NIFI-9448 Improve S2S HTTP Extend Transaction Exception Handling

[exceptionfactory]

Description of PR

NIFI-9448 Improves Site-to-Site HTTP client processing for extend transaction exception handling.

As part of sending or receive files over an HTTP S2S connection, the SiteToSiteRestApiClient starts a background command to request extension of the current transaction on a periodic basis. In situations when the underlying HTTP connection pool throws an IllegalStateException, the background command attempts to close the parent SiteToSiteRestApiClient. This behavior short-circuits potential retry attempts that might otherwise be allowed according to the configured timeout settings.

These changes include refactoring the background command to a separate ExtendTransactionCommand class for easier testing. With the main SiteToSiteRestApiClient already wrapping an Apache HttpComponents HttpClient instance and connection pool, the refactored ExtendTransactionCommand reuses the same client and connection pool as opposed to creating a new instance of SiteToSiteRestApiClient. Additional changes include a null check in PeerSelector to avoid a potential NullPointerException when the Peer Persistence property is not configured. This pull request also removes the SiteToSiteClientIT as it does not provide significant value beyond the existing unit tests.

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[gresockj]

Thanks for this improvement, @exceptionfactory. I think this makes the transaction extension code more intuitive, and removes the additional client created for transaction extension. I was able to reproduce the issue on a cluster under heavy S2S load by using a remote debugger, closing the HttpClient right before extending the transaction, and did not detect any adverse behavior.

Will merge.