Created attachment 994295 [details]
test.jsp (reproducer)

### Description of problem:

Getting the following "org.postgresql.util.PSQLException: Cannot change transaction isolation" at calling java.sql.Connection#setTransactionIsolation() when enabling connection validation for data source in EAP 6:

~~~
WARN  [org.jboss.jca.adapters.jdbc.local.LocalManagedConnectionFactory] (http-/127.0.0.1:8080-1) Error resetting transaction isolation : org.postgresql.util.PSQLException: Cannot change transaction isolation level in the middle of a transaction.
    at org.postgresql.jdbc2.AbstractJdbc2Connection.setTransactionIsolation(AbstractJdbc2Connection.java:929)
    at org.jboss.jca.adapters.jdbc.BaseWrapperManagedConnection.cleanup(BaseWrapperManagedConnection.java:357) [ironjacamar-jdbc-1.0.28.Final-redhat-1.jar:1.0.28.Final-redhat-1]
    at org.jboss.jca.core.connectionmanager.pool.mcp.SemaphoreArrayListManagedConnectionPool.returnConnection(SemaphoreArrayListManagedConnectionPool.java:479)
    at org.jboss.jca.core.connectionmanager.pool.AbstractPool.returnConnection(AbstractPool.java:586)
    at org.jboss.jca.core.connectionmanager.AbstractConnectionManager.returnManagedConnection(AbstractConnectionManager.java:452)
    at org.jboss.jca.core.connectionmanager.listener.NoTxConnectionListener.connectionClosed(NoTxConnectionListener.java:90)
    at org.jboss.jca.adapters.jdbc.BaseWrapperManagedConnection.closeHandle(BaseWrapperManagedConnection.java:584) [ironjacamar-jdbc-1.0.28.Final-redhat-1.jar:1.0.28.Final-redhat-1]
    at org.jboss.jca.adapters.jdbc.WrappedConnection.close(WrappedConnection.java:265) [ironjacamar-jdbc-1.0.28.Final-redhat-1.jar:1.0.28.Final-redhat-1]
    at org.apache.jsp.test_jsp._jspService(test_jsp.java:116)
  ...(snip)...
~~~


### How reproducible:

Anytime when using PostgreSQL doing the following steps to reproduce.


### Steps to Reproduce:

1. Configure the following data source in EAP 6 standalone.xml

                <datasource jndi-name="java:jboss/PostgresDS" pool-name="PostgresDS" enabled="true">
                    <connection-url>jdbc:postgresql://localhost:5432/jboss</connection-url>
                    <driver>postgresql</driver>
                    <pool>
                        <min-pool-size>1</min-pool-size>
                        <max-pool-size>1</max-pool-size>
                        <prefill>true</prefill>
                        <use-strict-min>true</use-strict-min>
                    </pool>
                    <security>
                        <user-name>postgres</user-name>
                        <password>postgres</password>
                    </security>
                    <validation>
                        <check-valid-connection-sql>select 1</check-valid-connection-sql>
                        <!-- <valid-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.postgres.PostgreSQLValidConnectionChecker"/> -->
                        <validate-on-match>true</validate-on-match>
                        <exception-sorter class-name="org.jboss.jca.adapters.jdbc.extensions.postgres.PostgreSQLExceptionSorter"/>
                    </validation>
                </datasource>


2. Access to the attached test.jsp **twice**. The first request will succeed without any exception but the second one will fail with the exception.


### Actual results:

Connection#setTransactionIsolation() failed with "org.postgresql.util.PSQLException: Cannot change transaction isolation"


### Expected results:

Connection#setTransactionIsolation() succeed wihtout "org.postgresql.util.PSQLException: Cannot change transaction isolation"


### Additional info:

 - Setting min-pool-size/max-pool-size to 1 was used here for ease to reproduce this issue.
 - Same issue occurred with <valid-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.postgres.PostgreSQLValidConnectionChecker"/> instead of using <check-valid-connection-sql>.
 - This exception did not occur when connection validation is not enabled.
 - This issue did not occur with Oracle and MySQL
   (I guess this different result is just caused by the implementation difference between JDBC driver. As noted in  https://docs.oracle.com/javase/6/docs/api/java/sql/Connection.html#setTransactionIsolation(int), it says "If this method is called during a transaction, the result is implementation-defined.")
 - The application does not use JTA transaction but just use JDBC transaction.
 - This issue occurred regardless of the "jta" attribute configuration (same issue occurred with jta="true" (default) and jta="false".)
 - This issue is originally reported in EAP 6.1.0 but same issue is reproducible with the latest EAP 6.3.3