plotter/tests/test_gl_context.cpp

// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: 2026 Martino Ferrari

#include "harness.hpp"
#include "egl_headless.hpp"

#include <EGL/egl.h>  // eglGetProcAddress: the context-aware entry point resolver

#include <plot/gl/caps.hpp>
#include <plot/gl/handle.hpp>
#include <plot/gl/loader.hpp>
#include <plot/render/embedded_mono_font.hpp>
#include <plot/render/primitives.hpp>
#include <plot/render/program.hpp>
#include <plot/render/shaders.hpp>
#include <plot/render/text_atlas.hpp>
#include <plot/render/vertex_array.hpp>

#include <cstddef>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <dlfcn.h>
#include <utility>
#include <vector>

#include <fcntl.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <unistd.h>

namespace {

/// `eglGetProcAddress` is the context-aware resolver of the EGL context: in a
/// surfaceless context there is no GLX/WGL dispatch to fall back on, so this is
/// the path that must work.
plot::gl::ProcLoader egl_loader() noexcept {
  return reinterpret_cast<plot::gl::ProcLoader>(eglGetProcAddress);
}

constexpr const char* kVertexSrc = R"(#version 330 core
layout(location = 0) in vec2 a_pos;
uniform vec2 u_offset;
void main() { gl_Position = vec4(a_pos + u_offset, 0.0, 1.0); }
)";

constexpr const char* kFragmentSrc = R"(#version 330 core
uniform float u_alpha;
out vec4 o_color;
void main() { o_color = vec4(1.0, 0.0, 0.0, u_alpha); }
)";

/// Deliberately unparseable GLSL, to exercise the failure path of
/// `Program::from_source`.
constexpr const char* kBrokenFragmentSrc = R"(#version 330 core
out vec4 o_color;
void main() { o_color = this is not glsl; }
)";

/// Valid GLSL on both sides but an interface that cannot link: the fragment
/// reads a `vec3` where the vertex writes a `vec2`, so the link has to fail and
/// carry the driver's log through `Status::message`.
constexpr const char* kMismatchedVertexSrc = R"(#version 330 core
layout(location = 0) in vec2 a_pos;
out vec2 v_value;
void main() { v_value = a_pos; gl_Position = vec4(a_pos, 0.0, 1.0); }
)";

constexpr const char* kMismatchedFragmentSrc = R"(#version 330 core
in vec3 v_value;
out vec4 o_color;
void main() { o_color = vec4(v_value, 1.0); }
)";

/// Holds one GL entry point at a stub for as long as it lives, the render-smoke
/// stubs' save/restore pattern made generic over the slot's function-pointer
/// type so this file needs one class rather than one per entry point. The
/// pointer is restored by the destructor, which also runs while a failed
/// `PLOT_CHECK` unwinds, so a stub can never leak into the next case.
template <class Fn>
class EntryGuard {
 public:
  EntryGuard(Fn& slot, Fn value) noexcept : slot_(slot), saved_(slot) { slot_ = value; }
  ~EntryGuard() { slot_ = saved_; }
  EntryGuard(const EntryGuard&) = delete;
  EntryGuard& operator=(const EntryGuard&) = delete;

 private:
  Fn& slot_;
  Fn saved_;
};

// --- the caps probe, driven by fake strings ----------------------------------

/// Reported by the stubbed `glGetString`; each case sets them before probing.
const char* g_fake_version = "2.1";
const char* g_fake_extensions = "";
bool g_fake_version_missing = false;

const plot::gl::GLubyte* fake_get_string(plot::gl::GLenum name) noexcept {
  if (name == plot::gl::GL_VERSION) {
    return g_fake_version_missing ? nullptr
                                  : reinterpret_cast<const plot::gl::GLubyte*>(g_fake_version);
  }
  if (name == plot::gl::GL_EXTENSIONS) {
    return g_fake_extensions == nullptr
               ? nullptr
               : reinterpret_cast<const plot::gl::GLubyte*>(g_fake_extensions);
  }
  if (name == plot::gl::GL_RENDERER) {
    return reinterpret_cast<const plot::gl::GLubyte*>("fake renderer");
  }
  if (name == plot::gl::GL_VENDOR) {
    return reinterpret_cast<const plot::gl::GLubyte*>("fake vendor");
  }
  return nullptr;
}

/// The indexed list's length and the profile mask the fake `glGetIntegerv`
/// reports; only the 3.x indexed path reads them.
void fake_get_integer_v(plot::gl::GLenum name, plot::gl::GLint* out) noexcept {
  *out = name == plot::gl::GL_NUM_EXTENSIONS ? 2 : 0;  // 0 = no core-profile bit
}

/// What the stubbed `glGetStringi` reports: 0 = a null pointer, 1 = the
/// extension the probe looks for, 2 = some other one.
int g_fake_stringi_mode = 0;

const plot::gl::GLubyte* fake_get_string_i(plot::gl::GLenum, plot::gl::GLuint) noexcept {
  if (g_fake_stringi_mode == 0) {
    return nullptr;
  }
  return reinterpret_cast<const plot::gl::GLubyte*>(
      g_fake_stringi_mode == 1 ? "GL_ARB_buffer_storage" : "GL_ARB_something_else");
}

// --- entry points that hand out no name, for the constructors' failure paths --

void zero_gen_buffers(plot::gl::GLsizei, plot::gl::GLuint* out) noexcept { *out = 0; }
void zero_gen_vertex_arrays(plot::gl::GLsizei, plot::gl::GLuint* out) noexcept { *out = 0; }
void zero_gen_textures(plot::gl::GLsizei, plot::gl::GLuint* out) noexcept { *out = 0; }
void zero_gen_framebuffers(plot::gl::GLsizei, plot::gl::GLuint* out) noexcept { *out = 0; }
plot::gl::GLuint zero_create_shader(plot::gl::GLenum) noexcept { return 0; }
plot::gl::GLuint zero_create_program() noexcept { return 0; }

