2574

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

* integration: ensure last_seen is set

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

---------

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

hscontrol/db/db.go

@@ -672,7 +672,24 @@ AND auth_key_id NOT IN (
 			{
 				ID: "202502171819",
 				Migrate: func(tx *gorm.DB) error {
-					_ = tx.Migrator().DropColumn(&types.Node{}, "last_seen")
+					// This migration originally removed the last_seen column
+					// from the node table, but it was added back in
+					// 202505091439.
+					return nil
+				},
+				Rollback: func(db *gorm.DB) error { return nil },
+			},
+			// Add back last_seen column to node table.
+			{
+				ID: "202505091439",
+				Migrate: func(tx *gorm.DB) error {
+					// Add back last_seen column to node table if it does not exist.
+					// This is a workaround for the fact that the last_seen column
+					// was removed in the 202502171819 migration, but only for some
+					// beta testers.
+					if !tx.Migrator().HasColumn(&types.Node{}, "last_seen") {
+						_ = tx.Migrator().AddColumn(&types.Node{}, "last_seen")
+					}
 
 					return nil
 				},

hscontrol/db/node.go

@@ -251,6 +251,20 @@ func SetApprovedRoutes(
 	return nil
 }
 
+// SetLastSeen sets a node's last seen field indicating that we
+// have recently communicating with this node.
+func (hsdb *HSDatabase) SetLastSeen(nodeID types.NodeID, lastSeen time.Time) error {
+	return hsdb.Write(func(tx *gorm.DB) error {
+		return SetLastSeen(tx, nodeID, lastSeen)
+	})
+}
+
+// SetLastSeen sets a node's last seen field indicating that we
+// have recently communicating with this node.
+func SetLastSeen(tx *gorm.DB, nodeID types.NodeID, lastSeen time.Time) error {
+	return tx.Model(&types.Node{}).Where("id = ?", nodeID).Update("last_seen", lastSeen).Error
+}
+
 // RenameNode takes a Node struct and a new GivenName for the nodes
 // and renames it. If the name is not unique, it will return an error.
 func RenameNode(tx *gorm.DB,