PyGPU: Show error for unsupported POLYLINE shader configurations

In Blender 4.4 (since commit 00a8d006fe), polyline shaders stopped
using geometry shaders and now rely on SSBOs.

In C++, workarounds allow these shaders to function as before, albeit
with some limitations.

To address these known limitations, error messages are now raised when
a polyline shader is used with unsupported configurations.

Pull Request: https://projects.blender.org/blender/blender/pulls/131055

source/blender/python/gpu/gpu_py_batch.cc

@@ -224,6 +224,62 @@ static PyObject *pygpu_batch_program_set(BPyGPUBatch *self, BPyGPUShader *py_sha
   Py_RETURN_NONE;
 }
 
+/**
+ * Verify if the Shader is compatible with the batch and can be used for rendering.
+ * Derived from `polyline_draw_workaround` in `gpu_immediate.cc`.
+ */
+static const char *pygpu_shader_check_compatibility(blender::gpu::Batch *batch)
+{
+  if (!batch->shader) {
+    return nullptr;
+  }
+
+  /* Currently only POLYLINE shaders are checked. */
+  if (!bpygpu_shader_is_polyline(batch->shader)) {
+    return nullptr;
+  }
+
+  /* Check batch compatibility with shader. */
+  for (auto vert : blender::Span(batch->verts, ARRAY_SIZE(batch->verts))) {
+    if (!vert) {
+      continue;
+    }
+    GPUVertFormat &format = vert->format;
+    if ((format.stride % 4) != 0) {
+      return "For POLYLINE shaders, only 4-byte aligned formats are supported";
+    }
+
+    int pos_attr_id = -1;
+    int col_attr_id = -1;
+    for (uint a_idx = 0; a_idx < format.attr_len; a_idx++) {
+      const GPUVertAttr *a = &format.attrs[a_idx];
+      if ((a->offset % 4) != 0) {
+        return "For POLYLINE shaders, only 4-byte aligned attributes are supported";
+      }
+      const blender::StringRefNull name = GPU_vertformat_attr_name_get(&format, a, 0);
+      if (pos_attr_id == -1 && name == "pos") {
+        if (!ELEM(a->comp_type, GPU_COMP_F32)) {
+          return "For POLYLINE shaders, the 'pos' attribute needs to be 'F32'";
+        }
+        if (!ELEM(a->fetch_mode, GPU_FETCH_FLOAT)) {
+          return "For POLYLINE shaders, the 'pos' attribute must use the 'FLOAT' fetch type";
+        }
+        pos_attr_id = a_idx;
+      }
+      else if (col_attr_id == -1 && name == "color") {
+        if (!ELEM(a->comp_type, GPU_COMP_F32, GPU_COMP_U8)) {
+          return "For POLYLINE shaders, the 'color' attribute needs to be 'F32' or 'U8'";
+        }
+        col_attr_id = a_idx;
+      }
+      if (pos_attr_id != -1 && col_attr_id != -1) {
+        break;
+      }
+    }
+  }
+  return nullptr;
+}
+
 PyDoc_STRVAR(
     /* Wrap. */
     pygpu_batch_draw_doc,
@@ -262,6 +318,11 @@ static PyObject *pygpu_batch_draw(BPyGPUBatch *self, PyObject *args)
     GPU_batch_set_shader(self->batch, py_shader->shader);
   }
 
+  if (const char *error = pygpu_shader_check_compatibility(self->batch)) {
+    PyErr_SetString(PyExc_RuntimeError, error);
+    return nullptr;
+  }
+
   GPU_batch_draw(self->batch);
   Py_RETURN_NONE;
 }