Fix connection pool shutdown by reusing cached SSL connections

Use computeIfAbsent() instead of compute() to prevent creating a new
PoolingHttpClientConnectionManager on every createClient() call. This
fixes "Connection pool shut down" exceptions that occurred when multiple
HttpClients shared a connection manager that was inadvertently replaced.

Author: kuntzed

token-client/src/main/java/com/sap/cloud/security/client/DefaultHttpClientFactory.java

@@ -56,8 +56,8 @@ public CloseableHttpClient createClient(ClientIdentity clientIdentity) throws Ht
 		HttpClientBuilder httpClientBuilder = HttpClients.custom().setDefaultRequestConfig(requestConfig);
 
 		if (clientId != null && clientIdentity.isCertificateBased()) {
-			SslConnection connectionPool = sslConnectionPool.compute(clientId,
-					(s, c) -> new SslConnection(clientIdentity));
+			SslConnection connectionPool = sslConnectionPool.computeIfAbsent(clientId,
+					s -> new SslConnection(clientIdentity));
 			return httpClientBuilder
 					.setConnectionManager(connectionPool.poolingConnectionManager)
 					.setSSLContext(connectionPool.context)