Platform: only initialize/terminate EGLDisplay once in a shared set.

Co-authored-by: Vladimír Vondruš <mosra@centrum.cz>

src/Magnum/Platform/WindowlessEglApplication.cpp

@@ -82,18 +82,30 @@ bool extensionSupported(const char* const extensions, Containers::ArrayView<cons
 #endif
 
 WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, GLContext* const magnumContext) {
+    #ifndef MAGNUM_TARGET_WEBGL
+    /* The user provided a shared context, use the associated display
+       directly. We don't call eglInitialize() in this case either -- the
+       context we share with already did that on the provided display */
+    if(configuration.sharedContext() != EGL_NO_CONTEXT && configuration.sharedDisplay() != EGL_NO_DISPLAY) {
+        _display = configuration.sharedDisplay();
+        _sharedContext = true;
+    } else
+    #endif
+    /* Otherwise find the display and initialize EGL */
+    {
         #ifndef MAGNUM_TARGET_WEBGL
         /* If relevant extensions are supported, try to find some display using
            those APIs, as that works reliably also when running headless. This
            would ideally use EGL 1.5 APIs but since we still want to support
            systems which either have old EGL headers or old EGL implementation,
            we'd need to have a code path for 1.4 *and* 1.5, plus do complicated
-       version parsing from a string. Not feeling like doing that today, no. */
+           version parsing from a string. Not feeling like doing that today,
+           no. */
         const char* const extensions = eglQueryString(EGL_NO_DISPLAY, EGL_EXTENSIONS);
         if(extensions &&
-        /* eglQueryDevicesEXT(). NVidia exposes only EGL_EXT_device_base, which
+            /* eglQueryDevicesEXT(). NVidia exposes only EGL_EXT_device_base,
-           is an older version of EGL_EXT_device_enumeration before it got
+               which is an older version of EGL_EXT_device_enumeration before
-           split to that and EGL_EXT_device_query, so test for both. */
+               it got split to that and EGL_EXT_device_query, so test for both. */
             (extensionSupported(extensions, "EGL_EXT_device_enumeration") || extensionSupported(extensions, "EGL_EXT_device_base")) &&
 
             /* eglGetPlatformDisplayEXT() */
@@ -106,9 +118,9 @@ WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, G
             /* When libEGL_nvidia.so is present on a system w/o a NV GPU,
                eglQueryDevicesEXT() fails there with EGL_BAD_ALLOC, but that is
                never propagated to the glvnd wrapper. Enable debug output if
-           --magnum-gpu-validation is enabled because otherwise it's fucking
+               --magnum-gpu-validation is enabled because otherwise it's
-           hard to discover what's to blame (lost > 3 hours already). See class
+               fucking hard to discover what's to blame (lost > 3 hours
-           docs for more info and a workaround. */
+               already). See class docs for more info and a workaround. */
             if(extensionSupported(extensions, "EGL_KHR_debug") && magnumContext && (magnumContext->internalFlags() & GL::Context::InternalFlag::GpuValidation)) {
                 auto eglDebugMessageControl = reinterpret_cast<EGLint(*)(EGLDEBUGPROCKHR, const EGLAttrib*)>(eglGetProcAddress("eglDebugMessageControlKHR"));
                 const EGLAttrib debugAttribs[] = {
@@ -145,8 +157,8 @@ WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, G
                 Debug{} << "Platform::WindowlessEglApplication: found" << count << "EGL devices, choosing device" << configuration.device();
             }
 
-        /* Assuming the same thing won't suddenly start failing when called the
+            /* Assuming the same thing won't suddenly start failing when called
-           second time */
+               the second time */
             Containers::Array<EGLDeviceEXT> devices{configuration.device() + 1};
             CORRADE_INTERNAL_ASSERT_OUTPUT(eglQueryDevices(configuration.device() + 1, devices, &count));
 
@@ -154,10 +166,10 @@ WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, G
                 Error{} << "Platform::WindowlessEglApplication::tryCreateContext(): cannot get platform display for a device:" << Implementation::eglErrorString(eglGetError());
                 return;
             }
-    } else
+        }
-    #endif
         /* Otherwise initialize the classic way. WebGL doesn't have any of the
            above, so no need to compile that at all. */
+        #endif
         {
             #ifndef MAGNUM_TARGET_WEBGL
             if(configuration.device() != 0) {
@@ -176,6 +188,7 @@ WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, G
             Error() << "Platform::WindowlessEglApplication::tryCreateContext(): cannot initialize EGL:" << Implementation::eglErrorString(eglGetError());
             return;
         }
+    }
 
     const EGLenum api =
         #ifndef MAGNUM_TARGET_GLES
@@ -299,7 +312,15 @@ WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, G
     #endif
 }
 
-WindowlessEglContext::WindowlessEglContext(WindowlessEglContext&& other): _display{other._display}, _context{other._context} {
+WindowlessEglContext::WindowlessEglContext(WindowlessEglContext&& other):
+    #ifndef MAGNUM_TARGET_WEBGL
+    _sharedContext{other._sharedContext},
+    #endif
+    _display{other._display}, _context{other._context}
+{
+    #ifndef MAGNUM_TARGET_WEBGL
+    other._sharedContext = false;
+    #endif
     other._display = {};
     other._context = {};
 }
@@ -309,11 +330,23 @@ WindowlessEglContext::~WindowlessEglContext() {
     #if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_WEBGL)
     if(_surface) eglDestroySurface(_display, _surface);
     #endif
-    if(_display) eglTerminate(_display);
+
+    /* Don't terminate EGL if we're a shared context as it would kill all
+       others as well. In case of a shared context it's expected that the
+       first instance of WindowlessEglContext in the shared chain is destroyed
+       last, calling eglTerminate() after all others are gone. */
+    if(
+        #ifndef MAGNUM_TARGET_WEBGL
+        !_sharedContext &&
+        #endif
+        _display) eglTerminate(_display);
 }
 
