mod_auth_digest: Detect during startup when AuthDigestProvider

is configured to use an incompatible provider via AuthnProviderAlias.
PR 45196 



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@672639 13f79535-47bb-0310-9956-ffa450edef68

include/mod_auth.h

@@ -93,6 +93,10 @@ typedef struct {
      */
     authn_status (*get_realm_hash)(request_rec *r, const char *user,
                                    const char *realm, char **rethash);
+
+    /* OK if provider can satisfy get_realm_hash(), APR_ENOTIMPL otherwise. */
+    apr_status_t (*has_realm_hash)(cmd_parms *cmd, const char *provider_name);
+
 } authn_provider;
 
 /* A linked-list of authn providers. */