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This code defines a feature gate implementation that uses environment variables to control feature states.

Key Components:

• envVarFeatureGates struct: This struct holds the logic for managing feature gates, including:
• known: A map storing all registered features and their default values and lock settings.
• enabledViaEnvVar: An atomic value containing a map of features and their states obtained from environment variables.
• enabledViaSetMethod: A map of features and their states explicitly set using the Set method.
• readEnvVarsOnce: A sync.Once used to ensure environment variable reading happens only once.
• readEnvVars: An atomic boolean indicating if environment variables have been read.
• callSiteName: The name of the file where the envVarFeatureGates instance is created, used for debugging.

Functionality:

1. Initialization:

• newEnvVarFeatureGates: Creates a new envVarFeatureGates instance. It initializes the known map from the provided features argument, and sets up the enabledViaEnvVar and enabledViaSetMethod maps.
• getEnabledMapFromEnvVar: This function is executed only once, using sync.Once. It reads environment variables of the form KUBE_FEATURE_<feature_name> and populates the enabledViaEnvVar map. If an environment variable cannot be parsed as a boolean or conflicts with a locked feature, an error is logged but ignored.

2. Feature State Retrieval:

• Enabled: This method checks if a given feature is enabled. It first looks for the feature in enabledViaSetMethod. If found, the value from enabledViaSetMethod is returned. Otherwise, it retrieves the value from enabledViaEnvVar. If the feature is not found in either map, the default value from known is used.

3. Feature State Modification:

• Set: Allows setting a feature’s state explicitly. It validates the feature name and ensures that locked features cannot be set to a value different from their default.

Key Points:

• Feature Lock: The LockToDefault field in FeatureSpec prevents changing a feature’s state from its default value.
• Environment Variable Preference: The Set method overrides values set by environment variables, giving explicit control to users.
• Error Handling: Errors during environment variable parsing or setting locked features are logged but ignored to prevent fatal failures.

Use Cases:

This code is designed to manage feature flags in applications that require flexible control over functionality.

• Development and Testing: Feature gates enable toggling features during development and testing without code changes.
• Production Rollout: Gradual rollouts of new features can be done by enabling them for a subset of users through environment variables.
• Feature Toggles: This allows for quick disabling or enabling of features in production, providing flexibility and control over functionality.

Overall:

The envVarFeatureGates implementation provides a robust and flexible way to manage feature flags in Go applications, leveraging environment variables and explicit control for feature toggling.