[Fix, Test] (MG_State, MG_Backend/DirectVulkan, MG_Backend/DirectGLES, MG_IntegrationTest): a respecified capture buffer left the transform feedback writing one store and the readback reading another

MobileGL/MG_Backend/DirectGLES/Managers.cpp

@@ -603,7 +603,18 @@ namespace MobileGL::MG_Backend::DirectGLES {
             void Ops_Respecify(BufferObject& bufferObject) {
                 auto* resource = ResourceOf(bufferObject);
                 if (!resource) return; // lazy: EnsureBufferResource full-uploads on creation
-                if (resource->persistentMapped) return; // immutable persistent storage is never respecified
+                if (resource->persistentMapped) {
+                    // The frontend writes straight into the storage mapped here, and it
+                    // renewed that mapping for the redefined store before writing to it
+                    // (BufferObject::RedefineStorage), so the new contents already are
+                    // where a respecification would put them.
+                    if (bufferObject.IsBackendPersistentMapped()) return;
+                    // Renewal declined (a zero-sized store, or the map could not be
+                    // retaken): the buffer is back on its CPU shadow and needs the
+                    // ordinary respecification below.
+                    resource->persistentMapped = false;
+                    resource->persistentPtr = nullptr;
+                }
                 if (!CanTouchGLNow() || resource->id == 0 ||
                     resource->contextGeneration != g_bufferContextGeneration) {
                     resource->pendingRespecify = true;

MobileGL/MG_Backend/DirectVulkan/Renderer/VkBufferManager.cpp

@@ -379,6 +379,23 @@ namespace MobileGL::MG_Backend::DirectVulkan {
         BumpSliceEpoch(*resource);
         // Any cached streaming slice refers to the previous contents.
         resource->transientFrameSerial = 0;
+        if (resource->persistentMapped) {
+            if (bufferObject.IsBackendPersistentMapped()) {
+                // The frontend renewed its adoption of this storage for the redefined
+                // store before writing a byte of it (BufferObject::RedefineStorage), so
+                // the new contents are already HERE and there is no second copy to
+                // update. Swapping the storage is what must not happen: the mapping the
+                // frontend holds, and every read that resolves through it, would keep
+                // addressing the storage being released - which is how a transform
+                // feedback capture came to be written to one buffer and read back out
+                // of another.
+                return;
+            }
+            // The renewal did not happen (a zero-sized store, or the storage could not
+            // be created): the frontend is back on its CPU shadow, so this is an
+            // ordinary resident buffer again and the handling below applies.
+            resource->persistentMapped = false;
+        }
         if (!resource->buffer.IsValid()) {
             return; // streaming-only resource: shadow + serial are enough
         }

MobileGL/MG_IntegrationTest/CMakeLists.txt

@@ -71,6 +71,7 @@ add_executable(MobileGLIntegrationTest
     Scenarios/FragmentOutputArrayIndexScenario.cpp
     Scenarios/BufferTextureScenario.cpp
     Scenarios/VertexAttribBindingScenario.cpp
+    Scenarios/XfbCaptureBufferReuseScenario.cpp
 )
 