/// Probes caps once with the fake string table installed and `GL_EXTENSIONS`
/// reporting `extensions` (null meaning the query returns no string). The
/// `load()` first clears the cached probe *and* restores the real entry point
/// -- the previous probe's guard has already run -- so each call reads the stub.
/// Returning a copy keeps the probe valid after the guard restores the table.
plot::gl::Caps probe_legacy_caps(const char* extensions) noexcept {
  plot::gl::load(nullptr);
  EntryGuard<decltype(plot::gl::glGetString)> guard(plot::gl::glGetString, &fake_get_string);
  g_fake_version = "2.1";
  g_fake_version_missing = false;
  g_fake_extensions = extensions;
  return plot::gl::caps();
}

/// Probes caps once on a 3.2+ context, where the indexed list supplies the
/// names: `GL_NUM_EXTENSIONS` and the profile mask come from the fake
/// `glGetIntegerv`, each name from the fake `glGetStringi` (`stringi_mode`:
/// 0 = null pointer, 1 = the extension sought, 2 = another).
plot::gl::Caps probe_indexed_caps(int stringi_mode) noexcept {
  plot::gl::load(nullptr);
  EntryGuard<decltype(plot::gl::glGetString)> string_guard(plot::gl::glGetString,
                                                           &fake_get_string);
  EntryGuard<decltype(plot::gl::glGetIntegerv)> int_guard(plot::gl::glGetIntegerv,
                                                          &fake_get_integer_v);
  EntryGuard<decltype(plot::gl::glGetStringi)> stringi_guard(plot::gl::glGetStringi,
                                                             &fake_get_string_i);
  g_fake_version = "3.3";
  g_fake_version_missing = false;
  g_fake_stringi_mode = stringi_mode;
  return plot::gl::caps();
}

}  // namespace

// Must stay the first GL case in this translation unit: `caps()` caches after
// the first successful query, so "before any load()" is observable only once.
PLOT_TEST(caps_before_load_is_zeroed) {
  const plot::gl::Caps& caps = plot::gl::caps();
  PLOT_CHECK(caps.major == 0 && caps.minor == 0);
  PLOT_CHECK(!caps.compute && !caps.buffer_storage && !caps.persistent_map);
  PLOT_CHECK(!caps.core_profile);
  PLOT_CHECK(caps.renderer != nullptr && caps.renderer[0] == '\0');
  PLOT_CHECK(caps.vendor != nullptr && caps.vendor[0] == '\0');
}

// The `kNoLoader` family and the `load()` failure branch need `available()` to
// be false, and `load()` is one-way and process-wide: once this suite has
// resolved the table -- the very next case does -- a failure cannot be produced
// in-process. So the whole case runs in a forked child, where poisoning the
// table poisons only its own copy. On a host that ships libGL.so.1 the only way
// left to make a required entry point unresolvable is to block `dlopen` by
// exhausting the file descriptors, which is what the child does first.
//
// It must run before `headless_context_provides_gl33`: after that case the
// process has opened libGL.so.1, and the excluded descriptors would no longer
// stop `dlopen` from handing the loader the table it wants.
PLOT_TEST(a_gl_failure_without_a_loader_is_safe_in_a_forked_child) {
  std::fflush(nullptr);  // so the child's exit() cannot re-flush the parent's stdout
  const pid_t child = ::fork();
  PLOT_CHECK(child >= 0);
  if (child == 0) {
    int rc = 0;
    try {
      struct rlimit original {};
      PLOT_CHECK(::getrlimit(RLIMIT_NOFILE, &original) == 0);
      struct rlimit lowered = original;
      lowered.rlim_cur = original.rlim_max < 64 ? original.rlim_max : 64;
      PLOT_CHECK(::setrlimit(RLIMIT_NOFILE, &lowered) == 0);
      std::vector<int> held;
      int fd = -1;
      while ((fd = ::open("/dev/null", O_RDONLY)) >= 0) {
        held.push_back(fd);
      }
      // Every descriptor is spent, so dlopen("libGL.so.1") fails and no required
      // entry point can be resolved: load() reports the first one it could not.
      PLOT_CHECK(!plot::gl::load(nullptr));
      PLOT_CHECK(!plot::gl::available());

      plot::Status st;
      plot::gl::Buffer buffer;
      PLOT_CHECK(!buffer.create(&st));
      PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
      PLOT_CHECK(st.message != nullptr);
      PLOT_CHECK(buffer.id() == 0);
      plot::gl::VertexArray vao;
      PLOT_CHECK(!vao.create(&st));
      PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
      PLOT_CHECK(vao.id() == 0);
      plot::gl::Program program = plot::gl::Program::from_source(kVertexSrc, kFragmentSrc, &st);
      PLOT_CHECK(!st);
      PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
      PLOT_CHECK(program.id() == 0);
      plot::gl::Texture rgba;
      PLOT_CHECK(!rgba.create_rgba(4, 4, &st));
      PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
      PLOT_CHECK(rgba.id() == 0);
      plot::gl::Texture r8;
      PLOT_CHECK(!r8.create_r8(4, 4, &st));
      PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
      PLOT_CHECK(r8.id() == 0);
      plot::gl::Framebuffer fbo;
      PLOT_CHECK(!fbo.create(&st));
      PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
      PLOT_CHECK(fbo.id() == 0);
      plot::gl::Fence fence;
      PLOT_CHECK(!fence.create(&st));
      PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
      PLOT_CHECK(fence.id() == nullptr);

