Platform: prefer EGL_EXT_device_enumeration to find an EGL display.

Makes things work on Mesa 19.2 when DISPLAY is not defined. And opens the
possibilities for *sane* GPU selection. No more environment variables,
globally exported symbols or other BAD SHIT.

doc/changelog.dox

@@ -401,6 +401,12 @@ See also:
     application. Users are encouraged to call
     @ref Platform::BasicScreen::hasApplication() to check if the application is
     accessible.
+-   @ref Platform::WindowlessEglApplication now prefers to use the
+    @m_class{m-doc-external} [EGL_EXT_device_enumeration](https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_device_enumeration.txt),
+    @m_class{m-doc-external} [EGL_EXT_platform_base](https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_platform_base.txt) and
+    @m_class{m-doc-external} [EGL_EXT_platform_device](https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_platform_device.txt) extensions where
+    available instead of `EGL_DEFAULT_DISPLAY` to work better on headless
+    setups
 
 @subsubsection changelog-latest-changes-text Text library
 

src/Magnum/Platform/WindowlessEglApplication.cpp

@@ -25,6 +25,7 @@
 
 #include "WindowlessEglApplication.h"
 
+#include <cstring>
 #include <string>
 #include <Corrade/Utility/Assert.h>
 #include <Corrade/Utility/Debug.h>
@@ -38,11 +39,80 @@
 #include <Corrade/Containers/ArrayView.h>
 #endif
 
+#ifndef EGL_EXT_device_base
+typedef void *EGLDeviceEXT;
+#endif
+
+#ifndef EGL_EXT_platform_device
+#define EGL_PLATFORM_DEVICE_EXT 0x313F
+#endif
+
 namespace Magnum { namespace Platform {
 
+namespace {
+
+bool extensionSupported(const char* const extensions, Containers::ArrayView<const char> extension) {
+    CORRADE_INTERNAL_ASSERT(extensions);
+    const char* pos = std::strstr(extensions, extension);
+    /* Extension is supported if its string is delimited by a space or end of
+       the extension list. The extension.size() is the whole C array including
+       a 0-terminator, so subtract 1 to look at one character after. */
+    return pos && (pos[extension.size() - 1] == ' ' || pos[extension.size() - 1] == '\0');
+}
+
+}
+
 WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, GLContext* const magnumContext) {
-    /* Initialize */
-    _display = eglGetDisplay(EGL_DEFAULT_DISPLAY);
+    /* 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. */
+    const char* const extensions = eglQueryString(EGL_NO_DISPLAY, EGL_EXTENSIONS);
+    if(extensions &&
+        /* eglQueryDevicesEXT() */
+        extensionSupported(extensions, "EGL_EXT_device_enumeration") &&
+
+        /* eglGetPlatformDisplayEXT() */
+        extensionSupported(extensions, "EGL_EXT_platform_base") &&
+
+        /* EGL_PLATFORM_DEVICE_EXT (FFS, why it has to be scattered over a
+           thousand extensions?!). This is supported only since Mesa 19.2. */
+        extensionSupported(extensions, "EGL_EXT_platform_device")
+    ) {
+        EGLint count;
+        auto eglQueryDevices = reinterpret_cast<EGLBoolean(*)(EGLint, EGLDeviceEXT*, EGLint*)>(eglGetProcAddress("eglQueryDevicesEXT"));
+        if(!eglQueryDevices(0, nullptr, &count)) {
+            Error{} << "Platform::WindowlessEglApplication::tryCreateContext(): cannot query EGL devices:" << Implementation::eglErrorString(eglGetError());
+            return;
+        }
+
+        if(!count) {
+            Error{} << "Platform::WindowlessEglApplication::tryCreateContext(): no EGL devices found";
+            return;
+        }
+
+        if(magnumContext && (magnumContext->internalFlags() >= GL::Context::InternalFlag::DisplayVerboseInitializationLog)) {
+            Debug{} << "Platform::WindowlessEglApplication: found" << count << "EGL devices, choosing the first one";
+        }
+
+        /* Assuming the same thing won't suddenly start failing when called the
+           second time */
+        EGLDeviceEXT device;
+        CORRADE_INTERNAL_ASSERT_OUTPUT(eglQueryDevices(1, &device, &count));
+
+        if(!(_display = reinterpret_cast<EGLDisplay(*)(EGLenum, void*, const EGLint*)>(eglGetProcAddress("eglGetPlatformDisplayEXT"))(EGL_PLATFORM_DEVICE_EXT, device, nullptr))) {
+            Error{} << "Platform::WindowlessEglApplication::tryCreateContext(): cannot get platform display for a device:" << Implementation::eglErrorString(eglGetError());
+            return;
+        }
+
+    /* Otherwise initialize the classic way */
+    } else if(!(_display = eglGetDisplay(EGL_DEFAULT_DISPLAY))) {
+        Error{} << "Platform::WindowlessEglApplication::tryCreateContext(): cannot get default EGL display:" << Implementation::eglErrorString(eglGetError());
+        return;
+    }
+
     if(!eglInitialize(_display, nullptr, nullptr)) {
         Error() << "Platform::WindowlessEglApplication::tryCreateContext(): cannot initialize EGL:" << Implementation::eglErrorString(eglGetError());
         return;

src/Magnum/Platform/WindowlessEglApplication.h

@@ -349,6 +349,15 @@ MAGNUM_WINDOWLESSEGLAPPLICATION_MAIN(MyApplication)
 If no other application header is included, this class is also aliased to
 @cpp Platform::WindowlessApplication @ce and the macro is aliased to
 @cpp MAGNUM_WINDOWLESSAPPLICATION_MAIN() @ce to simplify porting.
+
+@subsection Platform-WindowlessEglApplication-usage-device-enumeration EGL device enumeration
+
+The application prefers to use the @m_class{m-doc-external}
+[EGL_EXT_device_enumeration](https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_device_enumeration.txt),
+@m_class{m-doc-external} [EGL_EXT_platform_base](https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_platform_base.txt) and
+@m_class{m-doc-external} [EGL_EXT_platform_device](https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_platform_device.txt)
+where available instead of `EGL_DEFAULT_DISPLAY` to work better on headless
+setups. The application always chooses the first found device.
 */
 class WindowlessEglApplication {
     public: