Add path used by pip's build isolation procedure to DLL search

Luthaf · Luthaf · commit f2b49a99849f · 2024-07-22T18:19:05.000+02:00
Without this, trying to `import torch` in a downstream `setup.py` file
would result in

```
The specified module could not be found. Error loading "C:\...\pip-build-env-himl3xh3\normal\Lib\site-packages\torch\lib\shm.dll" or one of its dependencies."
```

This seems to be because pip does not use a full virtualenv for build
isolation, instead creating directories and manually adding them to
`sys.path`
diff --git a/torch/__init__.py b/torch/__init__.py
@@ -154,7 +154,7 @@ def _load_dll_libraries() -> None:
         pfiles_path = os.getenv("ProgramFiles", r"C:\Program Files")
         py_dll_path = os.path.join(sys.exec_prefix, "Library", "bin")
         th_dll_path = os.path.join(os.path.dirname(__file__), "lib")
-        usebase_path = os.path.join(
+        userbase_path = os.path.join(
             sysconfig.get_config_var("userbase"), "Library", "bin"
         )
 
@@ -167,9 +167,16 @@ def _load_dll_libraries() -> None:
         else:
             base_py_dll_path = ""
 
+        # when using pip install on a project that depends on PyTorch, pip installs
+        # torch & mkl in a custom path that's included in sys.path but does not match
+        # any of the other DLLs path above, so we need to manually add it here.
+        mkl_dll_path = os.path.join(
+            os.path.dirname(__file__), "..", "..", "..", "Library", "bin"
+        )
+
         dll_paths = [
             p
-            for p in (th_dll_path, py_dll_path, base_py_dll_path, usebase_path)
+            for p in (th_dll_path, py_dll_path, base_py_dll_path, userbase_path, mkl_dll_path)
             if os.path.exists(p)
         ]
 
