Prevent write-to-closed-channel panics

[mkysel]

First the easy stuff:

• got rid of the FixedRegistry

Now the hard stuff:

• removed the ability to cancel a sub. It is extremely difficult to reason about channels being closed by the reader. It is an anti-pattern that can lead to panics.
• stop rebuilding the sub on every rebuild
• made the notifier use a buffered channel of 1 which removes the need for concurrent go routines.

[graphite-app]

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[macroscopeapp]

Remove CancelSubscription return values from NodeRegistry interface methods to prevent write-to-closed-channel panics

The NodeRegistry interface has been modified to remove CancelSubscription return values from OnNewNodes() and OnChangedNode() methods in pkg/registry/interface.go. The notifier implementation has been replaced with SingleNotificationNotifier that uses buffered channels and synchronous writes in pkg/registry/notifier.go. A new NodeRegistryWatcher component has been introduced in pkg/sync/nodeRegistryWatcher.go to handle node change subscriptions. The syncWorker has been refactored to use the new watcher component in pkg/sync/syncWorker.go. All implementations, mocks, and tests have been updated to match the new interface, and the FixedNodeRegistry implementation has been removed entirely.

📍Where to Start

Start with the modified NodeRegistry interface in pkg/registry/interface.go to understand the core interface changes, then review the new SingleNotificationNotifier implementation in pkg/registry/notifier.go.

Changes since #933 opened

• Refactored locking mechanism in NodeRegistryWatcher.triggerCancel method [c44adbd]
• Added documentation to NodeRegistryWatcher type and methods in sync package [e852668]
• Reformatted NodeRegistration struct initialization in syncWorker.subscribeToNode method [e852668]

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Macroscope summarized e852668.