 target_include_directories(MobileGLIntegrationTest PRIVATE

MobileGL/MG_IntegrationTest/Scenarios/XfbCaptureBufferReuseScenario.cpp

@@ -0,0 +1,307 @@
+// MobileGL - MobileGL/MG_IntegrationTest/Scenarios/XfbCaptureBufferReuseScenario.cpp
+// Copyright (c) 2025-2026 MobileGL-Dev
+// Licensed under the GNU Lesser General Public License v3.0:
+//   https://www.gnu.org/licenses/gpl-3.0.txt
+//   https://www.gnu.org/licenses/lgpl-3.0.txt
+// SPDX-License-Identifier: LGPL-3.0-only
+// End of Source File Header
+//
+// ONE capture buffer, SEVERAL capture spans - the shape most KHR-GL4x cases that
+// use transform feedback as a readback channel are built on. They allocate the
+// capture buffer once in a setup step and then run span after span through it,
+// so a defect that only shows from the second span onwards fails the whole case
+// while the first span (and every single-span scenario in this suite) stays
+// green. The first thing checked here is therefore not the capture itself but
+// that the bytes the capture wrote are the bytes the readback reads.
+//
+// Two ways of reusing the buffer, because they exercise different machinery:
+//
+//   * respecified between spans (glBufferData while the buffer is still bound to
+//     the transform-feedback binding point), which is what a test helper that
+//     poisons its capture buffer before every span does;
+//   * allocated ONCE with immutable storage and never touched again, which is
+//     what KHR-GL45.direct_state_access.vertex_arrays_enable_disable_attributes
+//     does - glBufferStorage(4 bytes) in its setup, then two draws.
+//
+// The negative control (a fresh buffer object per span) is a separate case
+// rather than a parameter: it is the configuration that already worked, so it
+// has to keep working for the others to mean anything.
+
+#include <cstdio>
+#include <cstring>
+#include <string>
+#include <vector>
+
+#include "../Harness/HeadlessGL.h"
+#include "../Harness/ScenarioFixture.h"
+
+#ifdef GLAPI
+#undef GLAPI
+#endif
+#define GL_GLEXT_PROTOTYPES
+#include <GL/gl.h>
+#include <GL/glcorearb.h>
+#undef GL_GLEXT_PROTOTYPES
+
+namespace MGITest {
+    namespace {
+
+        constexpr float kPoison = -1234.0f;
+        // One vec4 per point, one point per draw.
+        constexpr std::size_t kCaptureFloats = 4;
+        constexpr std::size_t kCaptureBytes = kCaptureFloats * sizeof(float);
+
+        GLuint CompileShader(GLenum type, const std::string& source, std::string* log) {
+            const GLuint shader = glCreateShader(type);
+            const char* text = source.c_str();
+            glShaderSource(shader, 1, &text, nullptr);
+            glCompileShader(shader);
+            GLint status = GL_FALSE;
+            glGetShaderiv(shader, GL_COMPILE_STATUS, &status);
+            if (status == GL_FALSE) {
+                GLint length = 0;
+                glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &length);
+                std::vector<char> buffer(static_cast<std::size_t>(length) + 1, '\0');
+                glGetShaderInfoLog(shader, length + 1, nullptr, buffer.data());
+                if (log != nullptr) *log = buffer.data();
+                glDeleteShader(shader);
+                return 0;
+            }
+            return shader;
+        }
+
+        // Vertex-only capture program: whatever the draw fetched at location 0 comes
+        // straight back out through the capture. Runs under GL_RASTERIZER_DISCARD, so
+        // there is no fragment stage.
+        GLuint BuildCaptureProgram(std::string* log) {
+            const std::string vertexSource = R"(#version 430 core
+layout(location = 0) in vec4 vs_in_value;
+out vec4 vs_out_value;
+void main() {
+  vs_out_value = vs_in_value;
+}
+)";
+            const GLuint vertexShader = CompileShader(GL_VERTEX_SHADER, vertexSource, log);
+            if (vertexShader == 0) return 0;
+            const GLuint program = glCreateProgram();
+            glAttachShader(program, vertexShader);
+            const char* varying = "vs_out_value";
+            glTransformFeedbackVaryings(program, 1, &varying, GL_INTERLEAVED_ATTRIBS);
+            glLinkProgram(program);
+            glDeleteShader(vertexShader);
+            GLint status = GL_FALSE;
+            glGetProgramiv(program, GL_LINK_STATUS, &status);
+            if (status == GL_FALSE) {
+                GLint length = 0;
+                glGetProgramiv(program, GL_INFO_LOG_LENGTH, &length);
+                std::vector<char> buffer(static_cast<std::size_t>(length) + 1, '\0');
+                glGetProgramInfoLog(program, length + 1, nullptr, buffer.data());
+                if (log != nullptr) *log = buffer.data();
+                glDeleteProgram(program);
+                return 0;
+            }
+            return program;
+        }
+
+        class XfbCaptureBufferReuseScenario : public ScenarioTest {
+        protected:
+            void SetUp() override {
+                ScenarioTest::SetUp();
