plotter/tests/egl_headless.cpp

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

#include "egl_headless.hpp"

#include <plot/gl/api.hpp>
#include <plot/gl/loader.hpp>

#include <EGL/egl.h>
#include <EGL/eglext.h>

#include <cstdio>
#include <cstdlib>

namespace plottest {
namespace {

/// `eglGetPlatformDisplayEXT`, declared locally so the harness does not depend
/// on the exact spelling of the EGL_EXT_platform_base typedef.
using GetPlatformDisplayEXT = EGLDisplay (*)(EGLenum platform, void* native_display,
                                            const EGLint* attrib_list);

/// Keeps the text of the last EGL failure alive for the returned Status.
thread_local char g_message[512];

/// Records the pending EGL error in `g_message`, echoes it to stderr so a
/// failing CTest run shows the driver's own error code, and returns false.
bool fail(plot::Status* st, const char* what) noexcept {
  const EGLint error = eglGetError();
  std::snprintf(g_message, sizeof g_message, "%s (EGL error 0x%04X)", what,
                static_cast<unsigned>(error));
  std::fprintf(stderr, "plot_tests: headless EGL: %s\n", g_message);
  if (st != nullptr) {
    st->code = plot::ErrorCode::GLFailure;
    st->message = g_message;
  }
  return false;
}

}  // namespace

HeadlessContext::HeadlessContext(u32 w, u32 h) noexcept : width(w), height(h) {}

HeadlessContext::~HeadlessContext() { destroy(); }

bool HeadlessContext::create(plot::Status* st) noexcept {
  if (framebuffer_.id() != 0) {
    return true;  // already created
  }
  if (width == 0 || height == 0) {
    if (st != nullptr) {
      st->code = plot::ErrorCode::InvalidArgument;
      st->message = "headless context size must be non-zero";
    }
    return false;
  }

  auto abort = [&](const char* what) {
    const bool reported = fail(st, what);
    destroy();
    return reported;
  };

  // 1. Prefer the surfaceless platform: no window system at all. Fall back to
  //    the default display (still surfaceless when it can be), and only then
  //    accept a pbuffer surface.
  const GetPlatformDisplayEXT get_platform_display = reinterpret_cast<GetPlatformDisplayEXT>(
      eglGetProcAddress("eglGetPlatformDisplayEXT"));
  EGLDisplay display = EGL_NO_DISPLAY;
  if (get_platform_display != nullptr) {
    display = get_platform_display(EGL_PLATFORM_SURFACELESS_MESA, EGL_DEFAULT_DISPLAY, nullptr);
  }
  if (display == EGL_NO_DISPLAY) {
    display = eglGetDisplay(EGL_DEFAULT_DISPLAY);
  }
  if (display == EGL_NO_DISPLAY) {
    return abort("eglGetPlatformDisplayEXT(EGL_PLATFORM_SURFACELESS_MESA) and eglGetDisplay failed");
  }
  display_ = display;

  // 2. Initialise the display for the desktop GL API.
  EGLint egl_major = 0;
  EGLint egl_minor = 0;
  if (eglInitialize(display, &egl_major, &egl_minor) != EGL_TRUE) {
    return abort("eglInitialize failed");
  }
  if (eglBindAPI(EGL_OPENGL_API) != EGL_TRUE) {
    return abort("eglBindAPI(EGL_OPENGL_API) failed");
  }

  // 3. An RGBA8 config that can back an OpenGL context.
  const EGLint config_attribs[] = {
      EGL_SURFACE_TYPE,    EGL_PBUFFER_BIT, EGL_RENDERABLE_TYPE, EGL_OPENGL_BIT,
      EGL_RED_SIZE,        8,               EGL_GREEN_SIZE,      8,
      EGL_BLUE_SIZE,       8,               EGL_ALPHA_SIZE,      8,
      EGL_DEPTH_SIZE,      0,               EGL_NONE,
  };
  EGLConfig config = nullptr;
  EGLint config_count = 0;
  if (eglChooseConfig(display, config_attribs, &config, 1, &config_count) != EGL_TRUE ||
      config_count == 0) {
    return abort("no EGL config offers 8-bit RGBA with EGL_OPENGL_BIT");
  }

