From cc0e92a3c359cb34079b8e5b258ceef813ea684a Mon Sep 17 00:00:00 2001
From: Nicolas Williams <nico@cryptonector.com>
Date: Mon, 30 Nov 2015 00:47:54 -0500
Subject: [PATCH] base: correct Windows heim_base_once_f semantics

As implemented by Jeffrey Altman heim_base_once_f() is a variant of the
"double-checked gate lock pattern".   Full memory barriers must be used
when determining whether or not to call SwitchToThread().

Change-Id: I2f8446a56c50a37c921d6e993433c9a3f7488f50
---
 lib/base/heimbase.c | 19 +++++++++++++++----
 lib/base/heimbase.h |  5 ++---
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/lib/base/heimbase.c b/lib/base/heimbase.c
index 8f5694d27..5642b1cd3 100644
--- a/lib/base/heimbase.c
+++ b/lib/base/heimbase.c
@@ -369,12 +369,23 @@ void
 heim_base_once_f(heim_base_once_t *once, void *ctx, void (*func)(void *))
 {
 #if defined(WIN32)
-    if (InterlockedCompareExchange(&once->started, 1L, 0L) == 0L) {
-	once->running = 1L;
+    /*
+     * State 0 means that func() has never executed.
+     * State 1 means that func() is executing.
+     * State 2 means that func() has completed execution.
+     */
+    if (InterlockedCompareExchange(&once->state, 1L, 0L) == 0L) {
+	/* State is now 1 */
 	(*func)(ctx);
-	once->running = 0L;
+	(void)InterlockedExchange(&once->state, 2L);
+	/* State is now 2 */
     } else {
-	while (once->running)
+	/*
+	 * The InterlockedCompareExchange is being used to fetch
+	 * the current state under a full memory barrier.  As long
+	 * as the current state is 1 continue to spin.
+	 */
+	while (InterlockedCompareExchange(&once->state, 2L, 0L) == 1L)
 	    SwitchToThread();
     }
 #elif defined(HAVE_DISPATCH_DISPATCH_H)
diff --git a/lib/base/heimbase.h b/lib/base/heimbase.h
index c617a6344..7a4ec385d 100644
--- a/lib/base/heimbase.h
+++ b/lib/base/heimbase.h
@@ -58,10 +58,9 @@ typedef heim_object_t heim_bool_t;
 typedef heim_object_t heim_null_t;
 #if defined(WIN32)
 typedef struct {
-    LONG started;
-    LONG running;
+    LONG state;
 } heim_base_once_t;
-# define HEIM_BASE_ONCE_INIT {0L, 0L}
+# define HEIM_BASE_ONCE_INIT {0L}
 #else
 # define HEIM_BASE_ONCE_INIT 0
 typedef long heim_base_once_t; /* XXX arch dependant */