[FlashAttention] Symlink FA4 instead of copying when using `VLLM_FLASH_ATTN_SRC_DIR` (#38814)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

cmake/external_projects/vllm_flash_attn.cmake

@@ -87,10 +87,21 @@ endforeach()
 #
 add_custom_target(_vllm_fa4_cutedsl_C)
 
-# Copy flash_attn/cute directory (needed for FA4) and transform imports
-# The cute directory uses flash_attn.cute imports internally, which we replace
-# with vllm.vllm_flash_attn.cute to match our package structure.
-install(CODE "
+# Install flash_attn/cute directory (needed for FA4).
+# When using a local source dir (VLLM_FLASH_ATTN_SRC_DIR), create a symlink
+# so edits to cute-dsl Python files take effect immediately without rebuilding.
+# Otherwise, copy files and transform flash_attn.cute imports to
+# vllm.vllm_flash_attn.cute to match our package structure.
+if(VLLM_FLASH_ATTN_SRC_DIR)
+  install(CODE "
+    set(LINK_TARGET \"${vllm-flash-attn_SOURCE_DIR}/flash_attn/cute\")
+    set(LINK_NAME \"\${CMAKE_INSTALL_PREFIX}/vllm/vllm_flash_attn/cute\")
+    file(MAKE_DIRECTORY \"\${CMAKE_INSTALL_PREFIX}/vllm/vllm_flash_attn\")
+    file(REMOVE_RECURSE \"\${LINK_NAME}\")
+    file(CREATE_LINK \"\${LINK_TARGET}\" \"\${LINK_NAME}\" SYMBOLIC)
+  " COMPONENT _vllm_fa4_cutedsl_C)
+else()
+  install(CODE "
     file(GLOB_RECURSE CUTE_PY_FILES \"${vllm-flash-attn_SOURCE_DIR}/flash_attn/cute/*.py\")
     foreach(SRC_FILE \${CUTE_PY_FILES})
       file(RELATIVE_PATH REL_PATH \"${vllm-flash-attn_SOURCE_DIR}/flash_attn/cute\" \${SRC_FILE})
@@ -101,4 +112,5 @@ install(CODE "
       string(REPLACE \"flash_attn.cute\" \"vllm.vllm_flash_attn.cute\" FILE_CONTENTS \"\${FILE_CONTENTS}\")
       file(WRITE \${DST_FILE} \"\${FILE_CONTENTS}\")
     endforeach()
-" COMPONENT _vllm_fa4_cutedsl_C)
+  " COMPONENT _vllm_fa4_cutedsl_C)
+endif()

vllm/vllm_flash_attn/__init__.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from vllm.vllm_flash_attn.flash_attn_interface import (
+import importlib.machinery
+import os
+import sys
+import types
+
+# In symlink mode (VLLM_FLASH_ATTN_SRC_DIR), cute/ is a symlink to the real
+# source tree and its files use `flash_attn.cute.*` imports (not rewritten).
+# Register a virtual `flash_attn` package so those imports resolve.
+_cute_dir = os.path.join(os.path.dirname(__file__), "cute")
+if os.path.islink(_cute_dir) and "flash_attn" not in sys.modules:
+    _fa_mod = types.ModuleType("flash_attn")
+    _fa_mod.__path__ = [os.path.dirname(os.path.realpath(_cute_dir))]
+    _fa_mod.__package__ = "flash_attn"
+    _fa_mod.__spec__ = importlib.machinery.ModuleSpec(
+        "flash_attn", None, is_package=True
+    )
+    _fa_mod.__spec__.submodule_search_locations = _fa_mod.__path__
+    sys.modules["flash_attn"] = _fa_mod
+
+from vllm.vllm_flash_attn.flash_attn_interface import (  # noqa: E402
     FA2_AVAILABLE,
     FA3_AVAILABLE,
     fa_version_unsupported_reason,