      // An empty object's guarded operations must not reach the null table.
      plot::gl::Buffer empty_buffer;
      empty_buffer.storage(16, plot::gl::GL_MAP_WRITE_BIT);  // entry point null: no-op
      plot::gl::Program empty_program;
      empty_program.set_uniform_int("u_value", 1);  // id 0: no-op
      PLOT_CHECK(fence.wait(0));                    // nothing in flight

      // Hand the descriptors back so the coverage dump at exit can open its file.
      for (const int held_fd : held) {
        ::close(held_fd);
      }
      PLOT_CHECK(::setrlimit(RLIMIT_NOFILE, &original) == 0);
    } catch (...) {
      rc = 1;
    }
    std::exit(rc);
  }
  int status = 0;
  PLOT_CHECK(::waitpid(child, &status, 0) == child);
  PLOT_CHECK(WIFEXITED(status));
  PLOT_CHECK(WEXITSTATUS(status) == 0);
}

// `resolve()`'s second source, `dlsym(RTLD_DEFAULT, name)`, only answers when a
// GL library sits in the global symbol scope -- the usual arrangement on a host
// without glvnd's local dispatch. This host is the other kind, so the case
// promotes libGL into that scope itself. The promotion is one-way and
// process-wide, which is exactly why it happens in a forked child: the parent's
// namespace is untouched, and the child's exit() flushes the counters for the
// branch it walked.
PLOT_TEST(a_globally_visible_gl_library_satisfies_the_fallback_resolver) {
  std::fflush(nullptr);
  const pid_t child = ::fork();
  PLOT_CHECK(child >= 0);
  if (child == 0) {
    int rc = 0;
    try {
      void* handle = ::dlopen("libGL.so.1", RTLD_NOW | RTLD_GLOBAL);
      // Not every host ships a GL library to promote; where there is none the
      // case simply cannot reach the branch rather than pretending it did.
      if (handle != nullptr && ::dlsym(RTLD_DEFAULT, "glGetString") != nullptr) {
        PLOT_CHECK(plot::gl::load(nullptr));
        PLOT_CHECK(plot::gl::available());
      }
    } catch (...) {
      rc = 1;
    }
    std::exit(rc);
  }
  int status = 0;
  PLOT_CHECK(::waitpid(child, &status, 0) == child);
  PLOT_CHECK(WIFEXITED(status));
  PLOT_CHECK(WEXITSTATUS(status) == 0);
}

PLOT_TEST(headless_context_provides_gl33) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));  // fails loudly if surfaceless EGL is unavailable

  // Primary path: resolve through the current context.
  PLOT_CHECK(plot::gl::load(egl_loader()));

  const plot::gl::Caps& caps = plot::gl::caps();
  PLOT_CHECK(caps.major > 3 || (caps.major == 3 && caps.minor >= 3));
  PLOT_CHECK(caps.renderer != nullptr && caps.renderer[0] != '\0');
  PLOT_CHECK(caps.vendor != nullptr && caps.vendor[0] != '\0');
  // EGL_CONTEXT_OPENGL_CORE_PROFILE_BIT_KHR is requested, so this checks our profile-mask parsing.
  PLOT_CHECK(caps.core_profile);
  const char* version = reinterpret_cast<const char*>(plot::gl::glGetString(plot::gl::GL_VERSION));
  PLOT_CHECK(version != nullptr);
  const int major = caps.major;
  const int minor = caps.minor;
  std::printf("[info] GL %d.%d, renderer \"%s\", vendor \"%s\", %s profile, EGL path \"%s\"\n",
              major, minor, caps.renderer, caps.vendor, caps.core_profile ? "core" : "compat",
              ctx.egl_path);
  std::printf("[info] caps: compute=%d buffer_storage=%d persistent_map=%d core_profile=%d\n",
              caps.compute ? 1 : 0, caps.buffer_storage ? 1 : 0, caps.persistent_map ? 1 : 0,
              caps.core_profile ? 1 : 0);

  // The dlsym(RTLD_DEFAULT) / libGL.so.1 fallback must resolve the whole table
  // too, and its pointers must still dispatch to the current EGL context. Only
  // a host that ships desktop libGL can exercise it; on a GLES-only host there
  // is nothing to assert.
  if (dlopen("libGL.so.1", RTLD_LAZY) != nullptr) {
    PLOT_CHECK(plot::gl::load(nullptr));
    PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
    const char* fallback_version =
        reinterpret_cast<const char*>(plot::gl::glGetString(plot::gl::GL_VERSION));
    PLOT_CHECK(fallback_version != nullptr);
    PLOT_CHECK(std::strcmp(fallback_version, version) == 0);  // same context, same driver
    const plot::gl::Caps& reprobed = plot::gl::caps();  // load() invalidated the cache
    PLOT_CHECK(reprobed.major == major && reprobed.minor == minor);
    PLOT_CHECK(reprobed.renderer != nullptr && reprobed.renderer[0] != '\0');
  }

  // Hand the table back to the context-aware resolver so teardown (deleting the
  // framebuffer) runs through the entry points of the context it belongs to.
  PLOT_CHECK(plot::gl::load(egl_loader()));
  ctx.destroy();
}

PLOT_TEST(headless_framebuffer_reads_back_clear_color) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(egl_loader()));

  plot::gl::glClearColor(0.25f, 0.5f, 0.75f, 1.0f);
  plot::gl::glClear(plot::gl::GL_COLOR_BUFFER_BIT);

  std::uint8_t px[4] = {0, 0, 0, 0};
  PLOT_CHECK(ctx.read_pixel(0, 0, px));
  PLOT_CHECK_NEAR(px[0], 64, 2);  // round(0.25 * 255)
  PLOT_CHECK_NEAR(px[1], 128, 2);
  PLOT_CHECK_NEAR(px[2], 191, 2);
  PLOT_CHECK_NEAR(px[3], 255, 2);

