plotter/src/gl/loader.cpp

#include <plot/gl/loader.hpp>

#include <plot/gl/caps.hpp>

#include <cstdio>
#include <cstring>
#include <dlfcn.h>

// Debug-only diagnostic for the first entry point the loader cannot resolve;
// the failure itself is reported through load()'s return value, so this never
// aborts a release build or a test run.
#ifndef NDEBUG
#define PLOT_ASSERT(cond, message)                                                       \
  do {                                                                                   \
    if (!(cond)) {                                                                       \
      std::fprintf(stderr, "plot::gl::load: cannot resolve %s (%s:%d)\n", (message),      \
                   __FILE__, __LINE__);                                                  \
    }                                                                                    \
  } while (false)
#else
#define PLOT_ASSERT(cond, message) ((void)0)
#endif

namespace plot::gl {

// The one definition of the entry point table declared in api.hpp.
#define PLOT_GL_DEF(ret, name, args) ret (*name) args = nullptr;
PLOT_GL_FUNCS(PLOT_GL_DEF)
PLOT_GL_OPTIONAL_FUNCS(PLOT_GL_DEF)
#undef PLOT_GL_DEF

namespace {

/// False until a `load()` has resolved every required entry point of the table
/// above, which may stay partially populated until then. Read via available().
bool g_table_ready = false;

/// The system GL library, opened on the first name that neither the caller's
/// resolver nor the global namespace can supply. Never closed: the entry
/// points handed out must stay valid for the whole process.
void* system_gl() noexcept {
  static void* lib = nullptr;
  static bool opened = false;
  if (!opened) {
    opened = true;
    lib = dlopen("libGL.so.1", RTLD_LAZY | RTLD_LOCAL);
  }
  return lib;
}

void* resolve(ProcLoader loader, const char* name) noexcept {
  if (loader != nullptr) {
    if (void* symbol = loader(name)) {
      return symbol;
    }
  }
  if (void* symbol = dlsym(RTLD_DEFAULT, name)) {
    return symbol;
  }
  if (void* lib = system_gl()) {
    return dlsym(lib, name);
  }
  return nullptr;
}

// ---------------------------------------------------------------- caps probe

Caps g_caps;
bool g_caps_valid = false;

const char* gl_string(GLenum name) noexcept {
  const char* text = reinterpret_cast<const char*>(glGetString(name));
  return text != nullptr ? text : "";
}

/// True when `all` (the legacy space-separated GL_EXTENSIONS string) contains
/// `name` as a whole token.
bool has_token(const char* all, const char* name) noexcept {
  if (all == nullptr || name == nullptr) {
    return false;
  }
  const usize len = std::strlen(name);
  for (const char* hit = std::strstr(all, name); hit != nullptr; hit = std::strstr(hit + len, name)) {
    const bool at_start = hit == all || hit[-1] == ' ';
    const bool at_end = hit[len] == '\0' || hit[len] == ' ';
    if (at_start && at_end) {
      return true;
    }
  }
  return false;
}

/// Tests for an extension in the form the current context exposes it: the
/// indexed list on 3.0+, the legacy string before that (where
/// `glGetString(GL_EXTENSIONS)` is still legal).
bool has_extension(int major, const char* name) noexcept {
  if (major >= 3) {
    if (glGetStringi == nullptr) {
      return false;  // indexed list unavailable: the legacy string is illegal on 3.0+ core
    }
    GLint count = 0;
    glGetIntegerv(GL_NUM_EXTENSIONS, &count);
    for (GLint i = 0; i < count; ++i) {
      const char* ext =
          reinterpret_cast<const char*>(glGetStringi(GL_EXTENSIONS, static_cast<GLuint>(i)));
      if (ext != nullptr && std::strcmp(ext, name) == 0) {
        return true;
      }
    }
    return false;
  }
  // Before 3.0 the legacy space-separated string is the only enumeration.
  return has_token(reinterpret_cast<const char*>(glGetString(GL_EXTENSIONS)), name);
}

/// Fills `out` from the context that is current on this thread. Returns false
/// when there is no usable context, leaving `out` untouched.
bool query_caps(Caps& out) noexcept {
  if (glGetString == nullptr || glGetIntegerv == nullptr) {
    return false;
  }
  const char* version = reinterpret_cast<const char*>(glGetString(GL_VERSION));
  if (version == nullptr) {
    return false;  // no current context
  }
  int major = 0;
  int minor = 0;
  // Drivers report e.g. "4.6 (Compatibility Profile) Mesa 25.2.0"; only the
  // leading "major.minor" is part of the specification.
  if (std::sscanf(version, "%d.%d", &major, &minor) != 2) {
    return false;
  }

  out.major = major;
  out.minor = minor;
  out.renderer = gl_string(GL_RENDERER);
  out.vendor = gl_string(GL_VENDOR);

  const bool has_44 = major > 4 || (major == 4 && minor >= 4);
  out.compute = major > 4 || (major == 4 && minor >= 3);
  const bool storage = has_44 || has_extension(major, "GL_ARB_buffer_storage");
  out.buffer_storage = storage;
  out.persistent_map = storage;

  GLint profile = 0;
  if (major > 3 || (major == 3 && minor >= 2)) {
    // GL_CONTEXT_PROFILE_MASK only exists from 3.2 on; older contexts would
    // raise GL_INVALID_ENUM and leave the value at 0.
    glGetIntegerv(GL_CONTEXT_PROFILE_MASK, &profile);
  }
  out.core_profile = (static_cast<GLuint>(profile) & GL_CONTEXT_CORE_PROFILE_BIT) != 0u;
  return true;
}

}  // namespace

bool load(ProcLoader loader) noexcept {
  // A partially resolved table must not be used: the fallible constructors
  // consult available() and report Unsupported until the whole table is good.
  g_table_ready = false;
  const char* missing = nullptr;

#define PLOT_GL_RESOLVE(ret, name, args)               \
  do {                                                 \
    void* symbol = resolve(loader, #name);             \
    if (symbol != nullptr) {                           \
      name = reinterpret_cast<ret (*) args>(symbol);   \
    } else if (missing == nullptr) {                   \
      missing = #name;                                 \
    }                                                  \
  } while (false);

  PLOT_GL_FUNCS(PLOT_GL_RESOLVE)

  // Above-baseline entries: resolved the same way, but a name the driver does not
  // have leaves its pointer null instead of failing the load, so a 3.3-only
  // context keeps working. Callers gate them on caps() (see api.hpp).
  PLOT_GL_OPTIONAL_FUNCS(PLOT_GL_RESOLVE)

#undef PLOT_GL_RESOLVE

  // The table may have changed, so whatever caps() cached for the old one is
  // stale — including when this load() fails.
  g_caps_valid = false;
  if (missing != nullptr) {
    PLOT_ASSERT(false, missing);
    return false;
  }
  g_table_ready = true;
  return true;
}

bool available() noexcept { return g_table_ready; }

const Caps& caps() noexcept {
  if (!g_caps_valid) {
    if (query_caps(g_caps)) {
      g_caps_valid = true;
    } else {
      // No context (or no resolved entry points): report an empty probe rather
      // than stale or uninitialised values.
      g_caps = Caps{};
    }
  }
  return g_caps;
}

}  // namespace plot::gl