Unexpected error "New transaction is not allowed because there are other threads running in the session"

Hi,

I've found a problem that appears to be a driver or database bug.

While fetching a resultset in non-transactional context it is not allowed to start a transaction - this is well-known, you'll get a "New transaction is not allowed because there are other threads running in the session."

However, when using a specific type of COUNT()-query that should only have exactly one result record, the resultset appears to be left open even when sqlsrv_fetch_array() has been called thus leading to unexpected failures when calling sqlsrv_begin_transaction() afterwards.

I have written a reproduction script, please check:

###################################################################
<?php

try {
    $db_conn = get_db_conn_sqlsrv();

    echo '<br>php version: [' . phpversion() . ']';
    echo '<br>sapi name: [' . php_sapi_name() . ']';
    echo '<br>sqlsrv_client_info: [' . print_r(sqlsrv_client_info($db_conn), true) . ']';

    /*
     * symptoms:
     * - when trying to open a transaction, this error occurs:
     * "New transaction is not allowed because there are other threads running in the session."
     * even though the resultset was fetched entirely!
     *
     * required column properties for physical tables:
     *  - contains at least one NULL value
     *  - column has no index
     */
    $query = <<<SQL
        SELECT COUNT(*) AS num_records FROM
        (
            SELECT  MIN(foo.bar) AS baz
            FROM  
            (
                SELECT NULL AS bar
                
                  UNION
                  
                SELECT 'bar' AS bar
            ) foo
            
            UNION
            
            SELECT  NULL AS bar
        ) foobar
SQL;
    
    $ps = sqlsrv_prepare($db_conn, $query, array(), array('Scrollable' => SQLSRV_CURSOR_FORWARD));
    if (!$ps) {
        throw new Exception('no ps');
    }
    if (!sqlsrv_execute($ps)) {
        throw new Exception('execute failed');
    }
    
    $result = sqlsrv_fetch_array($ps);
    var_dump($result);
    
    // uncomment to workaround the unexpected error below:
    //$result = sqlsrv_fetch_array($ps);
    //var_dump($result);
    
    // this fails but should not fail
    if (!sqlsrv_begin_transaction($db_conn)) {
        throw new Exception(print_r(sqlsrv_errors(), true));
    }
    
} catch (Exception $e) {
    var_dump($e);
}

function get_db_conn_sqlsrv()
{
    $host = "127.0.0.1\SQLExpress,1433";
    $conn_info = array(
        'Database'  => '[to_be_replaced]',
        'MultipleActiveResultSets'  => true,
        'CharacterSet' => 'UTF-8',
        'UID' => '[to_be_replaced]',
        'PWD' => '[to_be_replaced]'
        );

    sqlsrv_configure('WarningsReturnAsErrors', 0);
    $conn = sqlsrv_connect($host, $conn_info);
    if (!$conn) {
        throw new Exception('no link');
    }

    return $conn;
}
###################################################################

Expected result: no error when calling sqlsrv_begin_transaction().
Actual result: "New transaction is not allowed because there are other threads running in the session."

php version: [7.0.15]
sqlsrv_client_info: [
(
    [DriverDllName] => msodbcsql11.dll
    [DriverODBCVer] => 03.80
    [DriverVer] => 12.00.5579
    [ExtensionVer] => 4.1.8930.1
)

Best regards
Lars