+                if (!Ready()) return;
+
+                std::string log;
+                m_program = BuildCaptureProgram(&log);
+                ASSERT_NE(m_program, 0u) << "capture program failed to build: " << log;
+
+                glGenVertexArrays(1, &m_vao);
+                glBindVertexArray(m_vao);
+                glGenBuffers(1, &m_vbo);
+                glBindBuffer(GL_ARRAY_BUFFER, m_vbo);
+                glBufferData(GL_ARRAY_BUFFER, kCaptureBytes, nullptr, GL_DYNAMIC_DRAW);
+                glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, nullptr);
+                glEnableVertexAttribArray(0);
+                glBindBuffer(GL_ARRAY_BUFFER, 0);
+            }
+
+            void TearDown() override {
+                if (!Ready()) return;
+                glBindVertexArray(0);
+                if (m_vbo != 0) glDeleteBuffers(1, &m_vbo);
+                if (m_vao != 0) glDeleteVertexArrays(1, &m_vao);
+                if (m_program != 0) glDeleteProgram(m_program);
+                glUseProgram(0);
+                ScenarioTest::TearDown();
+            }
+
+            // The vertex the next span will fetch and capture.
+            void SetVertex(float value) {
+                const float data[kCaptureFloats] = {value, value + 1.0f, value + 2.0f, value + 3.0f};
+                glBindBuffer(GL_ARRAY_BUFFER, m_vbo);
+                glBufferSubData(GL_ARRAY_BUFFER, 0, kCaptureBytes, data);
+                glBindBuffer(GL_ARRAY_BUFFER, 0);
+            }
+
+            // One capture span over the buffer currently bound to capture point 0.
+            void RunSpan() {
+                glEnable(GL_RASTERIZER_DISCARD);
+                glUseProgram(m_program);
+                glBindVertexArray(m_vao);
+                glBeginTransformFeedback(GL_POINTS);
+                glDrawArrays(GL_POINTS, 0, 1);
+                glEndTransformFeedback();
+                glDisable(GL_RASTERIZER_DISCARD);
+                glUseProgram(0);
+            }
+
+            static ::testing::AssertionResult CapturedIs(const float* data, float value) {
+                for (std::size_t i = 0; i < kCaptureFloats; ++i) {
+                    const float expected = value + static_cast<float>(i);
+                    const float got = data[i];
+                    if (got - expected > 0.01f || expected - got > 0.01f) {
+                        return ::testing::AssertionFailure()
+                               << "component " << i << " is " << got << ", expected " << expected
+                               << (got == kPoison ? " (the capture never reached these bytes)" : "");
+                    }
+                }
+                return ::testing::AssertionSuccess();
+            }
+
+            GLuint m_program = 0;
+            GLuint m_vao = 0;
+            GLuint m_vbo = 0;
+        };
+
+        // The negative control: one buffer object per span. This is the configuration
+        // every multi-span scenario in this suite works around the others with, so it
+        // has to hold or nothing below is interpretable.
+        TEST_F(XfbCaptureBufferReuseScenario, EverySpanIntoABufferObjectOfItsOwn) {
+            if (!Ready()) GTEST_SKIP();
+
+            for (int span = 0; span < 3; ++span) {
+                const float value = 10.0f * static_cast<float>(span + 1);
+                const std::vector<float> poison(kCaptureFloats, kPoison);
+
+                GLuint xfbBuffer = 0;
+                glGenBuffers(1, &xfbBuffer);
+                glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, xfbBuffer);
+                glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER, kCaptureBytes, poison.data(), GL_DYNAMIC_DRAW);
+
+                SetVertex(value);
+                RunSpan();
+
+                float readback[kCaptureFloats] = {kPoison, kPoison, kPoison, kPoison};
+                glGetBufferSubData(GL_TRANSFORM_FEEDBACK_BUFFER, 0, kCaptureBytes, readback);
+                EXPECT_TRUE(CapturedIs(readback, value)) << "span " << span;
+
+                glDeleteBuffers(1, &xfbBuffer);
+            }
+            EXPECT_EQ(glGetError(), GL_NO_ERROR);
+        }
+
+        // The same three spans through ONE buffer object, respecified before each of
+        // them WHILE it is bound to capture point 0 - a helper poisoning its capture
+        // buffer, which is what makes "captured nothing" legible in the first place.
+        //
+        // A respecification is free to replace the storage underneath (that is what
+        // orphaning is), and on a buffer whose bytes the backend has already handed
+        // the frontend a pointer into, the replacement has to reach that pointer too.
+        // It did not: the capture wrote the new storage and the readback kept reading
+        // the old one, so every span after the first came back poison.
+        TEST_F(XfbCaptureBufferReuseScenario, EverySpanIntoOneRespecifiedBufferObject) {
+            if (!Ready()) GTEST_SKIP();