  // A second corner proves the whole attachment is readable.
  PLOT_CHECK(ctx.read_pixel(ctx.width - 1, ctx.height - 1, px));
  PLOT_CHECK_NEAR(px[2], 191, 2);
  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
  ctx.destroy();
}

PLOT_TEST(raii_handles_round_trip) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(egl_loader()));

  const float verts[6] = {0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f};
  plot::gl::Buffer vbo;
  PLOT_CHECK(vbo.create(&st));
  PLOT_CHECK(vbo.id() != 0);
  vbo.upload(verts, sizeof verts, plot::gl::GL_STATIC_DRAW);
  vbo.upload_sub(verts, sizeof verts, 0);

  plot::gl::VertexArray vao;
  PLOT_CHECK(vao.create(&st));
  vao.bind();
  vbo.bind();
  vao.attrib(0, 2, 2 * sizeof(float), 0);

  const std::uint8_t texels[8 * 8 * 4] = {0};
  plot::gl::Texture color;
  PLOT_CHECK(color.create_rgba(8, 8, &st));
  color.upload_rgba(8, 8, texels);

  const std::uint8_t coverage[8 * 8] = {0};
  plot::gl::Texture mask;
  PLOT_CHECK(mask.create_r8(8, 8, &st));
  mask.upload_r8(8, 8, coverage);
  mask.set_filter(plot::gl::GL_NEAREST, plot::gl::GL_NEAREST);
  mask.bind();

  plot::gl::Framebuffer fbo;
  PLOT_CHECK(fbo.create(&st));
  fbo.attach_color(color);
  PLOT_CHECK(fbo.complete());

  plot::gl::Fence fence;
  PLOT_CHECK(fence.create(&st));
  plot::gl::glFinish();
  PLOT_CHECK(fence.wait(0));

  // Move-only: the source is emptied and the GL name survives.
  plot::gl::Buffer moved = std::move(vbo);
  PLOT_CHECK(moved.id() != 0);
  PLOT_CHECK(vbo.id() == 0);
  moved.reset();
  PLOT_CHECK(moved.id() == 0);

  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
  ctx.destroy();
}

PLOT_TEST(program_compiles_trivial_shaders) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(egl_loader()));

  plot::Status ok;
  plot::gl::Program program = plot::gl::Program::from_source(kVertexSrc, kFragmentSrc, &ok);
  PLOT_CHECK(bool(ok));
  PLOT_CHECK(program.id() != 0);

  program.use();
  program.set_uniform("u_offset", plot::Vec2{1.0f, 2.0f});
  program.set_uniform("u_alpha", 0.5f);
  program.set_uniform("u_missing", 0.25f);  // unknown name: silent no-op
  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);

  plot::Status bad;
  plot::gl::Program broken = plot::gl::Program::from_source(kVertexSrc, kBrokenFragmentSrc, &bad);
  PLOT_CHECK(!bad);
  PLOT_CHECK(bad.code == plot::ErrorCode::GLFailure);
  PLOT_CHECK(bad.message != nullptr && bad.message[0] != '\0');
  PLOT_CHECK(broken.id() == 0);
  ctx.destroy();
}

// The GLSL of both pipelines has to be accepted by the real driver: a typo that
// no CPU test can catch (a missing varying, a reserved word) fails here.
PLOT_TEST(shaders_compile_and_link) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(nullptr));
  plot::Status s0;
  plot::Status s1;
  plot::gl::Program line =
      plot::gl::Program::from_source(plot::render::kLineVS, plot::render::kLineFS, &s0);
  plot::gl::Program text =
      plot::gl::Program::from_source(plot::render::kTextVS, plot::render::kTextFS, &s1);
  PLOT_CHECK(bool(s0));
  PLOT_CHECK(bool(s1));
  PLOT_CHECK(line.id() != 0);
  PLOT_CHECK(text.id() != 0);
  ctx.destroy();
}

// A renderer wrapper that was never successfully created must be a harmless
// no-op: Task 7 checks `create()`'s bool and then keeps drawing with whatever
// else it has, so the setters must not reach an empty program.
PLOT_TEST(render_programs_are_inert_until_created) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(nullptr));

  plot::render::LineProgram line;
  plot::render::TextProgram text;
  PLOT_CHECK(!line.valid());
  PLOT_CHECK(!text.valid());
  line.bind();
  line.set_scale({1.0f, 1.0f});
  line.set_offset({0.0f, 0.0f});
  line.set_viewport({256.0f, 256.0f});
  line.set_width(2.0f);
  line.set_color(plot::Color::hex(0xFFFFFF));
  text.bind();
  text.set_viewport({256.0f, 256.0f});
  text.set_px_range({8.0f, 8.0f});
  text.set_glyph_uv({1.0f / 16.0f, 1.0f / 7.0f});
  text.set_texture_size({128.0f, 56.0f});
  text.set_color(plot::Color::hex(0xFFFFFF));
  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
  ctx.destroy();
}

// End to end on the real driver: the instanced attribute layout, the
// triangle-strip expansion, the single y flip between Rect pixels and GL, and
// the AA ramp that makes a thin trace legible.
PLOT_TEST(line_pipeline_draws_an_aa_strip) {
  plottest::HeadlessContext ctx(256, 256);
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(nullptr));

  plot::render::LineProgram program;
  PLOT_CHECK(program.create(&st));
  plot::render::LineVertexArray lines;
  PLOT_CHECK(lines.create(&st));

