Keep trying, you are nearly there.

src/txacme/challenges/_libcloud.py

@@ -12,6 +12,7 @@
 
 from txacme.errors import NotInZone, ZoneNotFound
 from txacme.interfaces import IResponder
+from txacme.util import const
 
 
 def _daemon_thread(*a, **kw):
@@ -135,7 +136,7 @@ def create(cls, reactor, driver_name, username, password, zone_name=None,
         """
         return cls(
             reactor=reactor,
-            thread_pool=pool(lambda: 1, threadFactory=_daemon_thread),
+            thread_pool=pool(const(1), threadFactory=_daemon_thread),
             driver=get_driver(driver_name)(username, password),
             zone_name=zone_name,
             settle_delay=settle_delay)

src/txacme/test/test_util.py

@@ -19,8 +19,8 @@
 from txacme.test.matchers import ValidForName
 from txacme.test.test_client import RSA_KEY_512, RSA_KEY_512_RAW
 from txacme.util import (
-    cert_cryptography_to_pyopenssl, csr_for_names, decode_csr, encode_csr,
-    generate_private_key, generate_tls_sni_01_cert,
+    cert_cryptography_to_pyopenssl, const, csr_for_names, decode_csr,
+    encode_csr, generate_private_key, generate_tls_sni_01_cert,
     key_cryptography_to_pyopenssl)
 
 
@@ -160,4 +160,15 @@ def test_common_name_too_long(self):
                         u'san.too.long.invalid')]))))
 
 
-__all__ = ['GeneratePrivateKeyTests', 'GenerateCertTests', 'CSRTests']
+class ConstTests(TestCase):
+    """
+    `~txacme.util.const` returns a function that always returns a constant
+    value.
+    """
+    @given(s.integers())
+    def test_const(self, x):
+        self.assertThat(const(x)(), Equals(x))
+
+
+__all__ = [
+    'GeneratePrivateKeyTests', 'GenerateCertTests', 'CSRTests', 'ConstTests']

src/txacme/util.py

@@ -179,8 +179,15 @@ def check_directory_url_type(url):
             'got {!r} instead'.format(url))
 
 
+def const(x):
+    """
+    Return a constant function.
+    """
+    return lambda: x
+
+
 __all__ = [
     'generate_private_key', 'generate_tls_sni_01_cert',
     'cert_cryptography_to_pyopenssl', 'key_cryptography_to_pyopenssl', 'tap',
     'encode_csr', 'decode_csr', 'csr_for_names', 'clock_now',
-    'check_directory_url_type']
+    'check_directory_url_type', 'const']