+
+            GLuint xfbBuffer = 0;
+            glGenBuffers(1, &xfbBuffer);
+            glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, xfbBuffer);
+
+            for (int span = 0; span < 3; ++span) {
+                const float value = 10.0f * static_cast<float>(span + 1);
+                const std::vector<float> poison(kCaptureFloats, kPoison);
+                glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER, kCaptureBytes, poison.data(), GL_DYNAMIC_DRAW);
+
+                SetVertex(value);
+                RunSpan();
+
+                float readback[kCaptureFloats] = {kPoison, kPoison, kPoison, kPoison};
+                glGetBufferSubData(GL_TRANSFORM_FEEDBACK_BUFFER, 0, kCaptureBytes, readback);
+                EXPECT_TRUE(CapturedIs(readback, value)) << "span " << span;
+            }
+
+            glDeleteBuffers(1, &xfbBuffer);
+            EXPECT_EQ(glGetError(), GL_NO_ERROR);
+        }
+
+        // A respecification that CHANGES the size, which is the case a re-pointing
+        // that only handled same-size storage would still get wrong - and, before the
+        // fix, the case that wrote the new (larger) contents through a mapping sized
+        // for the old ones.
+        TEST_F(XfbCaptureBufferReuseScenario, ARespecificationMayChangeTheCaptureBufferSize) {
+            if (!Ready()) GTEST_SKIP();
+
+            GLuint xfbBuffer = 0;
+            glGenBuffers(1, &xfbBuffer);
+            glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, xfbBuffer);
+
+            // Sized for one point, then for four, then back down to one.
+            const std::size_t pointCapacity[] = {1, 4, 1};
+            for (int span = 0; span < 3; ++span) {
+                const float value = 10.0f * static_cast<float>(span + 1);
+                const std::size_t floats = kCaptureFloats * pointCapacity[span];
+                const std::vector<float> poison(floats, kPoison);
+                glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER, static_cast<GLsizeiptr>(floats * sizeof(float)),
+                             poison.data(), GL_DYNAMIC_DRAW);
+
+                SetVertex(value);
+                RunSpan();
+
+                std::vector<float> readback(floats, kPoison);
+                glGetBufferSubData(GL_TRANSFORM_FEEDBACK_BUFFER, 0,
+                                   static_cast<GLsizeiptr>(floats * sizeof(float)), readback.data());
+                EXPECT_TRUE(CapturedIs(readback.data(), value)) << "span " << span;
+                // The bytes past the one point the draw produced must still be the
+                // poison the respecification put there, not whatever the previous
+                // (differently sized) storage held.
+                for (std::size_t i = kCaptureFloats; i < floats; ++i) {
+                    EXPECT_FLOAT_EQ(readback[i], kPoison) << "span " << span << " float " << i;
+                }
+            }
+
+            glDeleteBuffers(1, &xfbBuffer);
+            EXPECT_EQ(glGetError(), GL_NO_ERROR);
+        }
+
+        // The KHR-GL45.direct_state_access.vertex_arrays_enable_disable_attributes
+        // shape: the capture buffer gets IMMUTABLE storage once, in a setup step, and
+        // is never respecified - two spans simply run through it, each read back with
+        // glMapBuffer. Nothing here may depend on a respecification to reset the
+        // capture: glBeginTransformFeedback does that on its own.
+        TEST_F(XfbCaptureBufferReuseScenario, EverySpanIntoOneImmutableStorageBuffer) {
+            if (!Ready()) GTEST_SKIP();
+
+            GLuint xfbBuffer = 0;
+            glGenBuffers(1, &xfbBuffer);
+            glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, xfbBuffer);
+            glBufferStorage(GL_TRANSFORM_FEEDBACK_BUFFER, kCaptureBytes, nullptr, GL_MAP_READ_BIT);
+            ASSERT_EQ(glGetError(), GL_NO_ERROR) << "glBufferStorage on the capture buffer";
+            glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, xfbBuffer);
+
+            for (int span = 0; span < 3; ++span) {
+                const float value = 10.0f * static_cast<float>(span + 1);
+                SetVertex(value);
+                RunSpan();
+
+                const void* mapped = glMapBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, GL_READ_ONLY);
+                ASSERT_NE(mapped, nullptr) << "span " << span << ": glMapBuffer returned null";
+                float readback[kCaptureFloats] = {kPoison, kPoison, kPoison, kPoison};
+                std::memcpy(readback, mapped, kCaptureBytes);
+                glUnmapBuffer(GL_TRANSFORM_FEEDBACK_BUFFER);
+                EXPECT_TRUE(CapturedIs(readback, value)) << "span " << span;
+            }
+
+            glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER, 0);
+            glDeleteBuffers(1, &xfbBuffer);
+            EXPECT_EQ(glGetError(), GL_NO_ERROR);
+        }
+
+    } // namespace
+} // namespace MGITest