  // Three instances: two segments at different y, so a wrong attribute divisor
  // cannot pass (with a divisor of zero the second quad would be assembled from
  // the first instance's leftover attribute values), plus a degenerate one at
  // Rect (64, 140) — the shape a dense trace's min == max bucket stages, which
  // must still leave a mark instead of collapsing to zero area.
  const f32 segments[12] = {0.0f, 0.0f,  40.0f, 0.0f,   0.0f, 20.0f,
                            40.0f, 20.0f, 0.0f,  40.0f,  0.0f, 40.0f};
  lines.upload(segments, 3);
  PLOT_CHECK(lines.instance_count() == 3);

  plot::gl::glViewport(0, 0, static_cast<plot::gl::GLsizei>(ctx.width),
                       static_cast<plot::gl::GLsizei>(ctx.height));
  plot::gl::glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
  plot::gl::glClear(plot::gl::GL_COLOR_BUFFER_BIT);
  plot::gl::glEnable(plot::gl::GL_BLEND);
  plot::gl::glBlendFunc(plot::gl::GL_SRC_ALPHA, plot::gl::GL_ONE_MINUS_SRC_ALPHA);

  program.bind();
  program.set_scale({1.0f, 1.0f});
  program.set_offset({64.0f, 100.0f});
  program.set_viewport({static_cast<f32>(ctx.width), static_cast<f32>(ctx.height)});
  program.set_width(3.0f);
  program.set_color(plot::Color::hex(0xFF00FF));
  lines.bind();
  plot::gl::glDrawArraysInstanced(plot::gl::GL_TRIANGLE_STRIP, 0, 4, 3);

  plot::u8 px[4] = {};
  // Rect y = 100 is GL row 155.5, so rows 155 and 156 straddle the axis
  // (d = 0.5), rows 154 and 157 are the half-covered AA fringe (d = 1.5), and
  // row 152 is outside the quad. Getting the flip wrong, or applying it twice,
  // moves the whole strip to the other half of the target.
  PLOT_CHECK(ctx.read_pixel(84, 155, px));
  PLOT_CHECK(px[0] > 200 && px[1] < 60 && px[2] > 200);
  PLOT_CHECK(ctx.read_pixel(84, 156, px));
  PLOT_CHECK(px[0] > 200 && px[2] > 200);
  PLOT_CHECK(ctx.read_pixel(84, 157, px));
  PLOT_CHECK(px[0] > 60 && px[0] < 200);
  PLOT_CHECK(ctx.read_pixel(84, 152, px));
  PLOT_CHECK(px[0] + px[2] < 40);

  // The second instance is where its own segment says: Rect y = 120 is rows
  // 135/136, and the gap between the two strips stays background.
  PLOT_CHECK(ctx.read_pixel(84, 135, px));
  PLOT_CHECK(px[0] > 200 && px[2] > 200);
  PLOT_CHECK(ctx.read_pixel(84, 136, px));
  PLOT_CHECK(px[0] > 200 && px[2] > 200);
  PLOT_CHECK(ctx.read_pixel(84, 145, px));
  PLOT_CHECK(px[0] + px[2] < 40);
  // Past the end of the segment (flat cap) and far from both strips.
  PLOT_CHECK(ctx.read_pixel(110, 155, px));
  PLOT_CHECK(px[0] + px[2] < 40);
  // The degenerate instance: Rect y = 140 is rows 115/116 at x = 64, and the
  // mark must be visible rather than a zero-area quad, while staying inside its
  // own stroke width + fringe (columns 62..66).
  PLOT_CHECK(ctx.read_pixel(64, 115, px));
  PLOT_CHECK(px[0] > 200 && px[2] > 200);
  PLOT_CHECK(ctx.read_pixel(64, 116, px));
  PLOT_CHECK(px[0] > 200 && px[2] > 200);
  PLOT_CHECK(ctx.read_pixel(64, 121, px));
  PLOT_CHECK(px[0] + px[2] < 40);
  PLOT_CHECK(ctx.read_pixel(70, 115, px));
  PLOT_CHECK(px[0] + px[2] < 40);
  PLOT_CHECK(ctx.read_pixel(128, 0, px));
  PLOT_CHECK(px[0] + px[2] < 40);
  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
  ctx.destroy();
}

// The text pipeline end to end: the R8 atlas upload, the per-instance glyph UV
// rect, the NEAREST texel-centre sampling, the tint and the same single y flip.
// Every texel of the cell is checked against the backend's own coverage byte, so
// a wrong atlas cell, a wrong UV or a transposed axis cannot pass.
PLOT_TEST(text_pipeline_draws_a_glyph) {
  using plot::render::EmbeddedMonoFont;
  plottest::HeadlessContext ctx(256, 256);
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(nullptr));

  // The backend's atlas, through the owner the renderer uses: single channel,
  // GL_NEAREST, straight alpha, uploaded from the backend's own bytes.
  EmbeddedMonoFont font;
  plot::render::TextAtlas atlas;
  PLOT_CHECK(atlas.sync(font, &st));
  PLOT_CHECK(!atlas.empty());
  PLOT_CHECK_NEAR(atlas.texel_size().x, static_cast<plot::f32>(font.atlas_width()), 1e-6);
  PLOT_CHECK_NEAR(atlas.texel_size().y, static_cast<plot::f32>(font.atlas_height()), 1e-6);

  plot::render::TextProgram program;
  PLOT_CHECK(program.create(&st));
  plot::render::TextVertexArray glyphs;
  PLOT_CHECK(glyphs.create(&st));

  plot::render::TextBatch batch;
  batch.begin();
  // One cell at Rect (64, 100), at the backend's own size: `px` of 1 means one
  // font pixel per pixel, which is what makes the texel-to-pixel mapping below
  // an identity.
  batch.add_text("A", {64.0f, 100.0f}, 1.0f, font);
  PLOT_CHECK(batch.glyph_count() == 1);
  PLOT_CHECK(batch.data().size() == 6);
  glyphs.upload(batch.data().data(), batch.glyph_count());

