Closable processors are not really getting closed

[faec]

Some processors in libbeat have resources that need to be released when they're done. These processors implement the interface Closer in addition to the baseline Processor. However, it looks like the Close function is never getting called on such processors. This probably affects all Beats versions for at least the last ~2 years.

Here is the function that closes a processor, in libbeat/processors/processor.go:

// Close closes a processor if it implements the Closer interface
func Close(p Processor) error {
	if closer, ok := p.(Closer); ok {
		return closer.Close()
	}
	return nil
}

However, both Processor and Closer are interface types:

type Processor interface {
	Run(event *beat.Event) (*beat.Event, error)
	String() string
}

type Closer interface {
	Close() error
}

and interface casts like that only work if one interface is a refinement of the other, they do not check the underlying type. See for example this demonstration of the issue in a go playground. This probably means that the underlying struct for all closable processors is never receiving a Close call.

This is related to but independent from the recent error loops in #34716.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[faec]

Ah! A mistake in my testing, disregard.