  // 4. OpenGL 3.3 core, the baseline the library targets.
  const EGLint context_attribs[] = {
      EGL_CONTEXT_MAJOR_VERSION,
      3,
      EGL_CONTEXT_MINOR_VERSION,
      3,
      EGL_CONTEXT_OPENGL_PROFILE_MASK,
      EGL_CONTEXT_OPENGL_CORE_PROFILE_BIT_KHR,
      EGL_NONE,
  };
  const EGLContext context = eglCreateContext(display, config, EGL_NO_CONTEXT, context_attribs);
  if (context == EGL_NO_CONTEXT) {
    return abort("eglCreateContext(OpenGL 3.3 core) failed");
  }
  context_ = context;

  // 5. Surfaceless current context first; a 1x1 pbuffer only if the driver
  //    lacks EGL_KHR_surfaceless_context. PLOT_HEADLESS_FORCE_PBUFFER=1 skips
  //    straight to the fallback, so that path can be verified somewhere the
  //    surfaceless path would otherwise always win.
  const bool force_pbuffer = std::getenv("PLOT_HEADLESS_FORCE_PBUFFER") != nullptr;
  if (!force_pbuffer &&
      eglMakeCurrent(display, EGL_NO_SURFACE, EGL_NO_SURFACE, context) == EGL_TRUE) {
    egl_path = "egl surfaceless";
  } else {
    const EGLint pbuffer_attribs[] = {EGL_WIDTH, 1, EGL_HEIGHT, 1, EGL_NONE};
    const EGLSurface surface = eglCreatePbufferSurface(display, config, pbuffer_attribs);
    if (surface == EGL_NO_SURFACE ||
        eglMakeCurrent(display, surface, surface, context) != EGL_TRUE) {
      return abort("neither a surfaceless context nor a 1x1 pbuffer surface could be made current");
    }
    surface_ = surface;
    egl_path = "egl pbuffer";
  }

  // 6. Resolve the entry points against the context that is now current.
  if (!plot::gl::load(reinterpret_cast<plot::gl::ProcLoader>(eglGetProcAddress))) {
    if (st != nullptr) {
      st->code = plot::ErrorCode::GLFailure;
      st->message = "plot::gl::load(eglGetProcAddress) could not resolve every entry point";
    }
    std::fprintf(stderr, "plot_tests: headless EGL: %s\n", st != nullptr ? st->message : "");
    destroy();
    return false;
  }

  // 7. The off-screen target: an RGBA8 texture on colour attachment 0.
  plot::Status gl_status;
  if (!color_.create_rgba(width, height, &gl_status) ||
      !framebuffer_.create(&gl_status)) {
    if (st != nullptr) {
      *st = gl_status;
    }
    std::fprintf(stderr, "plot_tests: headless EGL: %s\n",
                 gl_status.message != nullptr ? gl_status.message : "framebuffer setup failed");
    destroy();
    return false;
  }
  framebuffer_.attach_color(color_);
  if (!framebuffer_.complete()) {
    std::fprintf(stderr, "plot_tests: headless EGL: framebuffer is not complete\n");
    if (st != nullptr) {
      st->code = plot::ErrorCode::GLFailure;
      st->message = "framebuffer is not complete";
    }
    destroy();
    return false;
  }
  return true;
}

void HeadlessContext::destroy() noexcept {
  // GL names belong to the context that created them, so drop them while it is
  // still current.
  framebuffer_.reset();
  color_.reset();

  if (display_ != nullptr) {
    const EGLDisplay display = static_cast<EGLDisplay>(display_);
    eglMakeCurrent(display, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT);
    if (context_ != nullptr) {
      eglDestroyContext(display, static_cast<EGLContext>(context_));
    }
    if (surface_ != nullptr) {
      eglDestroySurface(display, static_cast<EGLSurface>(surface_));
    }
    eglTerminate(display);
  }
  display_ = nullptr;
  context_ = nullptr;
  surface_ = nullptr;
  egl_path = "";
}

bool HeadlessContext::read_pixel(u32 x, u32 y, std::uint8_t rgba[4]) const noexcept {
  if (framebuffer_.id() == 0 || x >= width || y >= height) {
    return false;
  }
  framebuffer_.bind();
  plot::gl::glReadPixels(static_cast<plot::gl::GLint>(x), static_cast<plot::gl::GLint>(y), 1, 1,
                         plot::gl::GL_RGBA, plot::gl::GL_UNSIGNED_BYTE, rgba);
  return plot::gl::glGetError() == plot::gl::GL_NO_ERROR;
}

}  // namespace plottest