nss: Implement <nss_database.h>

This code manages the mappings of the available databases in NSS
(i.e. passwd, hosts, netgroup, etc) with the actions that should
be taken to do a query on those databases.

This is the main API between query functions scattered throughout
glibc and the underlying code (actions, modules, etc).

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

nss/nss_database.h

@@ -0,0 +1,88 @@
+/* Mapping NSS services to action lists.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _NSS_DATABASE_H
+#define _NSS_DATABASE_H
+
+#include <file_change_detection.h>
+
+/* Each "line" in nsswitch.conf maps a supported database (example:
+   passwd) to one or more name service providers (example: files dns).
+   Internally, each name service provider (example: dns) is a
+   dynamically loadable module (i.e. libnss_dns.so), managed by
+   nss_module.h.  The sequence of providers and rules (example: files
+   [SUCCESS=RETURN] dns) is mapped by nss_action.h to a cached entry
+   which encodes the sequence of modules and rules.  Keeping track of
+   all supported databases and their corresponding actions is done
+   here.
+
+   The key entry is __nss_database_get, which provides a set of
+   actions which can be used with nss_lookup_function() and
+   nss_next().  Callers should assume that these functions are fast,
+   and should not cache the result longer than needed.  */
+
+#include "nss_action.h"
+
+/* The enumeration literal in enum nss_database for the database NAME
+   (e.g., nss_database_hosts for hosts).  */
+#define NSS_DATABASE_LITERAL(name) nss_database_##name
+
+enum nss_database
+{
+#define DEFINE_DATABASE(name) NSS_DATABASE_LITERAL (name),
+#include "databases.def"
+#undef DEFINE_DATABASE
+
+  /* Total number of databases.  */
+  NSS_DATABASE_COUNT
+};
+
+
+/* Looks up the action list for DB and stores it in *ACTIONS.  Returns
+   true on success or false on failure.  Success can mean that
+   *ACTIONS is NULL.  */
+bool __nss_database_get (enum nss_database db, nss_action_list *actions)
+  attribute_hidden;
+
+/* Like __nss_database_get, but does not reload /etc/nsswitch.conf
+   from disk.  This assumes that there has been a previous successful
+   __nss_database_get call (which may not have returned any data).  */
+nss_action_list __nss_database_get_noreload (enum nss_database db)
+  attribute_hidden;
+
+/* Called from __libc_freeres.  */
+void __nss_database_freeres (void) attribute_hidden;
+
+/* Internal type.  Exposed only for fork handling purposes.  */
+struct nss_database_data
+{
+  struct file_change_detection nsswitch_conf;
+  nss_action_list services[NSS_DATABASE_COUNT];
+  int reload_disabled;          /* Actually bool; int for atomic access.  */
+  bool initialized;
+};
+
+/* Called by fork in the parent process, before forking.  */
+void __nss_database_fork_prepare_parent (struct nss_database_data *data)
+  attribute_hidden;
+
+/* Called by fork in the new subprocess, after forking.  */
+void __nss_database_fork_subprocess (struct nss_database_data *data)
+  attribute_hidden;
+
+#endif /* _NSS_DATABASE_H */