  plot::gl::glViewport(0, 0, static_cast<plot::gl::GLsizei>(ctx.width),
                       static_cast<plot::gl::GLsizei>(ctx.height));
  plot::gl::glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
  plot::gl::glClear(plot::gl::GL_COLOR_BUFFER_BIT);
  plot::gl::glEnable(plot::gl::GL_BLEND);
  plot::gl::glBlendFunc(plot::gl::GL_SRC_ALPHA, plot::gl::GL_ONE_MINUS_SRC_ALPHA);

  program.bind();
  program.set_viewport({static_cast<f32>(ctx.width), static_cast<f32>(ctx.height)});
  // The backend's own cell, on screen at 1:1: the draw's one rect size.
  const plot::f32 cell_w = static_cast<plot::f32>(EmbeddedMonoFont::cell_width());
  const plot::f32 cell_h = static_cast<plot::f32>(EmbeddedMonoFont::cell_height());
  program.set_px_range({cell_w, cell_h});
  program.set_glyph_uv(font.cell_uv());
  program.set_texture_size(atlas.texel_size());
  // White, so the blended pixel value *is* the coverage byte: the shader emits
  // `vec4(rgb, a * coverage)`, and a white tint over the black clear lands at
  // `1.0 * coverage` in every channel.
  program.set_color(plot::Color::hex(0xFFFFFF));
  atlas.bind();
  glyphs.bind();
  plot::gl::glDrawArraysInstanced(plot::gl::GL_TRIANGLE_STRIP, 0, 4, 1);

  // The cell is cell_w x cell_h texels and u_px_range is the same, so texel
  // (gx, gy) lands on exactly one target pixel: Rect x = 64 + gx, and Rect y =
  // 100 + gy is GL row 155 - gy. A transposed axis, a wrong cell or a wrong UV
  // all move ink away from where the coverage byte says it is.
  const plot::u32 slot = EmbeddedMonoFont::glyph_index(static_cast<plot::u32>('A'));
  const plot::Vec2 origin = EmbeddedMonoFont::atlas_origin(slot);
  const plot::u8* atlas_bytes = font.atlas_pixels();
  plot::u32 antialiased = 0;
  plot::u8 px[4] = {};
  for (plot::u32 gy = 0; gy < EmbeddedMonoFont::cell_height(); ++gy) {
    for (plot::u32 gx = 0; gx < EmbeddedMonoFont::cell_width(); ++gx) {
      const plot::u32 ax = static_cast<plot::u32>(origin.x) + gx;
      const plot::u32 ay = static_cast<plot::u32>(origin.y) + gy;
      const plot::u8 want = atlas_bytes[static_cast<std::size_t>(ay) * font.atlas_width() + ax];
      PLOT_CHECK(ctx.read_pixel(64u + gx, 155u - gy, px));
      PLOT_CHECK_NEAR(px[0], static_cast<plot::f32>(want), 2.0);
      PLOT_CHECK_NEAR(px[1], static_cast<plot::f32>(want), 2.0);
      PLOT_CHECK_NEAR(px[2], static_cast<plot::f32>(want), 2.0);
      if (want > 0u && want < 255u) ++antialiased;
    }
  }
  // 'A' has grey edge texels and they reached the framebuffer: a 1-bit atlas
  // could only ever have produced 0 or 255 here.
  PLOT_CHECK(antialiased > 0u);
  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
  ctx.destroy();
}

// The loader's legacy scanner: a 2.1 version string routes `caps()` down the
// pre-3.0 path, the only one that reads `GL_EXTENSIONS` as one space-separated
// string, so these probes go through `has_token` exactly as a real 2.1 driver
// would. The near-misses are the assertions that matter -- a `strstr`-only
// scanner would set the cap for both of them.
PLOT_TEST(legacy_extension_string_is_scanned_a_whole_token_at_a_time) {
  PLOT_CHECK(probe_legacy_caps("GL_ARB_other GL_ARB_buffer_storage GL_ARB_more").buffer_storage);
  // At the very start and the very end: the boundaries the scanner exists for.
  PLOT_CHECK(probe_legacy_caps("GL_ARB_buffer_storage GL_ARB_more").buffer_storage);
  PLOT_CHECK(probe_legacy_caps("GL_ARB_more GL_ARB_buffer_storage").buffer_storage);
  // Prefix-only near-misses must not count.
  PLOT_CHECK(!probe_legacy_caps("GL_ARB_buffer_storageX").buffer_storage);
  PLOT_CHECK(!probe_legacy_caps("XGL_ARB_buffer_storage").buffer_storage);
  // No string at all: the null guard.
  PLOT_CHECK(!probe_legacy_caps(nullptr).buffer_storage);

  const plot::gl::Caps caps = probe_legacy_caps("GL_ARB_buffer_storage");
  PLOT_CHECK(caps.major == 2 && caps.minor == 1);  // the stub really was read
  PLOT_CHECK(caps.buffer_storage && caps.persistent_map);
  PLOT_CHECK(std::strcmp(caps.renderer, "fake renderer") == 0);
}

// The 3.0+ indexed path: `GL_NUM_EXTENSIONS` and the profile mask come from
// `glGetIntegerv`, each name from `glGetStringi`. A null `glGetStringi` means
// the list is unavailable, and the probe must then refuse -- the legacy string
// is illegal on 3.0+ core -- rather than quietly report no support.
PLOT_TEST(indexed_extension_list_is_probed_on_a_3x_context) {
  PLOT_CHECK(probe_indexed_caps(1).buffer_storage);
  PLOT_CHECK(!probe_indexed_caps(2).buffer_storage);  // other names only
  PLOT_CHECK(!probe_indexed_caps(0).buffer_storage);  // the entries are null
  {
    plot::gl::load(nullptr);
    EntryGuard<decltype(plot::gl::glGetString)> string_guard(plot::gl::glGetString,
                                                             &fake_get_string);
    EntryGuard<decltype(plot::gl::glGetIntegerv)> int_guard(plot::gl::glGetIntegerv,
                                                            &fake_get_integer_v);
    EntryGuard<decltype(plot::gl::glGetStringi)> stringi_guard(plot::gl::glGetStringi, nullptr);
    g_fake_version = "3.3";
    g_fake_version_missing = false;
    g_fake_extensions = "GL_ARB_buffer_storage";  // present, but must not be consulted
    PLOT_CHECK(!plot::gl::caps().buffer_storage);
  }
  const plot::gl::Caps caps = probe_indexed_caps(1);
  PLOT_CHECK(caps.major == 3 && caps.minor == 3);
}

// Neither a context that answers no version string nor one whose version does
// not parse may leave stale caps behind: both must report the zeroed probe.
PLOT_TEST(caps_are_zeroed_when_the_version_is_missing_or_unparsable) {
  {
    plot::gl::load(nullptr);
    EntryGuard<decltype(plot::gl::glGetString)> guard(plot::gl::glGetString, &fake_get_string);
    g_fake_version_missing = true;
    const plot::gl::Caps caps = plot::gl::caps();
    PLOT_CHECK(caps.major == 0 && caps.minor == 0);
    PLOT_CHECK(caps.renderer != nullptr && caps.renderer[0] == '\0');
  }
  {
    plot::gl::load(nullptr);
    EntryGuard<decltype(plot::gl::glGetString)> guard(plot::gl::glGetString, &fake_get_string);
    g_fake_version_missing = false;
    g_fake_version = "not a version";
    const plot::gl::Caps caps = plot::gl::caps();
    PLOT_CHECK(caps.major == 0 && caps.minor == 0);
  }
  g_fake_version = "2.1";
  g_fake_version_missing = false;
  g_fake_extensions = "";
}

// Every fallible constructor's refusal branches: a driver that hands out name 0
// (`GLFailure`), a table with the entry point absent (`Unsupported`), and the
// argument checks. Each must report the documented code and leave an object
// that is empty yet safe to destroy. The stubs save and restore the slot, so a
// failure cannot leak into the next block.
PLOT_TEST(fallible_handles_report_a_refused_driver_name_and_stay_empty) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(egl_loader()));

  {
    EntryGuard<decltype(plot::gl::glGenBuffers)> guard(plot::gl::glGenBuffers, &zero_gen_buffers);
    plot::gl::Buffer buffer;
    PLOT_CHECK(!buffer.create(&st));
    PLOT_CHECK(st.code == plot::ErrorCode::GLFailure);
    PLOT_CHECK(std::strcmp(st.message, "glGenBuffers returned 0") == 0);
    PLOT_CHECK(buffer.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glGenBuffers)> guard(plot::gl::glGenBuffers, nullptr);
    plot::gl::Buffer buffer;
    PLOT_CHECK(!buffer.create(&st));
    PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
    PLOT_CHECK(buffer.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glGenVertexArrays)> guard(plot::gl::glGenVertexArrays,
                                                            &zero_gen_vertex_arrays);
    plot::gl::VertexArray vao;
    PLOT_CHECK(!vao.create(&st));
    PLOT_CHECK(st.code == plot::ErrorCode::GLFailure);
    PLOT_CHECK(std::strcmp(st.message, "glGenVertexArrays returned 0") == 0);
    PLOT_CHECK(vao.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glGenVertexArrays)> guard(plot::gl::glGenVertexArrays, nullptr);
    plot::gl::VertexArray vao;
    PLOT_CHECK(!vao.create(&st));
    PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
    PLOT_CHECK(vao.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glGenTextures)> guard(plot::gl::glGenTextures,
                                                        &zero_gen_textures);
    plot::gl::Texture tex;
    PLOT_CHECK(!tex.create_rgba(4, 4, &st));
    PLOT_CHECK(st.code == plot::ErrorCode::GLFailure);
    PLOT_CHECK(std::strcmp(st.message, "glGenTextures returned 0") == 0);
    PLOT_CHECK(tex.id() == 0);
    PLOT_CHECK(!tex.create_r8(4, 4, &st));
    PLOT_CHECK(st.code == plot::ErrorCode::GLFailure);
    PLOT_CHECK(tex.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glGenTextures)> guard(plot::gl::glGenTextures, nullptr);
    plot::gl::Texture tex;
    PLOT_CHECK(!tex.create_rgba(4, 4, &st));
    PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
    PLOT_CHECK(!tex.create_r8(4, 4, &st));
    PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
  }
  {
    plot::gl::Texture tex;
    PLOT_CHECK(!tex.create_rgba(0, 4, &st));
    PLOT_CHECK(st.code == plot::ErrorCode::InvalidArgument);
    PLOT_CHECK(!tex.create_r8(4, 0, &st));
    PLOT_CHECK(st.code == plot::ErrorCode::InvalidArgument);
    PLOT_CHECK(tex.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glGenFramebuffers)> guard(plot::gl::glGenFramebuffers,
                                                            &zero_gen_framebuffers);
    plot::gl::Framebuffer fbo;
    PLOT_CHECK(!fbo.create(&st));
    PLOT_CHECK(st.code == plot::ErrorCode::GLFailure);
    PLOT_CHECK(std::strcmp(st.message, "glGenFramebuffers returned 0") == 0);
    PLOT_CHECK(fbo.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glGenFramebuffers)> guard(plot::gl::glGenFramebuffers, nullptr);
    plot::gl::Framebuffer fbo;
    PLOT_CHECK(!fbo.create(&st));
    PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
    PLOT_CHECK(fbo.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glFenceSync)> guard(plot::gl::glFenceSync, nullptr);
    plot::gl::Fence fence;
    PLOT_CHECK(!fence.create(&st));
    PLOT_CHECK(st.code == plot::ErrorCode::Unsupported);
    PLOT_CHECK(fence.id() == nullptr);
  }

  // Program: a table without glCreateShader, a null source, a shader the driver
  // refuses to create and a program it refuses to create.
  {
    EntryGuard<decltype(plot::gl::glCreateShader)> guard(plot::gl::glCreateShader, nullptr);
    plot::Status bad;
    plot::gl::Program program = plot::gl::Program::from_source(kVertexSrc, kFragmentSrc, &bad);
    PLOT_CHECK(!bad);
    PLOT_CHECK(bad.code == plot::ErrorCode::Unsupported);
    PLOT_CHECK(program.id() == 0);
  }
  {
    plot::Status bad;
    plot::gl::Program program = plot::gl::Program::from_source(nullptr, kFragmentSrc, &bad);
    PLOT_CHECK(!bad);
    PLOT_CHECK(bad.code == plot::ErrorCode::InvalidArgument);
    PLOT_CHECK(program.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glCreateShader)> guard(plot::gl::glCreateShader,
                                                         &zero_create_shader);
    plot::Status bad;
    plot::gl::Program program = plot::gl::Program::from_source(kVertexSrc, kFragmentSrc, &bad);
    PLOT_CHECK(!bad);
    PLOT_CHECK(bad.code == plot::ErrorCode::GLFailure);
    PLOT_CHECK(std::strcmp(bad.message, "glCreateShader returned 0") == 0);
    PLOT_CHECK(program.id() == 0);
  }
  {
    EntryGuard<decltype(plot::gl::glCreateProgram)> guard(plot::gl::glCreateProgram,
                                                          &zero_create_program);
    plot::Status bad;
    plot::gl::Program program = plot::gl::Program::from_source(kVertexSrc, kFragmentSrc, &bad);
    PLOT_CHECK(!bad);
    PLOT_CHECK(bad.code == plot::ErrorCode::GLFailure);
    PLOT_CHECK(std::strcmp(bad.message, "glCreateProgram returned 0") == 0);
    PLOT_CHECK(program.id() == 0);
  }
  plot::gl::Program empty;
  empty.set_uniform_int("u_value", 1);  // an empty program's setter is a no-op
  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
  ctx.destroy();
}

// A program whose stages each compile but cannot be linked together: the
// constructor has to delete the program and report the driver's own log, not a
// bare failure. This is the path the link-failure branch implements.
PLOT_TEST(a_program_that_cannot_link_reports_the_driver_log) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(egl_loader()));

  plot::Status bad;
  plot::gl::Program program =
      plot::gl::Program::from_source(kMismatchedVertexSrc, kMismatchedFragmentSrc, &bad);
  PLOT_CHECK(!bad);
  PLOT_CHECK(bad.code == plot::ErrorCode::GLFailure);
  PLOT_CHECK(bad.message != nullptr);
  PLOT_CHECK(std::strncmp(bad.message, "program link failed", std::strlen("program link failed")) ==
             0);
  // The driver's log follows the prefix: a real reason, not an empty tail. Mesa's
  // linker names the mismatched varying.
  PLOT_CHECK(std::strlen(bad.message) > std::strlen("program link failed: "));
  PLOT_CHECK(std::strstr(bad.message, "v_value") != nullptr);
  PLOT_CHECK(program.id() == 0);
  ctx.destroy();
}

// A vertex array that could not be fully created must not be left behind: the
// pair fails when the vertex array itself is refused, and again when the
// instance buffer is refused after the vertex array exists, and in both cases
// the object reports nothing to draw.
PLOT_TEST(vertex_arrays_leave_nothing_behind_when_create_fails) {
  plottest::HeadlessContext ctx;
  plot::Status st;
  PLOT_CHECK(ctx.create(&st));
  PLOT_CHECK(plot::gl::load(egl_loader()));

  {
    EntryGuard<decltype(plot::gl::glGenVertexArrays)> guard(plot::gl::glGenVertexArrays,
                                                            &zero_gen_vertex_arrays);
    plot::Status bad;
    plot::render::LineVertexArray lines;
    PLOT_CHECK(!lines.create(&bad));
    PLOT_CHECK(!lines.valid());
    plot::render::TextVertexArray glyphs;
    PLOT_CHECK(!glyphs.create(&bad));
    PLOT_CHECK(!glyphs.valid());
  }
  {
    EntryGuard<decltype(plot::gl::glGenBuffers)> guard(plot::gl::glGenBuffers, &zero_gen_buffers);
    plot::Status bad;
    plot::render::LineVertexArray lines;
    PLOT_CHECK(!lines.create(&bad));
    PLOT_CHECK(!lines.valid());
    plot::render::TextVertexArray glyphs;
    PLOT_CHECK(!glyphs.create(&bad));
    PLOT_CHECK(!glyphs.valid());
  }
  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
  ctx.destroy();
}

// An empty batch -- a null pointer or a count of zero, with no vertex array
// created -- stages nothing and draws nothing, without touching the GL table.
PLOT_TEST(vertex_arrays_stage_nothing_for_an_empty_batch) {
  plot::render::LineVertexArray lines;
  lines.upload(nullptr, 3);
  PLOT_CHECK(lines.instance_count() == 0);
  const f32 segments[4] = {0.0f, 0.0f, 1.0f, 1.0f};
  lines.upload(segments, 0);
  PLOT_CHECK(lines.instance_count() == 0);

  plot::render::TextVertexArray glyphs;
  glyphs.upload(nullptr, 4);
  PLOT_CHECK(glyphs.instance_count() == 0);
  const f32 glyph_data[6] = {0.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f};
  glyphs.upload(glyph_data, 0);
  PLOT_CHECK(glyphs.instance_count() == 0);
}