Flatten build variants to build/<variant>

Prior to this change, kernels were stored in `build/<variant>/<extname>`.
However, this was fragile because the extension name had to correspond to
the repository name. This change flattens kernels to be stored inside
`build/<variant>`. We add a symbolic link `build/<variant>/<extname>`
for compatibility with older versions of `kernels`.

Author: danieldk

lib/torch-extension/arch.nix

@@ -223,21 +223,26 @@ stdenv.mkDerivation (prevAttrs: {
   postInstall = ''
     (
       cd ..
-      cp -r torch-ext/${extensionName} $out/
+      cp -r torch-ext/${extensionName}/* $out/
     )
-    cp $out/_${extensionName}_*/* $out/${extensionName}
-    rm -rf $out/_${extensionName}_*
+    mv $out/_${extensionName}_*/* $out/
+    rm -d $out/_${extensionName}_${rev}
+
+    # Set up compatibility symlink for older kernels versions, remove
+    # the updated kernels has been around for a while. ln -r is not
+    # supported on macOS.
+    ( cd $out ; ln -sf . $out/${extensionName} )
   ''
   + (lib.optionalString (stripRPath && stdenv.hostPlatform.isLinux)) ''
-    find $out/${extensionName} -name '*.so' \
+    find $out/ -name '*.so' \
       -exec patchelf --set-rpath "" {} \;
   ''
   + (lib.optionalString (stripRPath && stdenv.hostPlatform.isDarwin)) ''
-    find $out/${extensionName} -name '*.so' \
+    find $out/ -name '*.so' \
       -exec rewrite-nix-paths-macho {} \;
 
     # Stub some rpath.
-    find $out/${extensionName} -name '*.so' \
+    find $out/ -name '*.so' \
       -exec install_name_tool -add_rpath "@loader_path/lib" {} \;
   '';
 

lib/torch-extension/no-arch.nix

@@ -46,7 +46,8 @@ stdenv.mkDerivation (prevAttrs: {
 
   installPhase = ''
     mkdir -p $out
-    cp -r torch-ext/${extensionName} $out/
+    cp -r torch-ext/${extensionName}/* $out/
+    ( cd $out ; ln -sf . $out/${extensionName} )
   '';
 
   doInstallCheck = true;