fix(gl): gate fallible constructors on a complete loader table

- loader: add available(), true only after load() has resolved every
  required entry point (false at the start of load()). Program::from_source
  and the other fallible constructors return ErrorCode::Unsupported while it
  is false, so a failed load() with a partially populated table can no longer
  fault on an unresolved entry point.
- caps: GL_CONTEXT_PROFILE_MASK only exists from 3.2, so query it only on
  3.2+ contexts. A 3.0/3.1 query latches GL_INVALID_ENUM in the context, which
  the next unrelated glGetError() (e.g. read_pixel) then reports as failure.
- tests: gate the dlsym/libGL.so.1 fallback assertions on libGL being
  resolvable, so the GL suite also passes on a GLES-only host.

include/plot/gl/loader.hpp

@@ -25,4 +25,11 @@ using ProcLoader = void* (*)(const char*);
 /// context.
 bool load(ProcLoader loader = nullptr) noexcept;
 
+/// True once a `load()` has resolved every required entry point of the global
+/// table. This is the gate superseding per-pointer null checks: while it is
+/// false the table may be only partially populated (see load()), so no `gl*`
+/// entry point may be called. The fallible handle constructors report
+/// `ErrorCode::Unsupported` until it becomes true.
+bool available() noexcept;
+
 }  // namespace plot::gl

src/gl/handle.cpp

@@ -1,5 +1,7 @@
 #include <plot/gl/handle.hpp>
 
+#include <plot/gl/loader.hpp>
+
 #include <cstdint>
 #include <cstdio>
 
@@ -78,6 +80,9 @@ GLuint alloc_texture(GLenum internal_format, GLenum format, u32 width, u32 heigh
 // ------------------------------------------------------------------- Buffer
 
 bool Buffer::create(Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenBuffers == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -109,6 +114,9 @@ void Buffer::destroy(GLuint id) noexcept { glDeleteBuffers(1, &id); }
 // -------------------------------------------------------------- VertexArray
 
 bool VertexArray::create(Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenVertexArrays == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -139,6 +147,10 @@ void VertexArray::destroy(GLuint id) noexcept { glDeleteVertexArrays(1, &id); }
 Program Program::from_source(const char* vertex_src, const char* fragment_src,
                              Status* st) noexcept {
   Program program;
+  if (!available()) {
+    fail(st, ErrorCode::Unsupported, kNoLoader);
+    return program;
+  }
   if (glCreateShader == nullptr || glCreateProgram == nullptr) {
     fail(st, ErrorCode::Unsupported, kNoLoader);
     return program;
@@ -219,6 +231,9 @@ void Program::destroy(GLuint id) noexcept { glDeleteProgram(id); }
 // ------------------------------------------------------------------ Texture
 
 bool Texture::create_rgba(u32 width, u32 height, Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenTextures == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -235,6 +250,9 @@ bool Texture::create_rgba(u32 width, u32 height, Status* st) noexcept {
 }
 
 bool Texture::create_r8(u32 width, u32 height, Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenTextures == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -280,6 +298,9 @@ void Texture::destroy(GLuint id) noexcept { glDeleteTextures(1, &id); }
 // -------------------------------------------------------------- Framebuffer
 
 bool Framebuffer::create(Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenFramebuffers == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -310,6 +331,9 @@ void Framebuffer::destroy(GLuint id) noexcept { glDeleteFramebuffers(1, &id); }
 // -------------------------------------------------------------------- Fence
 
 bool Fence::create(Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glFenceSync == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }

src/gl/loader.cpp

@@ -30,6 +30,10 @@ PLOT_GL_FUNCS(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.
@@ -138,7 +142,7 @@ bool query_caps(Caps& out) noexcept {
   out.persistent_map = storage;
 
   GLint profile = 0;
-  if (major >= 3) {
+  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);
@@ -150,6 +154,9 @@ bool query_caps(Caps& out) noexcept {
 }  // 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)               \
@@ -173,9 +180,12 @@ bool load(ProcLoader loader) noexcept {
     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)) {

tests/test_gl_context.cpp

@@ -10,6 +10,7 @@
 #include <cstdint>
 #include <cstdio>
 #include <cstring>
+#include <dlfcn.h>
 #include <utility>
 
 namespace {
@@ -65,7 +66,8 @@ PLOT_TEST(headless_context_provides_gl33) {
   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');
-  PLOT_CHECK(caps.core_profile);  // the context was created as 3.3 core
+  // 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;
@@ -78,16 +80,20 @@ PLOT_TEST(headless_context_provides_gl33) {
               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.
-  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');
+  // 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.