chore: Fix unexpected connection-pool shut-down

[newtork]

Context

Related

• Apache HTTP Client: Connection pool shut down #1093
• Apache HTTP Client: Connection pool shut down #1058
• Apache HTTP Client: Connection pool shut down #806
• Repeated "Connection pool shut down" errors after updating the Cloud SDK version #970

I was wondering whether the following rule always holds true...?

Tenant+Destination 1 <-> 1 (Cloud SDK) HttpClient 1 <-> 1 Connection Manager 1 <-> 1 Connection Pool

I thought that would be correct, until I realized there's one exception in HttpClientWrapper.
The following method creates a new HttpClient and inherits the connection-pool implicitly:

cloud-sdk-java/cloudplatform/connectivity-apache-httpclient4/src/main/java/com/sap/cloud/sdk/cloudplatform/connectivity/HttpClientWrapper.java

Lines 94 to 107 in 5b1c7fe

	HttpClientWrapper withDestination( final HttpDestinationProperties destination )
	{
	// explicitly check the reference equality, since equals doesn't check header providers
	// this is a slight improvement, avoiding unnecessary wrapper instantiation
	// in cases where destination objects are reused / served from cache
	if( !destination.equals(this.destination) ) {
	throw new ShouldNotHappenException(
	"This method must not be used outside of updating an instance of HttpClientWrapper for http clients served from the HttpClientCache.");
	}
	if( destination == this.destination ) {
	return this;
	}
	return new HttpClientWrapper(httpClient, destination);
	}

I'm able to reproduce the reported bug.
That even can explain the observed behavior from garbage collector.

Definition of Done

• Functionality scope stated & covered
• Tests cover the scope above
• Error handling created / updated & covered by the tests above
• Documentation updated
• Release notes updated

[Jonas-Isr]

Mostly LGTM :)