Fix #139191: GPU: Blender crashes on startup with addons that use pyGPU shader

This was caused by missing `shader->constants`.
Creating an empty `SpecializationConstants` fixes the issue.

source/blender/gpu/intern/gpu_shader.cc

@@ -132,6 +132,10 @@ GPUShader *GPU_shader_create_ex(const std::optional<StringRefNull> vertcode,
               computecode.has_value()));
 
   Shader *shader = GPUBackend::get()->shader_alloc(shname.c_str());
+  /* Needs to be called before init as GL uses the default specialization constants state to insert
+   * default shader inside a map. */
+  shader->constants = std::make_unique<const shader::SpecializationConstants>();
+  shader->init();
 
   if (vertcode) {
     Vector<StringRefNull> sources;

source/blender/gpu/intern/gpu_shader_private.hh

@@ -71,6 +71,8 @@ class Shader {
 
   /* TODO: Remove `is_batch_compilation`. */
   virtual void init(const shader::ShaderCreateInfo &info, bool is_batch_compilation) = 0;
+  /* Variant for legacy python shaders. To be removed, not supported in Vulkan or Metal. */
+  virtual void init() = 0;
 
   virtual void vertex_shader_from_glsl(MutableSpan<StringRefNull> sources) = 0;
   virtual void geometry_shader_from_glsl(MutableSpan<StringRefNull> sources) = 0;

source/blender/gpu/metal/mtl_shader.hh

@@ -234,6 +234,7 @@ class MTLShader : public Shader {
   ~MTLShader();
 
   void init(const shader::ShaderCreateInfo & /*info*/, bool is_batch_compilation) override;
+  void init() override {}
 
   /* Assign GLSL source. */
   void vertex_shader_from_glsl(MutableSpan<StringRefNull> sources) override;

source/blender/gpu/opengl/gl_shader.cc

@@ -82,6 +82,15 @@ void GLShader::init(const shader::ShaderCreateInfo &info, bool is_batch_compilat
   }
 }
 
+void GLShader::init()
+{
+  main_program_ = &program_cache_.lookup_or_add_default(constants->values);
+  if (!main_program_->program_id) {
+    main_program_->program_id = glCreateProgram();
+    debug::object_label(GL_PROGRAM, main_program_->program_id, name);
+  }
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/gpu/opengl/gl_shader.hh

@@ -145,6 +145,7 @@ class GLShader : public Shader {
   ~GLShader();
 
   void init(const shader::ShaderCreateInfo &info, bool is_batch_compilation) override;
+  void init() override;
 
   /** Return true on success. */
   void vertex_shader_from_glsl(MutableSpan<StringRefNull> sources) override;

source/blender/gpu/vulkan/vk_shader.hh

@@ -58,6 +58,7 @@ class VKShader : public Shader {
   virtual ~VKShader();
 
   void init(const shader::ShaderCreateInfo &info, bool is_batch_compilation) override;
+  void init() override {}
 
   void vertex_shader_from_glsl(MutableSpan<StringRefNull> sources) override;
   void geometry_shader_from_glsl(MutableSpan<StringRefNull> sources) override;