Make the schema cache TTL user-configurable

The API schema is not checked for changes until after a TTL
is expired. A one-hour TTL was hardcoded which makes development
tedious because the only way to force a schema update is to
remember to remove files between invocations.

This adds a new environment variable, schema_ttl, to configure
the TTL returned by the server to schema() calls. This can be
set low to ensure a frequent refresh during development.

If the client is in compat mode, that is if client is working
against a server that doesn't support the schema() command,
then use the client's schema_ttl instead so that the user still
has control.

Re-check validity before writing the cache. This saves us both
a disk write and the possibility of updating the expiration
with a ttl of 0. This can happen if the fingerprint is still
valid (not expired, no language change) the schema check is
skipped so we have no server-provided ttl.

https://pagure.io/freeipa/issue/8492

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-By: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipalib/constants.py

@@ -183,6 +183,9 @@ DEFAULT_CONFIG = (
     # How long to wait for a certmonger request to finish
     ('certmonger_wait_timeout', 300),
 
+    # Number of seconds before client should check for schema update.
+    ('schema_ttl', 3600),
+
     # Web Application mount points
     ('mount_ipa', '/ipa/'),