[BugFix] Fix cmake based incremental install (wrong vllm install dir) (#35773)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>
Co-authored-by: Wentao Ye <44945378+yewentao256@users.noreply.github.com>

cmake/external_projects/vllm_flash_attn.cmake

@@ -46,24 +46,20 @@ else()
   )
 endif()
 
-
-# Install rules for FA components need the install prefix nested under vllm/
-# These run at install time, before the FA library's own install rules
-foreach(_FA_COMPONENT _vllm_fa2_C _vllm_fa3_C)
-  install(CODE "set(CMAKE_INSTALL_LOCAL_ONLY FALSE)" COMPONENT ${_FA_COMPONENT})
-  install(CODE "set(OLD_CMAKE_INSTALL_PREFIX \"\${CMAKE_INSTALL_PREFIX}\")" COMPONENT ${_FA_COMPONENT})
-  install(CODE "set(CMAKE_INSTALL_PREFIX \"\${CMAKE_INSTALL_PREFIX}/vllm/\")" COMPONENT ${_FA_COMPONENT})
-endforeach()
+# Make sure vllm-flash-attn install rules are nested under vllm/
+# ALL_COMPONENTS ensures the save/modify/restore runs exactly once regardless
+# of how many components are being installed, avoiding double-append of /vllm/.
+install(CODE "set(CMAKE_INSTALL_LOCAL_ONLY FALSE)" ALL_COMPONENTS)
+install(CODE "set(OLD_CMAKE_INSTALL_PREFIX \"\${CMAKE_INSTALL_PREFIX}\")" ALL_COMPONENTS)
+install(CODE "set(CMAKE_INSTALL_PREFIX \"\${CMAKE_INSTALL_PREFIX}/vllm/\")" ALL_COMPONENTS)
 
 # Fetch the vllm-flash-attn library
 FetchContent_MakeAvailable(vllm-flash-attn)
 message(STATUS "vllm-flash-attn is available at ${vllm-flash-attn_SOURCE_DIR}")
 
 # Restore the install prefix after FA's install rules
-foreach(_FA_COMPONENT _vllm_fa2_C _vllm_fa3_C)
-  install(CODE "set(CMAKE_INSTALL_PREFIX \"\${OLD_CMAKE_INSTALL_PREFIX}\")" COMPONENT ${_FA_COMPONENT})
-  install(CODE "set(CMAKE_INSTALL_LOCAL_ONLY TRUE)" COMPONENT ${_FA_COMPONENT})
-endforeach()
+install(CODE "set(CMAKE_INSTALL_PREFIX \"\${OLD_CMAKE_INSTALL_PREFIX}\")" ALL_COMPONENTS)
+install(CODE "set(CMAKE_INSTALL_LOCAL_ONLY TRUE)" ALL_COMPONENTS)
 
 # Install shared Python files for both FA2 and FA3 components
 foreach(_FA_COMPONENT _vllm_fa2_C _vllm_fa3_C)