 WindowlessEglContext& WindowlessEglContext::operator=(WindowlessEglContext&& other) {
     using std::swap;
+    #ifndef MAGNUM_TARGET_WEBGL
+    swap(other._sharedContext, _sharedContext);
+    #endif
     swap(other._display, _display);
     swap(other._context, _context);
     return *this;
@@ -390,6 +423,16 @@ bool WindowlessEglApplication::tryCreateContext(const Configuration& configurati
     return true;
 }
 
+#ifndef MAGNUM_TARGET_WEBGL
+auto WindowlessEglContext::Configuration::setSharedContext(EGLDisplay display, EGLContext context) -> Configuration& {
+    CORRADE_ASSERT((context == EGL_NO_CONTEXT) == (display == EGL_NO_DISPLAY),
+        "Platform::WindowlessEglContext::Configuration::setSharedContext(): either both the context and the display have to be valid or both null", *this);
+    _sharedDisplay = display;
+    _sharedContext = context;
+    return *this;
+}
+#endif
+
 WindowlessEglApplication::~WindowlessEglApplication() = default;
 
 }}

src/Magnum/Platform/WindowlessEglApplication.h

@@ -132,6 +132,9 @@ class WindowlessEglContext {
         EGLContext glContext() { return _context; }
 
     private:
+        #ifndef MAGNUM_TARGET_WEBGL
+        bool _sharedContext = false;
+        #endif
         EGLDisplay _display{};
         EGLContext _context{};
         #if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_WEBGL)
@@ -259,7 +262,9 @@ class WindowlessEglContext::Configuration {
          * The device ID is expected to be smaller than the count of devices
          * reported by EGL. When using @ref WindowlessEglApplication, this is
          * also exposed as a `--magnum-device` command-line option and a
-         * `MAGNUM_DEVICE` environment variable.
+         * `MAGNUM_DEVICE` environment variable. If @ref setSharedContext() is
+         * set, this value is ignored and the device is picked to be the same
+         * as in the shared context instead.
          * @requires_gles Device selection is not available in WebGL.
          */
         Configuration& setDevice(UnsignedInt id) {
@@ -273,18 +278,25 @@ class WindowlessEglContext::Configuration {
          * @m_since_latest
          *
          * When set, the created context will share a subset of OpenGL objects
-         * with @p context, instead of being independent. Many caveats and
+         * with @p context and its associated @p display, instead of being
-         * limitations apply to shared OpenGL contexts, please consult the
+         * independent. Many caveats and limitations apply to shared OpenGL
-         * OpenGL specification for details. Default is `EGL_NO_CONTEXT`, i.e.
+         * contexts, please consult the OpenGL specification for details.
-         * no sharing.
+         * Default is `EGL_NO_CONTEXT`, i.e. no sharing. See
+         * @ref Platform-WindowlessEglApplication-shared-contexts for more
+         * information.
          * @see @ref WindowlessEglContext::glContext(),
          *      @ref WindowlessEglApplication::glContext()
          * @requires_gles Context sharing is not available in WebGL.
          */
-        Configuration& setSharedContext(EGLContext context) {
+        Configuration& setSharedContext(EGLDisplay display, EGLContext context);
-            _sharedContext = context;
+
-            return *this;
+        /**
-        }
+         * @brief Shared display
+         * @m_since_latest
+         *
+         * @requires_gles Context sharing is not available in WebGL.
+         */
+        EGLContext sharedDisplay() const { return _sharedDisplay; }
 
         /**
          * @brief Shared context
@@ -299,6 +311,7 @@ class WindowlessEglContext::Configuration {
         #ifndef MAGNUM_TARGET_WEBGL
         Flags _flags;
         UnsignedInt _device;
+        EGLDisplay _sharedDisplay = EGL_NO_DISPLAY;
         EGLContext _sharedContext = EGL_NO_CONTEXT;
         #endif
 };
@@ -465,6 +478,21 @@ using the `__EGL_VENDOR_LIBRARY_FILENAMES` environment variable, for example:
 __EGL_VENDOR_LIBRARY_FILENAMES=/usr/share/glvnd/egl_vendor.d/50_mesa.json ./my-application
 @endcode
 @endparblock
+
+@section Platform-WindowlessEglApplication-shared-contexts Shared EGL contexts
+
+Unlike with @ref WindowlessGlxApplication and @ref WindowlessWglApplication,
+you're expected to supply both the display and the context in
+@ref Configuration::setSharedContext(). This is done in order to ensure the
+same `EGLDisplay` is used for all shared contexts, especially when a
+non-default GPU device is selected via @ref Configuration::setDevice().
+
+Moreover, since `eglInitialize()` and `eglTerminate()` is expected to be called
+just once on a particular display, EGL initialization and termination is only
+done in the case of a non-shared @ref WindowlessEglApplication (or the first
+one created in a shared chain). Shared instances then reuse the already
+initialized `EGLDisplay` and expect that it's terminated only after all shared
+instances are gone.
 */
 class WindowlessEglApplication {
     public: