Add OpenXR bring-up: render the game as a floating quad in an immersive Quest session

- New android/app/src/main/cpp/xr_session.c (+.h): owns the OpenXR
  instance/session/local-space/swapchain and the per-frame
  xrWaitFrame/xrBeginFrame/xrEndFrame loop, submitting the game's
  existing flat render as a single head-tracked XrCompositionLayerQuad.
  No stereo rendering or controller input yet - that's steps B/C/D of
  the plan.
- 11-android-openxr-cmake.patch: links the OpenXR loader as a build
  dependency, staged into jniLibs the same way as gl4es/SDL2.
  build-image/Dockerfile and prepare-android-project.sh build and stage
  it.
- 12-android-openxr-present.patch: redirects OpenGL.cc's final present
  step (opengl_swap_and_restore / SDLDraw's software path) into the XR
  swapchain FBO instead of the window, on Android only. Two real gl4es
  bugs had to be worked around to get pixels on screen at all:
    - gl4es's own glBindFramebuffer errors on an FBO id it didn't create
      itself, even though the real driver-level bind succeeds - fixed by
      creating the swapchain FBO container via gl4es's own
      glGenFramebuffers/glBindFramebuffer, and only using the real
      (dlsym'd) driver call for attaching OpenXR's foreign swapchain
      texture, which gl4es's own attach can't handle.
    - gl4es's glBegin/glVertexAttrib/glVertex3f immediate-mode emulation
      only captures a fresh per-vertex value for attribute 0 (position,
      driven directly by glVertex3f) - custom attributes like this
      shader's texcoords/light are GLES2's *constant*-attribute API and
      applied once for the whole draw, not per vertex, silently
      collapsing the UI-overlay quad to a single sampled texel. Fixed by
      switching that one draw call to real vertex arrays
      (glVertexAttribPointer/glDrawArrays).
  Also flips the V texcoord to match SDL's top-down row order against
  GL's texture convention, and reuses a persistent texture object
  instead of a fresh gen/upload/delete every frame.
- AndroidManifest.xml: declares the immersive-HMD intent category and
  focus-aware metadata, drops the 2D-panel layout hint.
- README: documents the new OpenXR immersive mode.

android/app/build.gradle

@@ -115,15 +115,23 @@ android {
                 // build-image/Dockerfile's ANDROID_16KB_LDFLAGS, which does
                 // the same for the prebuilt SDL2/SDL2_mixer/fluidsynth-lite/
                 // gl4es .so's this links against.
+                // ANDROID_EXTRA_SOURCES is a semicolon-separated CMake list,
+                // not shell-split - AGP passes each "arguments" string
+                // straight through as one argv element to cmake, so the
+                // ';' below survives intact. ANDROID_EXTRA_INCLUDE_DIRS lets
+                // engine/'s own OpenGL.cc find "xr_session.h" (see
+                // android/engine-patches/11-android-openxr-cmake.patch,
+                // which adds it to include_directories()).
                 arguments "-DENABLE_SDL2=ON", "-DENABLE_SOUND=BUNDLED", "-DENABLE_FLUIDSYNTH=BUNDLED", \
                     "-DANDROID_PREBUILT_DIR=${prebuiltDir}", \
+                    "-DANDROID_EXTRA_INCLUDE_DIRS=${projectDir}/src/main/cpp", \
                     "-DCMAKE_PREFIX_PATH=${prebuiltDir}/sdl2", \
                     "-DCMAKE_FIND_ROOT_PATH=${prebuiltDir}/sdl2", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=BOTH", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=BOTH", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=BOTH", \
                     "-DCMAKE_SHARED_LINKER_FLAGS=-Wl,-z,max-page-size=16384", \
-                    "-DANDROID_EXTRA_SOURCES=${projectDir}/src/main/cpp/questshock_native.c"
+                    "-DANDROID_EXTRA_SOURCES=${projectDir}/src/main/cpp/questshock_native.c;${projectDir}/src/main/cpp/xr_session.c"
                 abiFilters 'arm64-v8a'
             }
         }

android/app/src/main/AndroidManifest.xml

@@ -12,6 +12,11 @@
     <!-- External mouse input events -->
     <uses-feature android:name="android.hardware.type.pc" android:required="false" />
 
+    <!-- Immersive OpenXR app now (see android/app/src/main/cpp/xr_session.c),
+         not a 2D Home panel - vr.headtracking is what actually declares that
+         to Horizon OS. -->
+    <uses-feature android:name="android.hardware.vr.headtracking" android:version="1" android:required="true" />
+
     <!-- Plain, unrestricted /sdcard access for the res/data,res/sound the
          user drops in and the shaders/soundfont this app extracts there on
          first run - see res/GET_ASSETS_QUEST.txt and QuestShockActivity.
@@ -28,6 +33,11 @@
         android:theme="@android:style/Theme.NoTitleBar.Fullscreen"
         android:hardwareAccelerated="true" >
 
+        <!-- Tells Horizon OS this activity wants the compositor for itself
+             (immersive VR), not the Home 2D-panel path - required for an
+             OpenXR session to actually start. -->
+        <meta-data android:name="com.oculus.vr.focusaware" android:value="true" />
+
         <activity android:name="de.ladkau.questshock.QuestShockActivity"
             android:label="@string/app_name"
             android:alwaysRetainTaskState="true"
@@ -40,23 +50,19 @@
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
+                <!-- The vendor-neutral, Khronos-defined way to declare an
+                     immersive OpenXR entry point (vs. e.g. Meta's own
+                     com.oculus.intent.category.VR) - see this project's
+                     "favor OpenXR" rule in the README. If Horizon OS still
+                     launches this as a flat panel on-device, the next thing
+                     to try is adding a com.oculus.supportedDevices meta-data
+                     listing the target Quest models - not confirmed
+                     necessary from static analysis alone. -->
+                <category android:name="org.khronos.openxr.intent.category.IMMERSIVE_HMD" />
             </intent-filter>
             <intent-filter>
                 <action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" />
             </intent-filter>
-            <!-- Without this, Quest's Home shell picks its own default 2D
-                 panel shape for this activity - a tall portrait rectangle,
-                 which crops a 4:3 landscape game like System Shock's classic
-                 640x480 down to a sliver. defaultWidth/defaultHeight (the
-                 standard Android multi-window sizing hint, which Quest's
-                 panel system honors for 2D apps) requests a properly
-                 landscape, 4:3-ish panel instead; minWidth/minHeight keep it
-                 from being resized below the game's native resolution. -->
-            <layout android:defaultWidth="1280dp"
-                android:defaultHeight="960dp"
-                android:minWidth="640dp"
-                android:minHeight="480dp"
-                android:gravity="center" />
         </activity>
     </application>
 

android/app/src/main/cpp/xr_session.c

@@ -0,0 +1,456 @@
+#include "xr_session.h"
+
+#include <dlfcn.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <android/log.h>
+#include <jni.h>
+
+#include <EGL/egl.h>
+#include <GLES3/gl3.h>
+
+#define XR_USE_PLATFORM_ANDROID 1
+#define XR_USE_GRAPHICS_API_OPENGL_ES 1
+#include <openxr/openxr.h>
+#include <openxr/openxr_platform.h>
+
+#include <SDL.h>
+
+#define TAG "QuestShock"
+#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, TAG, __VA_ARGS__)
+#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, TAG, __VA_ARGS__)
+
+// Fixed pose/size for the single game quad - 2m in front of the local space
+// origin, sized to preserve the game's own aspect ratio at a comfortable
+// visual angle. No per-eye view/projection work is needed for a quad layer
+// at all - the compositor handles reprojecting it per eye itself, which is
+// exactly why this milestone doesn't need xrLocateViews or any stereo
+// rendering (see the plan's step A notes).
+#define QUAD_DISTANCE_METERS 2.0f
+#define QUAD_WIDTH_METERS 1.6f
+
+// Up to this many swapchain images/FBOs - real runtimes report small counts
+// (2-4); this is just a fixed upper bound for the cache arrays below.
+#define MAX_SWAPCHAIN_IMAGES 8
+
+static XrInstance g_instance = XR_NULL_HANDLE;
+static XrSystemId g_system_id = XR_NULL_SYSTEM_ID;
+static XrSession g_session = XR_NULL_HANDLE;
+static XrSpace g_local_space = XR_NULL_HANDLE;
+static XrSessionState g_session_state = XR_SESSION_STATE_UNKNOWN;
+// Distinct from g_session_state - true from a successful xrBeginSession
+// until xrEndSession/loss/exit. Many runtimes only advance the *state*
+// (e.g. READY -> SYNCHRONIZED -> VISIBLE -> FOCUSED) in response to the
+// app actually calling xrWaitFrame/xrBeginFrame/xrEndFrame continuously
+// after xrBeginSession - gating frame submission on already having
+// reached SYNCHRONIZED+ (as this used to) is a chicken-and-egg deadlock,
+// since the runtime never progresses past READY without the frame loop
+// running in the first place. Frame calls just need the session to have
+// begun, per the standard OpenXR sample pattern - not any particular
+// later state.
+static bool g_session_running = false;
+
+static XrSwapchain g_game_swapchain = XR_NULL_HANDLE;
+static int g_game_width = 0;
+static int g_game_height = 0;
+static GLuint g_swapchain_fbos[MAX_SWAPCHAIN_IMAGES];
+static uint32_t g_swapchain_image_count = 0;
+
+static XrTime g_predicted_display_time = 0;
+static bool g_frame_should_render = false;
+static bool g_have_acquired_image = false;
+
+// This file's GL calls otherwise resolve to gl4es (the only GL symbol
+// provider linked into this binary - see android/engine-patches/
+// 02-android-opengl-es.patch), which is fine for anything shared with the
+// engine's own gl4es-routed rendering. But the swapchain images OpenXR
+// hands us are real driver texture objects gl4es never created itself,
+// and gl4es's own glFramebufferTexture2D can't attach a texture it has no
+// tracked metadata for. So the FBO *container* is created via gl4es's own
+// glGenFramebuffers/glBindFramebuffer (so gl4es recognizes the id as its
+// own and its own per-frame glBindFramebuffer succeeds), while the
+// texture-attach step - the specifically foreign part - goes through the
+// real driver directly, via dlsym against libGLESv2.so.
+typedef void (*PFNQSFRAMEBUFFERTEXTURE2D)(GLenum, GLenum, GLenum, GLuint, GLint);
+typedef GLenum (*PFNQSCHECKFRAMEBUFFERSTATUS)(GLenum);
+
+static PFNQSFRAMEBUFFERTEXTURE2D real_glFramebufferTexture2D;
+static PFNQSCHECKFRAMEBUFFERSTATUS real_glCheckFramebufferStatus;
+
+static bool xr_load_real_gles(void) {
+    void *lib = dlopen("libGLESv2.so", RTLD_NOW | RTLD_LOCAL);
+    if (lib == NULL) {
+        LOGE("XR: dlopen(libGLESv2.so) failed: %s", dlerror());
+        return false;
+    }
+    real_glFramebufferTexture2D = (PFNQSFRAMEBUFFERTEXTURE2D)dlsym(lib, "glFramebufferTexture2D");
+    real_glCheckFramebufferStatus =
+        (PFNQSCHECKFRAMEBUFFERSTATUS)dlsym(lib, "glCheckFramebufferStatus");
+    return real_glFramebufferTexture2D && real_glCheckFramebufferStatus;
+}
+
+static bool xr_check(XrResult result, const char *what) {
+    if (XR_SUCCEEDED(result))
+        return true;
+    char resultString[XR_MAX_RESULT_STRING_SIZE] = {0};
+    if (g_instance != XR_NULL_HANDLE)
+        xrResultToString(g_instance, result, resultString);
+    LOGE("XR: %s failed: %s (%d)", what, resultString[0] ? resultString : "?", (int)result);
+    return false;
+}
+
+static PFN_xrVoidFunction xr_get_proc(const char *name) {
+    PFN_xrVoidFunction fn = NULL;
+    // xrGetInstanceProcAddr itself works with a NULL instance for the
+    // handful of functions (like xrInitializeLoaderKHR) that must be called
+    // before an instance exists.
+    if (!xr_check(xrGetInstanceProcAddr(g_instance, name, &fn), name))
+        return NULL;
+    return fn;
+}
+
+// The Android OpenXR loader needs the JavaVM/context before anything else -
+// separate from XR_KHR_android_create_instance below, which only affects
+// xrCreateInstance itself. Skipping this is a common cause of
+// xrCreateInstance silently failing to find a runtime on Android.
+static bool xr_initialize_android_loader(JavaVM *vm, jobject activity) {
+    PFN_xrInitializeLoaderKHR initializeLoader =
+        (PFN_xrInitializeLoaderKHR)xr_get_proc("xrInitializeLoaderKHR");
+    if (initializeLoader == NULL) {
+        LOGE("XR: xrInitializeLoaderKHR not available - no Android OpenXR loader present?");
+        return false;
+    }
+
+    XrLoaderInitInfoAndroidKHR loaderInitInfo = {XR_TYPE_LOADER_INIT_INFO_ANDROID_KHR};
+    loaderInitInfo.applicationVM = vm;
+    loaderInitInfo.applicationContext = activity;
+    return xr_check(initializeLoader((const XrLoaderInitInfoBaseHeaderKHR *)&loaderInitInfo),
+                     "xrInitializeLoaderKHR");
+}
+
+// eglGetCurrentDisplay()/eglGetCurrentContext() hand us the display/context,
+// but not the EGLConfig they were created with - eglQueryContext's
+// EGL_CONFIG_ID plus eglChooseConfig is the standard way to recover it (the
+// same trick Khronos' own hello_xr sample uses).
+static EGLConfig xr_get_current_egl_config(EGLDisplay display, EGLContext context) {
+    EGLint configId = 0;
+    eglQueryContext(display, context, EGL_CONFIG_ID, &configId);
+
+    EGLint attribs[] = {EGL_CONFIG_ID, configId, EGL_NONE};
+    EGLConfig config = NULL;
+    EGLint numConfigs = 0;
+    eglChooseConfig(display, attribs, &config, 1, &numConfigs);
+    return config;
+}
+
+static bool xr_create_instance_and_session(int game_width, int game_height) {
+    JNIEnv *env = (JNIEnv *)SDL_AndroidGetJNIEnv();
+    jobject activity = (jobject)SDL_AndroidGetActivity();
+    if (env == NULL || activity == NULL) {
+        LOGE("XR: no JNIEnv/Activity from SDL - can't init OpenXR");
+        return false;
+    }
+    JavaVM *vm = NULL;
+    (*env)->GetJavaVM(env, &vm);
+
+    if (!xr_initialize_android_loader(vm, activity))
+        return false;
+
+    const char *extensions[] = {
+        XR_KHR_ANDROID_CREATE_INSTANCE_EXTENSION_NAME,
+        XR_KHR_OPENGL_ES_ENABLE_EXTENSION_NAME,
+    };
+
+    XrInstanceCreateInfoAndroidKHR androidInfo = {XR_TYPE_INSTANCE_CREATE_INFO_ANDROID_KHR};
+    androidInfo.applicationVM = vm;
+    androidInfo.applicationActivity = activity;
+
+    XrInstanceCreateInfo createInfo = {XR_TYPE_INSTANCE_CREATE_INFO};
+    createInfo.next = &androidInfo;
+    createInfo.enabledExtensionCount = sizeof(extensions) / sizeof(extensions[0]);
+    createInfo.enabledExtensionNames = extensions;
+    strncpy(createInfo.applicationInfo.applicationName, "QuestShock",
+            XR_MAX_APPLICATION_NAME_SIZE - 1);
+    strncpy(createInfo.applicationInfo.engineName, "Shockolate", XR_MAX_ENGINE_NAME_SIZE - 1);
+    createInfo.applicationInfo.applicationVersion = 1;
+    createInfo.applicationInfo.engineVersion = 1;
+    createInfo.applicationInfo.apiVersion = XR_CURRENT_API_VERSION;
+
+    if (!xr_check(xrCreateInstance(&createInfo, &g_instance), "xrCreateInstance"))
+        return false;
+
+    XrSystemGetInfo systemInfo = {XR_TYPE_SYSTEM_GET_INFO};
+    systemInfo.formFactor = XR_FORM_FACTOR_HEAD_MOUNTED_DISPLAY;
+    if (!xr_check(xrGetSystem(g_instance, &systemInfo, &g_system_id), "xrGetSystem"))
+        return false;
+
+    // Required by spec before creating an OpenGL ES-backed session, even
+    // though we don't gate on the returned min/max API version here.
+    PFN_xrGetOpenGLESGraphicsRequirementsKHR getGLESRequirements =
+        (PFN_xrGetOpenGLESGraphicsRequirementsKHR)xr_get_proc("xrGetOpenGLESGraphicsRequirementsKHR");
+    if (getGLESRequirements == NULL)
+        return false;
+    XrGraphicsRequirementsOpenGLESKHR glesRequirements = {XR_TYPE_GRAPHICS_REQUIREMENTS_OPENGL_ES_KHR};
+    if (!xr_check(getGLESRequirements(g_instance, g_system_id, &glesRequirements),
+                  "xrGetOpenGLESGraphicsRequirementsKHR"))
+        return false;
+
+    EGLDisplay display = eglGetCurrentDisplay();
+    EGLContext context = eglGetCurrentContext();
+    if (display == EGL_NO_DISPLAY || context == EGL_NO_CONTEXT) {
+        LOGE("XR: no current EGL display/context - call xr_init() after init_opengl()");
+        return false;
+    }
+
+    XrGraphicsBindingOpenGLESAndroidKHR graphicsBinding = {
+        XR_TYPE_GRAPHICS_BINDING_OPENGL_ES_ANDROID_KHR};
+    graphicsBinding.display = display;
+    graphicsBinding.config = xr_get_current_egl_config(display, context);
+    graphicsBinding.context = context;
+
+    XrSessionCreateInfo sessionInfo = {XR_TYPE_SESSION_CREATE_INFO};
+    sessionInfo.next = &graphicsBinding;
+    sessionInfo.systemId = g_system_id;
+    if (!xr_check(xrCreateSession(g_instance, &sessionInfo, &g_session), "xrCreateSession"))
+        return false;
+
+    XrReferenceSpaceCreateInfo spaceInfo = {XR_TYPE_REFERENCE_SPACE_CREATE_INFO};
+    spaceInfo.referenceSpaceType = XR_REFERENCE_SPACE_TYPE_LOCAL;
+    spaceInfo.poseInReferenceSpace.orientation.w = 1.0f;
+    if (!xr_check(xrCreateReferenceSpace(g_session, &spaceInfo, &g_local_space),
+                  "xrCreateReferenceSpace"))
+        return false;
+
+    // Prefer a plain linear 8-bit format over GL_SRGB8_ALPHA8: the source
+    // SDL surface pixels are already sRGB-encoded (as ordinary 8-bit image
+    // data conventionally is) and get uploaded/sampled as plain linear
+    // GL_RGBA with no decode step anywhere in this path, matching the
+    // desktop SDL_RenderCopy path this replaces - an sRGB swapchain format
+    // would auto-gamma-encode on write and double-encode already-encoded
+    // data.
+    uint32_t formatCount = 0;
+    xrEnumerateSwapchainFormats(g_session, 0, &formatCount, NULL);
+    int64_t *formats = (int64_t *)malloc(sizeof(int64_t) * formatCount);
+    xrEnumerateSwapchainFormats(g_session, formatCount, &formatCount, formats);
+    int64_t chosenFormat = formats[0];
+    for (uint32_t i = 0; i < formatCount; i++) {
+        LOGI("XR: swapchain format[%u] = 0x%llx", i, (unsigned long long)formats[i]);
+        if (formats[i] == GL_RGBA8) {
+            chosenFormat = GL_RGBA8;
+            break;
+        }
+    }
+    free(formats);
+
+    XrSwapchainCreateInfo swapchainInfo = {XR_TYPE_SWAPCHAIN_CREATE_INFO};
+    swapchainInfo.usageFlags = XR_SWAPCHAIN_USAGE_COLOR_ATTACHMENT_BIT | XR_SWAPCHAIN_USAGE_SAMPLED_BIT;
+    swapchainInfo.format = chosenFormat;
+    swapchainInfo.sampleCount = 1;
+    swapchainInfo.width = (uint32_t)game_width;
+    swapchainInfo.height = (uint32_t)game_height;
+    swapchainInfo.faceCount = 1;
+    swapchainInfo.arraySize = 1;
+    swapchainInfo.mipCount = 1;
+    if (!xr_check(xrCreateSwapchain(g_session, &swapchainInfo, &g_game_swapchain),
+                  "xrCreateSwapchain"))
+        return false;
+    g_game_width = game_width;
+    g_game_height = game_height;
+
+    uint32_t imageCount = 0;
+    xrEnumerateSwapchainImages(g_game_swapchain, 0, &imageCount, NULL);
+    if (imageCount > MAX_SWAPCHAIN_IMAGES) {
+        LOGE("XR: swapchain reports %u images, only room for %d", imageCount,
+             MAX_SWAPCHAIN_IMAGES);
+        return false;
+    }
+    // Zero-initialized, not just `.type` set per element - these structs
+    // also carry a `next` field the runtime may read, and an
+    // uninitialized stack array would leave it as garbage.
