Add a guard for the calls to MakeCurrent during initialization

src/win32/init.rs

@@ -9,6 +9,7 @@ use super::Window;
 use super::MonitorID;
 use super::ContextWrapper;
 use super::WindowWrapper;
+use super::make_current_guard::CurrentContextGuard;
 
 use Api;
 use BuilderAttribs;
@@ -143,24 +144,18 @@ unsafe fn init(title: Vec<u16>, builder: BuilderAttribs<'static>,
         let dummy_context = try!(create_context(None, &dummy_window, None));
 
         // making context current
-        gl::wgl::MakeCurrent(dummy_window.1 as *const libc::c_void,
-                             dummy_context.0 as *const libc::c_void);
+        let current_context = try!(CurrentContextGuard::make_current(&dummy_window,
+                                                                     &dummy_context));
 
         // loading the extra WGL functions
-        let extra_functions = gl::wgl_extra::Wgl::load_with(|addr| {
+        gl::wgl_extra::Wgl::load_with(|addr| {
             use libc;
 
             let addr = CString::new(addr.as_bytes()).unwrap();
             let addr = addr.as_ptr();
 
             gl::wgl::GetProcAddress(addr) as *const libc::c_void
-        });
-
-        // removing current context
-        gl::wgl::MakeCurrent(ptr::null(), ptr::null());
-
-        // returning the address
-        extra_functions
+        })
     };
 
     // creating the real window this time
@@ -237,15 +232,11 @@ unsafe fn init(title: Vec<u16>, builder: BuilderAttribs<'static>,
     // handling vsync
     if builder.vsync {
         if extra_functions.SwapIntervalEXT.is_loaded() {
-            gl::wgl::MakeCurrent(real_window.1 as *const libc::c_void,
-                                 context.0 as *const libc::c_void);
+            let guard = try!(CurrentContextGuard::make_current(&real_window, &context));
+
             if extra_functions.SwapIntervalEXT(1) == 0 {
                 return Err(OsError(format!("wglSwapIntervalEXT failed")));
             }
-
-            // it is important to remove the current context, otherwise you get very weird
-            // errors
-            gl::wgl::MakeCurrent(ptr::null(), ptr::null());
         }
     }
 

src/win32/make_current_guard.rs

@@ -0,0 +1,53 @@
+use std::marker::PhantomData;
+use std::os;
+
+use libc;
+use winapi;
+use CreationError;
+
+use super::gl;
+use super::ContextWrapper;
+use super::WindowWrapper;
+
+/// A guard for when you want to make the context current. Destroying the guard restores the
+/// previously-current context.
+pub struct CurrentContextGuard<'a, 'b> {
+    previous_hdc: winapi::HDC,
+    previous_hglrc: winapi::HGLRC,
+    marker1: PhantomData<&'a ()>,
+    marker2: PhantomData<&'b ()>,
+}
+
+impl<'a, 'b> CurrentContextGuard<'a, 'b> {
+    pub unsafe fn make_current(window: &'a WindowWrapper, context: &'b ContextWrapper)
+                               -> Result<CurrentContextGuard<'a, 'b>, CreationError>
+    {
+        let previous_hdc = gl::wgl::GetCurrentDC() as winapi::HDC;
+        let previous_hglrc = gl::wgl::GetCurrentContext() as winapi::HGLRC;
+
+        let result = gl::wgl::MakeCurrent(window.1 as *const libc::c_void,
+                                          context.0 as *const libc::c_void);
+
+        if result == 0 {
+            return Err(CreationError::OsError(format!("wglMakeCurrent function failed: {}",
+                                                      os::error_string(os::errno()))));
+        }
+
+        Ok(CurrentContextGuard {
+            previous_hdc: previous_hdc,
+            previous_hglrc: previous_hglrc,
+            marker1: PhantomData,
+            marker2: PhantomData,
+        })
+    }
+}
+
+#[unsafe_destructor]
+impl<'a, 'b> Drop for CurrentContextGuard<'a, 'b> {
+    fn drop(&mut self) {
+        unsafe {
+            gl::wgl::MakeCurrent(self.previous_hdc as *const libc::c_void,
+                                 self.previous_hglrc as *const libc::c_void);
+        }
+    }
+}

src/win32/mod.rs

@@ -21,6 +21,7 @@ mod event;
 mod gl;
 mod headless;
 mod init;
+mod make_current_guard;
 mod monitor;
 
 /// The Win32 implementation of the main `Window` object.