Add option to use atomic operations on stop_flag

This merges parts of 7717706cb5ab65953c5433bec57962b51ef56bc6 from xtne6f like mg_atomic_compare_and_swap

VisualStudio/civetweb_lua/civetweb_lua.vcxproj

@@ -102,7 +102,7 @@
       </PrecompiledHeader>
       <WarningLevel>Level3</WarningLevel>
       <Optimization>Disabled</Optimization>
-      <PreprocessorDefinitions>USE_HTTP2;MG_EXPERIMENTAL_INTERFACES;USE_SERVER_STATS;USE_DUKTAPE;USE_IPV6;LUA_COMPAT_ALL;USE_LUA;USE_LUA_SQLITE3;USE_LUA_FILE_SYSTEM;USE_LUA_LUAXML;USE_WEBSOCKET;WIN32;_DEBUG;_WINDOWS;_CRT_SECURE_NO_DEPRECATE;_CRT_SECURE_NO_WARNINGS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>STOP_FLAG_NEEDS_LOCK;OPENSSL_API_1_0;MG_EXPERIMENTAL_INTERFACES;USE_SERVER_STATS;USE_DUKTAPE;USE_IPV6;LUA_COMPAT_ALL;USE_LUA;USE_LUA_SQLITE3;USE_LUA_FILE_SYSTEM;USE_LUA_LUAXML;USE_WEBSOCKET;WIN32;_DEBUG;_WINDOWS;_CRT_SECURE_NO_DEPRECATE;_CRT_SECURE_NO_WARNINGS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <AdditionalIncludeDirectories>$(ProjectDir)..\..\include;$(ProjectDir)..\..\src\third_party;$(ProjectDir)..\..\src\third_party\lua-5.2.4\src;$(ProjectDir)..\..\src\third_party\duktape-1.5.2\src;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
     </ClCompile>
     <Link>
@@ -149,7 +149,7 @@
       <Optimization>MaxSpeed</Optimization>
       <FunctionLevelLinking>true</FunctionLevelLinking>
       <IntrinsicFunctions>true</IntrinsicFunctions>
-      <PreprocessorDefinitions>MG_EXPERIMENTAL_INTERFACES;USE_SERVER_STATS;USE_DUKTAPE;USE_IPV6;LUA_COMPAT_ALL;USE_LUA;USE_LUA_SQLITE3;USE_LUA_FILE_SYSTEM;USE_LUA_LUAXML;USE_WEBSOCKET;WIN32;_WINDOWS;_CRT_SECURE_NO_DEPRECATE;_CRT_SECURE_NO_WARNINGS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>OPENSSL_API_1_0;MG_EXPERIMENTAL_INTERFACES;USE_SERVER_STATS;USE_DUKTAPE;USE_IPV6;LUA_COMPAT_ALL;USE_LUA;USE_LUA_SQLITE3;USE_LUA_FILE_SYSTEM;USE_LUA_LUAXML;USE_WEBSOCKET;WIN32;_WINDOWS;_CRT_SECURE_NO_DEPRECATE;_CRT_SECURE_NO_WARNINGS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <AdditionalIncludeDirectories>$(ProjectDir)..\..\include;$(ProjectDir)..\..\src\third_party;$(ProjectDir)..\..\src\third_party\lua-5.2.4\src;$(ProjectDir)..\..\src\third_party\duktape-1.5.2\src;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
     </ClCompile>
     <Link>

src/civetweb.c

@@ -1333,6 +1333,34 @@ mg_atomic_add(volatile ptrdiff_t *addr, ptrdiff_t value)
 }
 
 
+FUNCTION_MAY_BE_UNUSED
+static int
+mg_atomic_compare_and_swap(volatile ptrdiff_t *addr,
+                           ptrdiff_t oldval,
+                           ptrdiff_t newval)
+{
+	ptrdiff_t ret;
+
+#if defined(_WIN64) && !defined(NO_ATOMICS)
+	ret = InterlockedCompareExchange64(addr, newval, oldval);
+#elif defined(_WIN32) && !defined(NO_ATOMICS)
+	ret = InterlockedCompareExchange(addr, newval, oldval);
+#elif defined(__GNUC__)                                                        \
+    && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ > 0)))           \
+    && !defined(NO_ATOMICS)
+	ret = __sync_val_compare_and_swap(addr, oldval, newval);
+#else
+	mg_global_lock();
+	ret = *addr;
+	if ((ret != newval) && (ret == oldval)) {
+		*addr = newval;
+	}
+	mg_global_unlock();
+#endif
+	return ret;
+}
+
+
 static void
 mg_atomic_max(volatile ptrdiff_t *addr, ptrdiff_t value)
 {
@@ -2817,41 +2845,43 @@ struct mg_domain_context {
 };
 
 
-/* Stop flag can be "volatile" or require a lock */
-typedef int volatile stop_flag_t;
-
+/* Stop flag can be "volatile" or require a lock.
+ * MSDN uses volatile for "Interlocked" operations, but also explicitly
+ * states a read operation for int is always atomic. */
 #ifdef STOP_FLAG_NEEDS_LOCK
+
+typedef ptrdiff_t volatile stop_flag_t;
+
 static int
 STOP_FLAG_IS_ZERO(stop_flag_t *f)
 {
-	int r;
-	mg_global_lock();
-	r = ((*f) == 0);
-	mg_global_unlock();
-	return r;
+	stop_flag_t sf = mg_atomic_add(f, 0);
+	return (sf == 0);
 }
 
 static int
 STOP_FLAG_IS_TWO(stop_flag_t *f)
 {
-	int r;
-	mg_global_lock();
-	r = ((*f) == 2);
-	mg_global_unlock();
-	return r;
+	stop_flag_t sf = mg_atomic_add(f, 0);
+	return (sf == 2);
 }
 
 static void
-STOP_FLAG_ASSIGN(stop_flag_t *f, int v)
+STOP_FLAG_ASSIGN(stop_flag_t *f, stop_flag_t v)
 {
-	mg_global_lock();
-	(*f) = v;
-	mg_global_unlock();
+	stop_flag_t sf;
+	do {
+		sf = mg_atomic_compare_and_swap(f, *f, v);
+	} while (sf != v);
 }
+
 #else /* STOP_FLAG_NEEDS_LOCK */
+
+typedef int volatile stop_flag_t;
 #define STOP_FLAG_IS_ZERO(f) ((*(f)) == 0)
 #define STOP_FLAG_IS_TWO(f) ((*(f)) == 2)
 #define STOP_FLAG_ASSIGN(f, v) ((*(f)) = (v))
+
 #endif /* STOP_FLAG_NEEDS_LOCK */