fix: metadata TTL

[shanduur]

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BUGFIX: This code change involves modifying the logic for handling metadata updates for Linode instances in a Kubernetes cluster. Previous behaviour spammed, did redundant work on Create and Update, and every time it was triggered, it pulled the node from the API. This prevents hitting the API too often.

1. instances.go:

   • The timeout variable is changed to be initialised with a default value of 15.
   • If the environment variable LINODE_INSTANCE_CACHE_TTL is set, the timeout is updated accordingly, but now it checks if the parsed value is greater than 0 before updating the timeout. This ensures that only positive values are considered valid for the timeout.

2. node_controller.go:

   • The nodeController structure is extended to include a sync.RWMutex for safe concurrent access to shared data.
   • Two new fields, metadataLastUpdate and ttl, are added to keep track of the last metadata update time and the time-to-live for metadata, respectively.
   • The timeout variable is introduced, which defaults to 300 seconds (5 minutes), and it can be overridden by the LINODE_METADATA_TTL environment variable.
   • Two new methods, LastMetadataUpdate and SetLastMetadataUpdate, are added to get and set the last metadata update time for a given node name in a thread safe manner.

   Changes in handleNodeAdded:

   • Before attempting to fetch Linode information, it checks the last update time and the TTL to decide whether a fresh metadata update is required.
   • The SetLastMetadataUpdate method is called after a successful update to store the current time as the last update time for that node.

   Changes in Run:

   • The UpdateFunc in the informer is removed. This function was handling updated nodes' metadata, but it's redundant.

In summary, the changes provide better control over the caching mechanism for Linode instance metadata. The introduction of a TTL for metadata and the ability to configure it through environment variables allow for more flexibility and optimisation in handling updates. Additionally, the use of a mutex ensures thread-safe access to shared data in a concurrent environment.