egl: allow for ordering display init paths

On certain platforms, initialization by query device is
preferred. On others, initialization by default device is
preferred. To accommodate for either, we use a bool to switch the
ordering if needed. PlatformEGLHeadless is assumed use the query
device path first, and PlatformEGL will still check the
default device/display first.

filament/backend/include/backend/platforms/PlatformEGL.h

@@ -83,6 +83,22 @@ protected:
      */
     Driver* createDriver(void* sharedContext, const DriverConfig& driverConfig) override;
 
+    /**
+     * The implementation of *createDriver*.
+     * @param sharedContext an optional shared context. This is not meaningful with all graphic
+     *                      APIs and platforms.
+     *                      For EGL platforms, this is an EGLContext.
+     *
+     * @param driverConfig  specifies driver initialization parameters
+     *
+     * @param initFirstbyquery determines the order of initialization. If true, then we'd query by
+     *                         eglQueryDevicesEXT first instead of using the default display. Useful
+     *                         for headless egl initialization.
+     * @return nullptr on failure, or a pointer to the newly created driver.
+     */
+    Driver* createDriverBase(void* sharedContext, const DriverConfig& driverConfig,
+            bool initFirstByQuery);
+
     /**
      * This returns zero. This method can be overridden to return something more useful.
      * @return zero

filament/backend/src/opengl/platforms/PlatformEGL.cpp

@@ -120,17 +120,28 @@ bool PlatformEGL::isOpenGL() const noexcept {
 PlatformEGL::ExternalImageEGL::~ExternalImageEGL() = default;
 
 Driver* PlatformEGL::createDriver(void* sharedContext, const DriverConfig& driverConfig) {
+    return createDriverBase(sharedContext, driverConfig, false /* initFirstByQuery */);
+}
+
+Driver* PlatformEGL::createDriverBase(void* sharedContext, const DriverConfig& driverConfig,
+        bool initFirstByQuery) {
     static constexpr int kMaxNumEGLDevices = 32;
 
     EGLint major, minor;
     EGLBoolean initialized = false;
 
-    PFNEGLQUERYDEVICESEXTPROC const eglQueryDevicesEXT =
-        PFNEGLQUERYDEVICESEXTPROC(eglGetProcAddress("eglQueryDevicesEXT"));
-    PFNEGLGETPLATFORMDISPLAYEXTPROC const getPlatformDisplay =
-        PFNEGLGETPLATFORMDISPLAYEXTPROC(eglGetProcAddress("eglGetPlatformDisplay"));
+    using InitFunc = std::function<void()>;
+
+    InitFunc queryInit = [&]() {
+        PFNEGLQUERYDEVICESEXTPROC const eglQueryDevicesEXT =
+                PFNEGLQUERYDEVICESEXTPROC(eglGetProcAddress("eglQueryDevicesEXT"));
+        PFNEGLGETPLATFORMDISPLAYEXTPROC const getPlatformDisplay =
+                PFNEGLGETPLATFORMDISPLAYEXTPROC(eglGetProcAddress("eglGetPlatformDisplay"));
+
+        if (!eglQueryDevicesEXT || !getPlatformDisplay) {
+            return;
+        }
 
-    if (eglQueryDevicesEXT != nullptr && getPlatformDisplay != nullptr) {
         EGLint numDevices = 0;
         EGLDeviceEXT eglDevices[kMaxNumEGLDevices];
         if (eglQueryDevicesEXT(kMaxNumEGLDevices, eglDevices, &numDevices)) {
@@ -139,12 +150,27 @@ Driver* PlatformEGL::createDriver(void* sharedContext, const DriverConfig& drive
                 initialized = eglInitialize(mEGLDisplay, &major, &minor);
             }
         }
+    };
+
+    InitFunc defaultInit = [&]() {
+        mEGLDisplay = eglGetDisplay(EGL_DEFAULT_DISPLAY);
+        initialized = eglInitialize(mEGLDisplay, &major, &minor);
+    };
+
+    // The order by which we check for display/device does matter because certain platforms have
+    // multiple displays/devices. We either return the first queried (and successfully init'd
+    // display) or just use the default display. Deciding which init path should go first is
+    // determined by the bool *initFirstByQuery*..
+    std::array<InitFunc, 2> initFuncs{ defaultInit, queryInit };
+    if (initFirstByQuery) {
+        std::swap(initFuncs[0], initFuncs[1]);
     }
 
-    if (!initialized) {
-        mEGLDisplay = eglGetDisplay(EGL_DEFAULT_DISPLAY);
-        assert_invariant(mEGLDisplay != EGL_NO_DISPLAY);
-        initialized = eglInitialize(mEGLDisplay, &major, &minor);
+    for (auto& initFunc: initFuncs) {
+        if (initialized) {
+            break;
+        }
+        initFunc();
     }
 
     if (UTILS_UNLIKELY(!initialized)) {

filament/backend/src/opengl/platforms/PlatformEGLHeadless.cpp

@@ -66,7 +66,7 @@ backend::Driver* PlatformEGLHeadless::createDriver(void* sharedContext,
         return nullptr;
     }
 
-    return PlatformEGL::createDriver(sharedContext, driverConfig);
+    return PlatformEGL::createDriverBase(sharedContext, driverConfig, true /* initFirstByQuery */);
 }
 
 } // namespace filament