GL,Platform: introduce GL::Context::Configuration::Flag::Windowless.

With this flag set (which is done implicitly for all windowless apps
and, conversely, not done for all windowed apps), the default
framebuffer state isn't touched in any way, which should avoid potential
race conditions with default framebuffer on another thread.

doc/changelog.dox

@@ -196,6 +196,13 @@ See also:
     now advertised as a @cpp "intel-windows-chatty-shader-compiler" @ce
     workaround, instead of being done silently. See @ref opengl-workarounds for
     more information.
+-   @ref Platform::WindowlessEglApplication "Platform::Windowless*Application"
+    instances no longer touch default framebuffer state in order to avoid
+    potential race conditions with a windowed context on another thread. This
+    is also exposed via a new @ref GL::Context::Configuration::Flag::Windowless
+    flag for integration with custom-created OpenGL contexts. See also
+    [mosra/magnum#493](https://github.com/mosra/magnum/pull/493) and
+    [mosra/magnum#494](https://github.com/mosra/magnum/pull/494).
 
 @subsubsection changelog-latest-changes-math Math library
 

src/Magnum/GL/Context.cpp

@@ -785,6 +785,10 @@ bool Context::tryCreate(const Configuration& configuration) {
     if(configuration.flags() & Configuration::Flag::GpuValidation)
         _configurationFlags |= Configuration::Flag::GpuValidation;
 
+    /* Same for windowless */
+    if(configuration.flags() & Configuration::Flag::Windowless)
+        _configurationFlags |= Configuration::Flag::Windowless;
+
     /* Driver workarounds get merged. Not using disableDriverWorkaround() here
        since the Configuration already contains the internal string views. */
     for(const Containers::StringView workaround: configuration.disabledWorkarounds())
@@ -974,9 +978,13 @@ bool Context::tryCreate(const Configuration& configuration) {
             if(!workaround.second) Debug(output) << "   " << workaround.first;
     }
 
-    /* Initialize functionality based on current OpenGL version and extensions */
     /** @todo Get rid of these */
-    DefaultFramebuffer::initializeContextBasedFunctionality(*this);
+    /* Initialize functionality based on current OpenGL version and extensions.
+       If we are on a windowless context don't touch the default framebuffer
+       to avoid potential race conditions with default framebuffer on another
+       thread. */
+    if(!(_configurationFlags & Configuration::Flag::Windowless))
+        DefaultFramebuffer::initializeContextBasedFunctionality(*this);
     Renderer::initializeContextBasedFunctionality();
 
     /* Enable GPU validation, if requested */

src/Magnum/GL/Context.h

@@ -70,6 +70,7 @@ namespace Implementation {
        Context before the Configuration class is defined, it has to be here */
     enum class ContextConfigurationFlag: UnsignedLong {
         /* Keeping the 32-bit range reserved for actual GL context flags */
+        Windowless = 1ull << 60,
         QuietLog = 1ull << 61,
         VerboseLog = 1ull << 62,
         GpuValidation = 1ull << 63
@@ -914,6 +915,19 @@ class MAGNUM_GL_EXPORT Context::Configuration {
             /* Docs only, keep in sync with
                Implementation::ContextConfigurationFlag please */
 
+            /**
+             * Treat the context as windowless, assume there's no default
+             * framebuffer and thus don't touch @ref defaultFramebuffer in any
+             * way. Useful for preventing race conditions when creating OpenGL
+             * contexts in background threads.
+             *
+             * This flag is implicitly enabled in all
+             * @ref Platform::WindowlessEglContext::Configuration "Platform::Windowless*Application::Configuration",
+             * and, conversely, not possible to enable in any
+             * @ref Platform::Sdl2Application::GLConfiguration::Flag "Platform::*Application::GLConfiguration".
+             */
+            Windowless = 1ull << 60,
+
             /**
              * Print only warnings and errors instead of the usual startup log
              * listing used extensions and workarounds. Ignored if

src/Magnum/GL/Framebuffer.cpp

@@ -120,18 +120,23 @@ Framebuffer::~Framebuffer() {
     if(!_id || !(_flags & ObjectFlag::DeleteOnDestruction)) return;
 
     /* If bound, remove itself from state */
-    Implementation::FramebufferState& state = Context::current().state().framebuffer;
+    Context& context = Context::current();
+    Implementation::FramebufferState& state = context.state().framebuffer;
     if(state.readBinding == _id) state.readBinding = 0;
 
-    /* For draw binding reset also viewport */
+    /* For draw binding reset also viewport. Don't do that for windowless
+       contexts to avoid potential race conditions with default framebuffer on
+       another thread. */
     if(state.drawBinding == _id) {
         state.drawBinding = 0;
 
-        /**
-         * @todo Less ugly solution (need to call setViewportInternal() to
-         * reset the viewport to size of default framebuffer)
-         */
-        defaultFramebuffer.bind();
+        if(!(context.configurationFlags() & Context::Configuration::Flag::Windowless)) {
+            /**
+             * @todo Less ugly solution (need to call setViewportInternal() to
+             * reset the viewport to size of default framebuffer)
+             */
+            defaultFramebuffer.bind();
+        }
     }
 
     glDeleteFramebuffers(1, &_id);

src/Magnum/Platform/AbstractXApplication.h

@@ -343,7 +343,9 @@ class AbstractXApplication::GLConfiguration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is not meant to be enabled for windowed apps.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {

src/Magnum/Platform/AndroidApplication.h

@@ -456,7 +456,9 @@ class AndroidApplication::GLConfiguration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is not meant to be enabled for windowed apps.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {

src/Magnum/Platform/EmscriptenApplication.h

@@ -942,7 +942,9 @@ class EmscriptenApplication::GLConfiguration: public GL::Context::Configuration
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is not meant to be enabled for windowed apps.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {

src/Magnum/Platform/GlfwApplication.h

@@ -784,7 +784,9 @@ class GlfwApplication::GLConfiguration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is not meant to be enabled for windowed apps.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {

src/Magnum/Platform/Sdl2Application.h

@@ -1236,7 +1236,9 @@ class Sdl2Application::GLConfiguration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is not meant to be enabled for windowed apps.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {

src/Magnum/Platform/WindowlessCglApplication.cpp

@@ -105,6 +105,10 @@ bool WindowlessCglContext::release() {
     return false;
 }
 
+WindowlessCglContext::Configuration::Configuration() {
+    GL::Context::Configuration::addFlags(GL::Context::Configuration::Flag::Windowless);
+}
+
 #ifndef DOXYGEN_GENERATING_OUTPUT
 WindowlessCglApplication::WindowlessCglApplication(const Arguments& arguments): WindowlessCglApplication{arguments, Configuration{}} {}
 #endif

src/Magnum/Platform/WindowlessCglApplication.h

@@ -166,7 +166,9 @@ class WindowlessCglContext::Configuration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is enabled implicitly by default.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {
@@ -196,6 +198,8 @@ class WindowlessCglContext::Configuration: public GL::Context::Configuration {
          */
         typedef Containers::EnumSet<Flag> Flags;
 
+        /*implicit*/ Configuration();
+
         /** @brief Context flags */
         Flags flags() const {
             return Flag(UnsignedLong(GL::Context::Configuration::flags()));

src/Magnum/Platform/WindowlessEglApplication.cpp

@@ -533,6 +533,7 @@ WindowlessEglContext::Configuration::Configuration()
     : _device{}
     #endif
 {
+    GL::Context::Configuration::addFlags(GL::Context::Configuration::Flag::Windowless);
     #ifndef MAGNUM_TARGET_GLES
     addFlags(Flag::ForwardCompatible);
     #endif

src/Magnum/Platform/WindowlessEglApplication.h

@@ -183,7 +183,9 @@ class WindowlessEglContext::Configuration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is enabled implicitly by default.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {

src/Magnum/Platform/WindowlessGlxApplication.cpp

@@ -274,6 +274,7 @@ bool WindowlessGlxContext::release() {
 }
 
 WindowlessGlxContext::Configuration::Configuration() {
+    GL::Context::Configuration::addFlags(GL::Context::Configuration::Flag::Windowless);
     #ifndef MAGNUM_TARGET_GLES
     addFlags(Flag::ForwardCompatible);
     #endif

src/Magnum/Platform/WindowlessGlxApplication.h

@@ -188,7 +188,9 @@ class WindowlessGlxContext::Configuration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is enabled implicitly by default.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {

src/Magnum/Platform/WindowlessIosApplication.h

@@ -157,7 +157,9 @@ class WindowlessIosContext::Configuration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is enabled implicitly by default.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {
@@ -187,6 +189,8 @@ class WindowlessIosContext::Configuration: public GL::Context::Configuration {
          */
         typedef Containers::EnumSet<Flag> Flags;
 
+        /*implicit*/ Configuration();
+
         /** @brief Context flags */
         Flags flags() const {
             return Flag(UnsignedLong(GL::Context::Configuration::flags()));

src/Magnum/Platform/WindowlessIosApplication.mm

@@ -82,6 +82,10 @@ bool WindowlessIosContext::release() {
     return false;
 }
 
+WindowlessIosContext::Configuration::Configuration() {
+    GL::Context::Configuration::addFlags(GL::Context::Configuration::Flag::Windowless);
+}
+
 #ifndef DOXYGEN_GENERATING_OUTPUT
 WindowlessIosApplication::WindowlessIosApplication(const Arguments& arguments): WindowlessIosApplication{arguments, Configuration{}} {}
 #endif

src/Magnum/Platform/WindowlessWglApplication.cpp

@@ -259,6 +259,7 @@ bool WindowlessWglContext::release() {
 }
 
 WindowlessWglContext::Configuration::Configuration() {
+    GL::Context::Configuration::addFlags(GL::Context::Configuration::Flag::Windowless);
     #ifndef MAGNUM_TARGET_GLES
     addFlags(Flag::ForwardCompatible);
     #endif

src/Magnum/Platform/WindowlessWglApplication.h

@@ -175,7 +175,9 @@ class WindowlessWglContext::Configuration: public GL::Context::Configuration {
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is enabled implicitly by default.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {

src/Magnum/Platform/WindowlessWindowsEglApplication.cpp

@@ -182,6 +182,10 @@ bool WindowlessWindowsEglContext::release() {
     return false;
 }
 
+WindowlessWindowsEglContext::Configuration::Configuration() {
+    GL::Context::Configuration::addFlags(GL::Context::Configuration::Flag::Windowless);
+}
+
 #ifndef DOXYGEN_GENERATING_OUTPUT
 WindowlessWindowsEglApplication::WindowlessWindowsEglApplication(const Arguments& arguments): WindowlessWindowsEglApplication{arguments, Configuration{}} {}
 #endif

src/Magnum/Platform/WindowlessWindowsEglApplication.h

@@ -161,7 +161,9 @@ class WindowlessWindowsEglContext::Configuration: public GL::Context::Configurat
         /**
          * @brief Context flag
          *
-         * Includes also everything from @ref GL::Context::Configuration::Flag.
+         * Includes also everything from @ref GL::Context::Configuration::Flag
+         * except for @relativeref{GL::Context::Configuration,Flag::Windowless},
+         * which is enabled implicitly by default.
          * @see @ref Flags, @ref setFlags(), @ref GL::Context::Flag
          */
         enum class Flag: UnsignedLong {
@@ -199,6 +201,8 @@ class WindowlessWindowsEglContext::Configuration: public GL::Context::Configurat
          */
         typedef Containers::EnumSet<Flag> Flags;
 
+        /*implicit*/ Configuration();
+
         /** @brief Context flags */
         Flags flags() const {
             return Flag(UnsignedLong(GL::Context::Configuration::flags()));