MobileGL/MG_State/GLState/BufferState/BufferObject.cpp

@@ -75,10 +75,39 @@ namespace MobileGL::MG_State::GLState {
         NotifySubData(offset, size);
     }
 
-    void BufferObject::Respecify(SizeT size, const void* data) {
+    // A (re)definition of the store is about to write `size` bytes through Bytes().
-        ReleaseMemory();
+    // Sizing the shadow is all that takes for a shadow-backed buffer, but a buffer
+    // whose bytes were adopted into backend GPU memory needs the adoption renewed
+    // first: the mapping it holds describes exactly the OLD store. Writing the new
+    // contents through it runs past its end the moment the store grows, and a backend
+    // that replaces the storage for the new store (which is what an orphaning
+    // respecification asks for) would leave that mapping - and therefore every later
+    // read of this buffer - addressing storage nothing writes to any more.
+    //
+    // Renewing rather than simply dropping is what keeps the common case free: a
+    // redefinition at the same size gets the same mapping back without any storage
+    // being created, which is also what makes the backend's respecify a no-op (the
+    // new bytes are already in the storage it would otherwise upload to).
+    void BufferObject::RedefineStorage(SizeT size) {
+        const Bool wasGpuResident = m_resource.IsGpuResident();
+        if (wasGpuResident) {
+            // A capture or a shader write may still be running against the very bytes
+            // that are about to be overwritten.
+            SyncGpuWrites();
+            m_resource.ReleasePersistentMap();
+        }
         m_size = size;
         m_resource.ResizeShadow(size);
+        if (wasGpuResident) {
+            // Declining is allowed (a zero-sized store, a backend without the op): the
+            // buffer simply goes back to the CPU-shadow model it had before adoption.
+            EnsureGpuResidentStorage();
+        }
+    }
+
+    void BufferObject::Respecify(SizeT size, const void* data) {
+        ReleaseMemory();
+        RedefineStorage(size);
         if (data && size > 0) {
             Memcpy(m_resource.Bytes(), data, size);
         }
@@ -96,8 +125,7 @@ namespace MobileGL::MG_State::GLState {
 
     void BufferObject::AllocateImmutableStorage(SizeT size, const void* data, GLbitfield storageFlags) {
         ReleaseMemory();
-        m_size = size;
+        RedefineStorage(size);
-        m_resource.ResizeShadow(size);
         if (data) {
             Memcpy(m_resource.Bytes(), data, size);
         } else if (size > 0) {

MobileGL/MG_State/GLState/BufferState/BufferObject.h

@@ -205,6 +205,9 @@ namespace MobileGL {
             void SetBackendResource(SharedPtr<BackendBufferResource> resource);
 
         private:
+            // Sizes the store for a (re)definition, renewing an adopted GPU-resident
+            // mapping across it. See the definition for why the renewal is not optional.
+            void RedefineStorage(SizeT size);
             void NotifyRespecify();
             void NotifySubData(SizeT offset, SizeT size);
             void NotifyFlushMappedRange(Range1D range, Flags<BufferMappingAccessBit> appAccess);

MobileGL/MG_State/GLState/BufferState/PipeResource.h

@@ -70,6 +70,15 @@ namespace MobileGL::MG_State::GLState {
             m_shadow->shrink_to_fit();
         }
 
+        // Give the adoption back: the bytes resolve against the shadow again (which
+        // the caller must (re)size, it was released on adoption). Used when the store
+        // itself is redefined - the mapping describes exactly the store that is going
+        // away, so it may neither be written through nor kept. It is NOT a general
+        // "unmap": a persistent map the application holds outlives every unmap by
+        // definition, and the calls that could redefine such a buffer's store are
+        // errors the frontend refuses before reaching here.
+        void ReleasePersistentMap() { m_gpuMapped = nullptr; }
+
         // Backend GPU resource, owned here in both modes.
         const SharedPtr<BackendBufferResource>& Backend() const { return m_backend; }
         void SetBackend(SharedPtr<BackendBufferResource> backend) { m_backend = std::move(backend); }