Use rendezvous variables for set_user post hooks

This patch changes the set_user hooks so that they're held in the rendezvous
variable hash map as function pointers, rather than in the set_user .so. This
way, we don't have to ensure that set_user comes before any extension that
implements them -- load order doesn't matter. Just query the hash map during the
hook execution and if there are hooks implemented, call them.

set_user.c

@@ -113,9 +113,6 @@ static Oid save_OldUserId = InvalidOid;
 static char *reset_token = NULL;
 static ProcessUtility_hook_type prev_hook = NULL;
 
-post_reset_user_hook_type post_reset_user_hook = NULL;
-post_set_user_hook_type post_set_user_hook = NULL;
-
 #ifdef HAS_ALTER_SYSTEM
 /* 9.4 & up */
 static bool Block_AS = false;
@@ -160,7 +157,7 @@ static void PU_hook(Node *parsetree, const char *queryString,
 #endif
 #endif
 
-extern void PostSetUserHook(bool is_reset, const char *newuser);
+static void PostSetUserHook(bool is_reset, const char *newuser);
 
 extern Datum set_user(PG_FUNCTION_ARGS);
 void _PG_init(void);
@@ -628,14 +625,24 @@ PU_hook(Node *parsetree, const char *queryString,
 /*
  * PostSetUserHook
  *
- * Handler for the post_set_user_hook
+ * Handler for set_user post hooks
  */
 void
 PostSetUserHook(bool is_reset, const char *username)
 {
-	if (post_reset_user_hook && is_reset)
-		(*post_reset_user_hook) ();
-	else if (post_set_user_hook)
-		(*post_set_user_hook) (username);
-	return;
+	/* Inter-extension hooks */
+	SetUserHooks **post_hooks;
+
+	post_hooks = (SetUserHooks **) find_rendezvous_variable(SET_USER_HOOKS_KEY);
+	if (*post_hooks)
+	{
+		if (!is_reset && (*post_hooks)->post_set_user)
+		{
+			(*post_hooks)->post_set_user(username);
+		}
+		else if ((*post_hooks)->post_reset_user)
+		{
+			(*post_hooks)->post_reset_user();
+		}
+	}
 }

set_user.h

@@ -1,11 +1,12 @@
 #ifndef SET_USER_H
 #define SET_USER_H
 
-/* Expose a hook for other extensions to use after reset_user finishes up. */
-typedef void (*post_reset_user_hook_type) (void);
-extern PGDLLIMPORT post_reset_user_hook_type post_reset_user_hook;
+typedef struct SetUserHooks
+{
+	void	(*post_set_user) (const char *username);
+	void	(*post_reset_user) ();
+} SetUserHooks;
+
+#define SET_USER_HOOKS_KEY	"SetUserHooks"
 
-/* Expose a hook for other extensions to use after set_user finishes up. */
-typedef void (*post_set_user_hook_type) (const char *username);
-extern PGDLLIMPORT post_set_user_hook_type post_set_user_hook;
 #endif