From f708eadcfdcbcea8a83c24ccd069b0955d5710d2 Mon Sep 17 00:00:00 2001
From: Jef Driesen <jefdriesen@users.sourceforge.net>
Date: Tue, 26 Sep 2017 22:58:59 +0200
Subject: [PATCH] Make the initialization thread-safe

Perform the initialization inside a critical section.

Unfortunately Windows critical sections, which are the simplest
synchronization mechanism available on Windows, do not support static
initialization. A call to InitializeCriticalSection is required.
Therefore a simple spinlock, with an implementation based on atomic
operations, is used as a workaround.
---
 configure.ac |  1 +
 src/usbhid.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/configure.ac b/configure.ac
index 17bd5f9..d0939b9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -154,6 +154,7 @@ AC_CHECK_HEADERS([linux/serial.h])
 AC_CHECK_HEADERS([IOKit/serial/ioss.h])
 AC_CHECK_HEADERS([getopt.h])
 AC_CHECK_HEADERS([sys/param.h])
+AC_CHECK_HEADERS([pthread.h])
 
 # Checks for global variable declarations.
 AC_CHECK_DECLS([optreset])
diff --git a/src/usbhid.c b/src/usbhid.c
index 0f7ba7b..f0c10df 100644
--- a/src/usbhid.c
+++ b/src/usbhid.c
@@ -24,6 +24,13 @@
 #endif
 
 #include <stdlib.h>
+#ifdef HAVE_PTHREAD_H
+#include <pthread.h>
+#endif
+#ifdef _WIN32
+#define NOGDI
+#include <windows.h>
+#endif
 
 #if defined(HAVE_HIDAPI)
 #define USE_HIDAPI
@@ -47,6 +54,14 @@
 #include "context-private.h"
 #include "platform.h"
 
+#ifdef _WIN32
+typedef LONG dc_mutex_t;
+#define DC_MUTEX_INIT 0
+#else
+typedef pthread_mutex_t dc_mutex_t;
+#define DC_MUTEX_INIT PTHREAD_MUTEX_INITIALIZER
+#endif
+
 struct dc_usbhid_t {
 	/* Library context. */
 	dc_context_t *context;
@@ -87,16 +102,41 @@ syserror(int errcode)
 #endif
 
 #ifdef USBHID
+static dc_mutex_t g_usbhid_mutex = DC_MUTEX_INIT;
 static size_t g_usbhid_refcount = 0;
 #ifdef USE_LIBUSB
 static libusb_context *g_usbhid_ctx = NULL;
 #endif
 
+static void
+dc_mutex_lock (dc_mutex_t *mutex)
+{
+#ifdef _WIN32
+	while (InterlockedCompareExchange (mutex, 1, 0) == 1) {
+		SleepEx (0, TRUE);
+	}
+#else
+	pthread_mutex_lock (mutex);
+#endif
+}
+
+static void
+dc_mutex_unlock (dc_mutex_t *mutex)
+{
+#ifdef _WIN32
+	InterlockedExchange (mutex, 0);
+#else
+	pthread_mutex_unlock (mutex);
+#endif
+}
+
 static dc_status_t
 dc_usbhid_init (dc_context_t *context)
 {
 	dc_status_t status = DC_STATUS_SUCCESS;
 
+	dc_mutex_lock (&g_usbhid_mutex);
+
 	if (g_usbhid_refcount == 0) {
 #if defined(USE_LIBUSB)
 		int rc = libusb_init (&g_usbhid_ctx);
@@ -119,12 +159,14 @@ dc_usbhid_init (dc_context_t *context)
 	g_usbhid_refcount++;
 
 error:
+	dc_mutex_unlock (&g_usbhid_mutex);
 	return status;
 }
 
 static dc_status_t
 dc_usbhid_exit (void)
 {
+	dc_mutex_lock (&g_usbhid_mutex);
 
 	if (--g_usbhid_refcount == 0) {
 #if defined(USE_LIBUSB)
@@ -135,6 +177,8 @@ dc_usbhid_exit (void)
 #endif
 	}
 
+	dc_mutex_unlock (&g_usbhid_mutex);
+
 	return DC_STATUS_SUCCESS;
 }
 #endif