Windows: properly handle symlink failures (#36003)

In the Windows filesystem logic for creating a symlink, we intend to
fall back to a copy when the symlink cannot be created (for some
configuration settings on Windows it is not possible for the user
to create a symlink). It turns out we were overly-broad in which
exceptions lead to this fallback, and the subsequent copy would
also fail: at least one case where this occurred is when we
attempted to create a symlink that already existed.

The updated logic expressly avoids falling back to a copy when the
file/symlink already exists.

lib/spack/llnl/util/filesystem.py

@@ -2407,7 +2407,7 @@ def _link(self, path, dest_dir):
             # For py2 compatibility, we have to catch the specific Windows error code
             # associate with trying to create a file that already exists (winerror 183)
             except OSError as e:
-                if sys.platform == "win32" and e.winerror == 183:
+                if sys.platform == "win32" and (e.winerror == 183 or e.errno == errno.EEXIST):
                     # We have either already symlinked or we are encoutering a naming clash
                     # either way, we don't want to overwrite existing libraries
                     already_linked = islink(dest_file)

lib/spack/llnl/util/symlink.py

@@ -30,9 +30,15 @@ def symlink(real_path, link_path):
         try:
             # Try to use junctions
             _win32_junction(real_path, link_path)
-        except OSError:
-            # If all else fails, fall back to copying files
-            shutil.copyfile(real_path, link_path)
+        except OSError as e:
+            if e.errno == errno.EEXIST:
+                # EEXIST error indicates that file we're trying to "link"
+                # is already present, don't bother trying to copy which will also fail
+                # just raise
+                raise
+            else:
+                # If all else fails, fall back to copying files
+                shutil.copyfile(real_path, link_path)
 
 
 def islink(path):