[PATCH] compat: Allow static initializer for pthreads on Windows

[Stefan Beller <sbeller@google.com>]

In Windows it is not possible to have a static initialized mutex as of
now, but that seems to be painful for the upcoming refactoring of the
attribute subsystem, as we have no good place to put the initialization
of the attr global lock.

The trick is to get a named mutex as CreateMutex[1] will return the
existing named mutex if it exists in a thread safe way, or return
a newly created mutex with that name.

Inside the critical section of the single named mutex, we need to double
check if the mutex was already initialized because the outer check is
not sufficient.
(e.g. 2 threads enter the first condition `(!a)` at the same time, but
only one of them will acquire the named mutex first and proceeds to
initialize the given mutex a. The second thread shall not re-initialize
the given mutex `a`, which is why we have the inner condition on `(!a)`.

Due to the use of memory barriers inside the critical section the mutex
`a` gets updated to other threads, such that any further invocation
will skip the initialization check code altogether on the first condition.

[1] https://msdn.microsoft.com/en-us/library/windows/desktop/ms682411(v=vs.85).aspx

Signed-off-by: Stefan Beller <sbeller@google.com>
---

 Flying blind here, i.e. not compiled, not tested. For a system I do not
 have deep knowledge of. The only help was the online documentation.

 compat/win32/pthread.h | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/compat/win32/pthread.h b/compat/win32/pthread.h
index 1c16408..a900513 100644
--- a/compat/win32/pthread.h
+++ b/compat/win32/pthread.h
@@ -21,9 +21,25 @@
 static inline int return_0(int i) {
 	return 0;
 }
+
+#define PTHREAD_MUTEX_INITIALIZER NULL
 #define pthread_mutex_init(a,b) return_0((InitializeCriticalSection((a)), 0))
 #define pthread_mutex_destroy(a) DeleteCriticalSection((a))
-#define pthread_mutex_lock EnterCriticalSection
+#define pthread_mutex_lock(a) \
+{ \
+	if (!a) { \
+		HANDLE p = CreateMutex(NULL, FALSE, "Git-Global-Windows-Mutex"); \
+		EnterCriticalSection(p); \
+		MemoryBarrier(); \
+		if (!a)
+			pthread_mutex_init(a); \
+		MemoryBarrier(); \
+		ReleaseMutex(p); \
+	} \
+	EnterCriticalSection(a); \
+}
+
+
 #define pthread_mutex_unlock LeaveCriticalSection
 
 typedef int pthread_mutexattr_t;
-- 
2.10.1.508.g6572022