[Bugfix][Pooling] Fix silent weight corruption with buffer-reusing iterators (#39650)

Signed-off-by: Pedram Razavi <pedram.razavi@gmail.com>

tests/models/test_adapters.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Tests for model adapter weight loading (adapters.py)."""
+
+import pytest
+import torch
+
+from vllm.model_executor.models.adapters import _create_pooling_model_cls
+from vllm.model_executor.models.utils import AutoWeightsLoader, StageMissingLayer
+
+pytestmark = pytest.mark.cpu_test
+
+
+class SimpleInnerModel(torch.nn.Module):
+    def __init__(self):
+        super().__init__()
+        self.embed = torch.nn.Linear(4, 8, bias=False)
+        self.layer0 = torch.nn.Linear(8, 8, bias=False)
+        self.layer1 = torch.nn.Linear(8, 8, bias=False)
+        self.norm = torch.nn.Linear(8, 4, bias=False)
+
+    def load_weights(self, weights):
+        params = dict(self.named_parameters())
+        loaded = set()
+        for name, tensor in weights:
+            if name in params:
+                params[name].data.copy_(tensor)
+                loaded.add(name)
+        return loaded
+
+
+class SimpleModel(torch.nn.Module):
+    def __init__(self):
+        super().__init__()
+        self.model = SimpleInnerModel()
+        self.lm_head = torch.nn.Linear(8, 16, bias=False)
+
+    def load_weights(self, weights):
+        loader = AutoWeightsLoader(self)
+        return loader.load_weights(weights)
+
+
+class PackedWeightInnerModel(torch.nn.Module):
+    """Remaps q_proj/k_proj into a fused qkv_proj (Qwen2/Llama pattern)."""
+
+    def __init__(self):
+        super().__init__()
+        self.qkv_proj = torch.nn.Linear(4, 16, bias=False)
+        self.out = torch.nn.Linear(8, 4, bias=False)
+
+    def load_weights(self, weights):
+        params = dict(self.named_parameters())
+        loaded = set()
+        for name, tensor in weights:
+            if name == "q_proj.weight":
+                params["qkv_proj.weight"].data[:8].copy_(tensor)
+                loaded.add("qkv_proj.weight")
+            elif name == "k_proj.weight":
+                params["qkv_proj.weight"].data[8:].copy_(tensor)
+                loaded.add("qkv_proj.weight")
+            elif name in params:
+                params[name].data.copy_(tensor)
+                loaded.add(name)
+        return loaded
+
+
+class PackedWeightModel(torch.nn.Module):
+    def __init__(self):
+        super().__init__()
+        self.model = PackedWeightInnerModel()
+        self.lm_head = torch.nn.Linear(4, 8, bias=False)
+
+    def load_weights(self, weights):
+        loader = AutoWeightsLoader(self)
+        return loader.load_weights(weights)
+
+
+def _buffer_reusing_iterator(weight_dict):
+    """Yield weights through a shared buffer overwritten each step.
+
+    Mimics ``runai_model_streamer`` with ``RUNAI_STREAMER_MEMORY_LIMIT=0``.
+    """
+    buf = None
+    for name, tensor in weight_dict.items():
+        if buf is None or buf.numel() < tensor.numel():
+            buf = torch.empty(tensor.numel(), dtype=tensor.dtype)
+        view = buf[: tensor.numel()].view(tensor.shape)
+        view.copy_(tensor)
+        yield name, view
+
+
+def _make_pooling_model(base_cls=SimpleModel):
+    PoolingModel = _create_pooling_model_cls(base_cls)
+    model = base_cls()
+    model.__class__ = PoolingModel
+    model.lm_head = StageMissingLayer("output", model.lm_head)
+    return model
+
+
+def _make_reference_weights():
+    torch.manual_seed(42)
+    return {
+        "model.embed.weight": torch.randn(8, 4),
+        "model.layer0.weight": torch.randn(8, 8),
+        "model.layer1.weight": torch.randn(8, 8),
+        "model.norm.weight": torch.randn(4, 8),
+        "lm_head.weight": torch.randn(16, 8),
+    }
+
+
+def _make_packed_reference_weights():
+    torch.manual_seed(42)
+    return {
+        "model.q_proj.weight": torch.randn(8, 4),
+        "model.k_proj.weight": torch.randn(8, 4),
+        "model.out.weight": torch.randn(4, 8),
+        "lm_head.weight": torch.randn(8, 4),
+    }
+
+
+def _load_and_compare(model, ref, expected):
+    for p in model.parameters():
+        p.data.zero_()
+    model.load_weights(_buffer_reusing_iterator(ref))
+    for name, param in model.named_parameters():
+        assert torch.equal(param.data, expected[name]), name
+
+
+def test_pooling_load_weights_with_buffer_reuse():
+    """Ensure ModelForPooling.load_weights works with buffer-reusing iterators."""
+    ref = _make_reference_weights()
+
+    ground_truth = SimpleModel()
+    ground_truth.load_weights(ref.items())
+    expected = {n: p.data.clone() for n, p in ground_truth.named_parameters()}
+
+    _load_and_compare(_make_pooling_model(), ref, expected)
+
+
+def test_pooling_load_weights_clones_probed_weights():
+    """Ensure probed weights survive buffer reuse during packed remapping."""
+    ref = _make_packed_reference_weights()
+
+    ground_truth = PackedWeightModel()
+    ground_truth.load_weights(ref.items())
+    expected = {n: p.data.clone() for n, p in ground_truth.named_parameters()}
+
+    _load_and_compare(_make_pooling_model(PackedWeightModel), ref, expected)

vllm/model_executor/models/adapters.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import itertools
 from collections.abc import Iterable
 from contextlib import contextmanager
 from typing import TYPE_CHECKING, Any, TypeVar, cast
@@ -181,7 +182,8 @@ def _create_pooling_model_cls(orig_cls: _T) -> _T:
 
             seen_weights = list[tuple[str, torch.Tensor]]()
             for name, loaded_weight in weights:
-                seen_weights.append((name, loaded_weight))
+                # Clone because the iterator may reuse the tensor buffer
+                seen_weights.append((name, loaded_weight.clone()))
 
                 try:
                     target_prefix = next(
@@ -208,9 +210,11 @@ def _create_pooling_model_cls(orig_cls: _T) -> _T:
                     self._get_name(),
                 )
 
+            # Lazy chain so buffer-reusing weight iterators (e.g.
+            # runai_streamer) are consumed one tensor at a time.
             mapped_weights = (
                 (target_prefix + name, weight)
-                for name, weight in (*seen_weights, *weights)
+                for name, weight in itertools.chain(seen_weights, weights)
             )
 
             def default_load_weights(weights):