+    XrSwapchainImageOpenGLESKHR images[MAX_SWAPCHAIN_IMAGES] = {0};
+    for (uint32_t i = 0; i < imageCount; i++)
+        images[i].type = XR_TYPE_SWAPCHAIN_IMAGE_OPENGL_ES_KHR;
+    if (!xr_check(xrEnumerateSwapchainImages(g_game_swapchain, imageCount, &imageCount,
+                                              (XrSwapchainImageBaseHeader *)images),
+                  "xrEnumerateSwapchainImages"))
+        return false;
+    g_swapchain_image_count = imageCount;
+
+    if (!xr_load_real_gles()) {
+        LOGE("XR: couldn't resolve real GLES FBO functions via dlsym");
+        return false;
+    }
+
+    // Wraps each swapchain-provided texture in its own framebuffer, matching
+    // how OpenGL.cc's own CreateFrameBuffer() wraps backupBuffer - just
+    // without a depth/stencil attachment, since the final composite draw
+    // (see opengl_swap_and_restore) never needs one. The Gen/Bind calls are
+    // gl4es's own (linked, not dlsym'd) - see the comment above
+    // real_glFramebufferTexture2D for why.
+    bool all_complete = true;
+    for (uint32_t i = 0; i < imageCount; i++) {
+        glGenFramebuffers(1, &g_swapchain_fbos[i]);
+        glBindFramebuffer(GL_FRAMEBUFFER, g_swapchain_fbos[i]);
+        real_glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D,
+                                     images[i].image, 0);
+        GLenum status = real_glCheckFramebufferStatus(GL_FRAMEBUFFER);
+        if (status != GL_FRAMEBUFFER_COMPLETE) {
+            LOGE("XR: swapchain FBO %u incomplete: 0x%x", i, status);
+            all_complete = false;
+        }
+    }
+    glBindFramebuffer(GL_FRAMEBUFFER, 0);
+    if (!all_complete)
+        return false;
+
+    LOGI("XR: instance/session/swapchain ready (%dx%d, %u images)", game_width, game_height,
+         imageCount);
+    return true;
+}
+
+bool xr_init(int game_width, int game_height) {
+    if (!xr_create_instance_and_session(game_width, game_height)) {
+        LOGE("XR: bring-up failed - falling back to the normal window present path");
+        xr_shutdown();
+        return false;
+    }
+    return true;
+}
+
+void xr_poll_events(void) {
+    if (g_instance == XR_NULL_HANDLE)
+        return;
+
+    while (true) {
+        XrEventDataBuffer event = {XR_TYPE_EVENT_DATA_BUFFER};
+        XrResult result = xrPollEvent(g_instance, &event);
+        if (result == XR_EVENT_UNAVAILABLE)
+            break;
+        if (!xr_check(result, "xrPollEvent"))
+            break;
+
+        if (event.type == XR_TYPE_EVENT_DATA_SESSION_STATE_CHANGED) {
+            XrEventDataSessionStateChanged *stateEvent = (XrEventDataSessionStateChanged *)&event;
+            g_session_state = stateEvent->state;
+            LOGI("XR: session state -> %d", (int)g_session_state);
+
+            if (g_session_state == XR_SESSION_STATE_READY) {
+                XrSessionBeginInfo beginInfo = {XR_TYPE_SESSION_BEGIN_INFO};
+                beginInfo.primaryViewConfigurationType = XR_VIEW_CONFIGURATION_TYPE_PRIMARY_STEREO;
+                g_session_running = xr_check(xrBeginSession(g_session, &beginInfo), "xrBeginSession");
+            } else if (g_session_state == XR_SESSION_STATE_STOPPING) {
+                xr_check(xrEndSession(g_session), "xrEndSession");
+                g_session_running = false;
