From 601ffbd700e7c11a101215d641ea09d0181d4771 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 1 Feb 2017 17:01:25 +0200
Subject: Fallback to normal mutex if shared_mutex not available

As is the case on MacOS prior to 10.12.
---
 build2/types | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

(limited to 'build2/types')

diff --git a/build2/types b/build2/types
index 7f39274..b9f3ce9 100644
--- a/build2/types
+++ b/build2/types
@@ -20,8 +20,11 @@
 
 #include <mutex>
 #include <future>
+
 #include <butl/ft/shared_mutex>
-#include <shared_mutex>
+#if defined(__cpp_lib_shared_mutex) || defined(__cpp_lib_shared_timed_mutex)
+#  include <shared_mutex>
+#endif
 
 #include <ios>           // ios_base::failure
 #include <exception>     // exception
@@ -77,15 +80,23 @@ namespace build2
   //
   using std::future;
 
-#ifdef __cpp_lib_shared_mutex
+#if   defined(__cpp_lib_shared_mutex)
   using shared_mutex = std::shared_mutex;
-#else
+  using ulock        = std::unique_lock<shared_mutex>;
+  using slock        = std::shared_lock<shared_mutex>;
+#elif defined(__cpp_lib_shared_timed_mutex)
   using shared_mutex = std::shared_timed_mutex;
+  using ulock        = std::unique_lock<shared_mutex>;
+  using slock        = std::shared_lock<shared_mutex>;
+#else
+  // Because we have this fallback, we need to be careful not to create
+  // multiple shared locks in the same thread.
+  //
+  using shared_mutex = std::mutex;
+  using ulock        = std::unique_lock<shared_mutex>;
+  using slock        = ulock;
 #endif
 
-  using slock = std::shared_lock<shared_mutex>;
-  using ulock = std::unique_lock<shared_mutex>;
-
   // Exceptions.
   //
   // While <exception> is included, there is no using for std::exception --
-- 
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