resolv: Introduce struct resolv_conf with extended resolver state

This change provides additional resolver configuration state which
is not exposed through the _res ABI.  It reuses the existing
initstamp field in the supposedly-private part of _res.  Some effort
is undertaken to avoid memory safety issues introduced by applications
which directly patch the _res object.

With this commit, only the initstamp field is moved into struct
resolv_conf.  Additional members will be added later, eventually
migrating the entire resolver configuration.

resolv/resolv_conf.h

@@ -0,0 +1,69 @@
+/* Extended resolver state separate from struct __res_state.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef RESOLV_STATE_H
+#define RESOLV_STATE_H
+
+#include <stdbool.h>
+#include <stddef.h>
+
+/* Extended resolver state associated with res_state objects.  Client
+   code can reach this state through a struct resolv_context
+   object.  */
+struct resolv_conf
+{
+  /* Used to propagate the effect of res_init across threads.  This
+     member is mutable and prevents sharing of the same struct
+     resolv_conf object among multiple struct __res_state objects.  */
+  unsigned long long int initstamp;
+
+  /* Reference counter.  The object is deallocated once it reaches
+     zero.  For internal use within resolv_conf only.  */
+  size_t __refcount;
+};
+
+/* The functions below are for use by the res_init resolv.conf parser
+   and the struct resolv_context facility.  */
+
+struct __res_state;
+
+/* Return the extended resolver state for *RESP, or NULL if it cannot
+   be determined.  A call to this function must be paired with a call
+   to __resolv_conf_put.  */
+struct resolv_conf *__resolv_conf_get (struct __res_state *) attribute_hidden;
+
+/* Converse of __resolv_conf_get.  */
+void __resolv_conf_put (struct resolv_conf *) attribute_hidden;
+
+/* Allocate a new struct resolv_conf object and copy the
+   pre-configured values from *INIT.  Return NULL on allocation
+   failure.  The object must be deallocated using
+   __resolv_conf_put.  */
+struct resolv_conf *__resolv_conf_allocate (const struct resolv_conf *init)
+  attribute_hidden __attribute__ ((nonnull (1), warn_unused_result));
+
+/* Associate an existing extended resolver state with *RESP.  Return
+   false on allocation failure.  In addition, update *RESP with the
+   overlapping non-extended resolver state.  */
+bool __resolv_conf_attach (struct __res_state *, struct resolv_conf *)
+  attribute_hidden;
+
+/* Detach the extended resolver state from *RESP.  */
+void __resolv_conf_detach (struct __res_state *resp) attribute_hidden;
+
+#endif /* RESOLV_STATE_H */