+            } else if (g_session_state == XR_SESSION_STATE_EXITING ||
+                       g_session_state == XR_SESSION_STATE_LOSS_PENDING) {
+                g_session_running = false;
+            }
+        }
+    }
+}
+
+bool xr_is_session_running(void) { return g_session_running; }
+
+bool xr_frame_begin(void) {
+    g_have_acquired_image = false;
+    if (!xr_is_session_running())
+        return false;
+
+    XrFrameWaitInfo waitInfo = {XR_TYPE_FRAME_WAIT_INFO};
+    XrFrameState frameState = {XR_TYPE_FRAME_STATE};
+    if (!xr_check(xrWaitFrame(g_session, &waitInfo, &frameState), "xrWaitFrame"))
+        return false;
+    g_predicted_display_time = frameState.predictedDisplayTime;
+    g_frame_should_render = frameState.shouldRender;
+
+    XrFrameBeginInfo beginInfo = {XR_TYPE_FRAME_BEGIN_INFO};
+    if (!xr_check(xrBeginFrame(g_session, &beginInfo), "xrBeginFrame"))
+        return false;
+
+    if (!g_frame_should_render)
+        return false;
+
+    uint32_t imageIndex = 0;
+    XrSwapchainImageAcquireInfo acquireInfo = {XR_TYPE_SWAPCHAIN_IMAGE_ACQUIRE_INFO};
+    if (!xr_check(xrAcquireSwapchainImage(g_game_swapchain, &acquireInfo, &imageIndex),
+                  "xrAcquireSwapchainImage"))
+        return false;
+
+    XrSwapchainImageWaitInfo waitImageInfo = {XR_TYPE_SWAPCHAIN_IMAGE_WAIT_INFO};
+    waitImageInfo.timeout = XR_INFINITE_DURATION;
+    if (!xr_check(xrWaitSwapchainImage(g_game_swapchain, &waitImageInfo), "xrWaitSwapchainImage"))
+        return false;
+
+    // gl4es's own (linked, not dlsym'd) bind - this targets an FBO id
+    // gl4es itself created (see xr_create_instance_and_session()), so its
+    // own "current FBO" bookkeeping updates correctly and its immediate-
+    // mode draw calls in android_draw_surface_as_quad() land in the right
+    // place.
+    glBindFramebuffer(GL_FRAMEBUFFER, g_swapchain_fbos[imageIndex]);
+    glViewport(0, 0, g_game_width, g_game_height);
+
+    g_have_acquired_image = true;
+    return true;
+}
+
+void xr_frame_end(void) {
+    if (g_session == XR_NULL_HANDLE)
+        return;
+
+    if (g_have_acquired_image) {
+        XrSwapchainImageReleaseInfo releaseInfo = {XR_TYPE_SWAPCHAIN_IMAGE_RELEASE_INFO};
+        xr_check(xrReleaseSwapchainImage(g_game_swapchain, &releaseInfo),
+                 "xrReleaseSwapchainImage");
+    }
+
+    if (!xr_is_session_running())
+        return;
+
+    XrCompositionLayerQuad quad = {XR_TYPE_COMPOSITION_LAYER_QUAD};
+    quad.space = g_local_space;
+    quad.eyeVisibility = XR_EYE_VISIBILITY_BOTH;
+    quad.subImage.swapchain = g_game_swapchain;
+    quad.subImage.imageRect.extent.width = g_game_width;
+    quad.subImage.imageRect.extent.height = g_game_height;
+    quad.pose.orientation.w = 1.0f;
+    quad.pose.position.z = -QUAD_DISTANCE_METERS;
+    quad.size.width = QUAD_WIDTH_METERS;
+    quad.size.height = QUAD_WIDTH_METERS * (float)g_game_height / (float)g_game_width;
+
+    const XrCompositionLayerBaseHeader *layers[1] = {(XrCompositionLayerBaseHeader *)&quad};
+
+    XrFrameEndInfo endInfo = {XR_TYPE_FRAME_END_INFO};
+    endInfo.displayTime = g_predicted_display_time;
+    endInfo.environmentBlendMode = XR_ENVIRONMENT_BLEND_MODE_OPAQUE;
+    endInfo.layerCount = g_have_acquired_image ? 1 : 0;
+    endInfo.layers = g_have_acquired_image ? layers : NULL;
