<nss_action.h>: New abstraction for combining NSS modules and NSS actions

nss_action manages a set of lists of actions; these are the portions
of the lines in nsswitch.conf to the right of the colons, like
"dns [!UNAVAIL=return] files".  Each permutation of actions and
conditionals is cached for reuse, which limits memory growth, and
refers to the static list of modules managed by nss_modules.

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

nss/nss_action.c

@@ -0,0 +1,116 @@
+/* NSS actions, elements in a nsswitch.conf configuration line.
+   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/>.  */
+
+#include <nss_action.h>
+
+#include <string.h>
+#include <libc-lock.h>
+
+/* Maintain a global list of NSS action lists.  Since most databases
+   use the same list of actions, this list is usually short.
+   Deduplication in __nss_action_allocate ensures that the list does
+   not grow without bounds.  */
+
+struct nss_action_list_wrapper
+{
+  /* The next element of the list.  */
+  struct nss_action_list_wrapper *next;
+
+  /* Number of elements in the list (excluding the terminator).  */
+  size_t count;
+
+  /* NULL-terminated list of actions.  */
+  struct nss_action actions[];
+};
+
+/* Toplevel list of allocated NSS action lists.  */
+static struct nss_action_list_wrapper *nss_actions;
+
+/* Lock covers the nss_actions list.  */
+__libc_lock_define (static, nss_actions_lock);
+
+/* Returns true if the actions are equal (same module, same actions
+   array).  */
+static bool
+actions_equal (const struct nss_action *a, const struct nss_action *b)
+{
+  return a->module == b->module && a->action_bits == b->action_bits;
+}
+
+
+/* Returns true if COUNT actions at A and B are equal (according to
+   actions_equal above). Caller must ensure that either A or B have at
+   least COUNT actions.  */
+static bool
+action_lists_equal (const struct nss_action *a, const struct nss_action *b,
+                    size_t count)
+{
+  for (size_t i = 0; i < count; ++i)
+    if (!actions_equal (a + i, b + i))
+      return false;
+  return true;
+}
+
+/* Returns a pre-allocated action list for COUNT actions at ACTIONS,
+   or NULL if no such list exists.  */
+static nss_action_list
+find_allocated (struct nss_action *actions, size_t count)
+{
+  for (struct nss_action_list_wrapper *p = nss_actions; p != NULL; p = p->next)
+    if (p->count == count && action_lists_equal (p->actions, actions, count))
+      return p->actions;
+  return NULL;
+}
+
+nss_action_list
+__nss_action_allocate (struct nss_action *actions, size_t count)
+{
+  nss_action_list result = NULL;
+  __libc_lock_lock (nss_actions_lock);
+
+  result = find_allocated (actions, count);
+  if (result == NULL)
+    {
+      struct nss_action_list_wrapper *wrapper
+        = malloc (sizeof (*wrapper) + sizeof (*actions) * count);
+      if (wrapper != NULL)
+        {
+          wrapper->next = nss_actions;
+          wrapper->count = count;
+          memcpy (wrapper->actions, actions, sizeof (*actions) * count);
+          nss_actions = wrapper;
+          result = wrapper->actions;
+        }
+    }
+
+  __libc_lock_unlock (nss_actions_lock);
+  return result;
+}
+
+void __libc_freeres_fn_section
+__nss_action_freeres (void)
+{
+  struct nss_action_list_wrapper *current = nss_actions;
+  while (current != NULL)
+    {
+      struct nss_action_list_wrapper *next = current->next;
+      free (current);
+      current = next;
+    }
+  nss_actions = NULL;
+}