+    xr_check(xrEndFrame(g_session, &endInfo), "xrEndFrame");
+}
+
+void xr_shutdown(void) {
+    for (uint32_t i = 0; i < g_swapchain_image_count; i++) {
+        if (g_swapchain_fbos[i] != 0)
+            glDeleteFramebuffers(1, &g_swapchain_fbos[i]);
+    }
+    g_swapchain_image_count = 0;
+
+    if (g_game_swapchain != XR_NULL_HANDLE)
+        xrDestroySwapchain(g_game_swapchain);
+    g_game_swapchain = XR_NULL_HANDLE;
+
+    if (g_local_space != XR_NULL_HANDLE)
+        xrDestroySpace(g_local_space);
+    g_local_space = XR_NULL_HANDLE;
+
+    if (g_session != XR_NULL_HANDLE)
+        xrDestroySession(g_session);
+    g_session = XR_NULL_HANDLE;
+
+    if (g_instance != XR_NULL_HANDLE)
+        xrDestroyInstance(g_instance);
+    g_instance = XR_NULL_HANDLE;
+
+    g_session_state = XR_SESSION_STATE_UNKNOWN;
+    g_session_running = false;
+}

android/app/src/main/cpp/xr_session.h

@@ -0,0 +1,56 @@
+// Minimal OpenXR bring-up for questshock's immersive Quest build. Keeps the
+// game's own rendering untouched (see android/engine-patches/
+// 12-android-openxr-present.patch) - this module only owns the OpenXR
+// instance/session/swapchain and the final "submit a quad layer instead of
+// presenting to a window" step. No stereo rendering, no controller input
+// yet (see the plan's steps B/C/D for those) - the game composite is shown
+// as a single flat quad floating in front of the viewer.
+#ifndef QUESTSHOCK_XR_SESSION_H
+#define QUESTSHOCK_XR_SESSION_H
+
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Call once, right after init_opengl() (OpenGL.cc) has created and made
+// current the GL context SDL/gl4es already use - creates the OpenXR
+// instance/session sharing that same EGL display/context, plus one
+// swapchain sized to the game's logical resolution (game_width/height, i.e.
+// grd_cap->w/h - see Shock.c's InitSDL()). Returns false if OpenXR bring-up
+// failed (e.g. no runtime installed) - callers should fall back to the
+// existing window-present path in that case.
+bool xr_init(int game_width, int game_height);
+
+// Pumps XR session-state events. Call once per frame, before
+// xr_frame_begin(). Must still be called even when xr_init() returned
+// false (no-op in that case).
+void xr_poll_events(void);
+
+// True once the session has reached a state where frames are expected
+// (XR_SESSION_STATE_SYNCHRONIZED or later) - i.e. it's safe/required to
+// start calling xr_frame_begin()/xr_frame_end() each frame.
+bool xr_is_session_running(void);
+
+// Begins the XR frame (xrWaitFrame/xrBeginFrame) and, if the runtime wants
+// this frame rendered, acquires the next swapchain image and binds its
+// framebuffer as the current render target - ready for the caller to draw
+// into exactly as it would have drawn to the default framebuffer. Returns
+// true if the caller should draw this frame; xr_frame_end() must be called
+// unconditionally afterward either way (a begun XR frame must always be
+// ended, rendered or not).
+bool xr_frame_begin(void);
+
+// Releases the swapchain image (if one was acquired this frame) and
+// submits it as a single XrCompositionLayerQuad positioned in front of the
+// local reference space's origin, then ends the XR frame.
+void xr_frame_end(void);
+
+void xr_shutdown(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif