Windows bugfix: safe rename if renaming file onto itself (#43456)

* Generally use os.replace on Windows and Linux
* Windows behavior for os.replace differs when the destination exists
  and is a symlink to a directory: on Linux the dst is replaced and
  on Windows this fails - this PR makes Windows behave like Linux
  (by deleting the dst before doing the rename unless src and dst
  are the same)

lib/spack/llnl/util/filesystem.py

@@ -198,15 +198,32 @@ def getuid():
         return os.getuid()
 
 
+def _win_rename(src, dst):
+    # os.replace will still fail if on Windows (but not POSIX) if the dst
+    # is a symlink to a directory (all other cases have parity Windows <-> Posix)
+    if os.path.islink(dst) and os.path.isdir(os.path.realpath(dst)):
+        if os.path.samefile(src, dst):
+            # src and dst are the same
+            # do nothing and exit early
+            return
+        # If dst exists and is a symlink to a directory
+        # we need to remove dst and then perform rename/replace
+        # this is safe to do as there's no chance src == dst now
+        os.remove(dst)
+    os.replace(src, dst)
+
+
 @system_path_filter
 def rename(src, dst):
     # On Windows, os.rename will fail if the destination file already exists
+    # os.replace is the same as os.rename on POSIX and is MoveFileExW w/
+    # the MOVEFILE_REPLACE_EXISTING flag on Windows
+    # Windows invocation is abstracted behind additonal logic handling
+    # remaining cases of divergent behavior accross platforms
     if sys.platform == "win32":
-        # Windows path existence checks will sometimes fail on junctions/links/symlinks
+        _win_rename(src, dst)
-        # so check for that case
+    else:
-        if os.path.exists(dst) or islink(dst):
+        os.replace(src, dst)
-            os.remove(dst)
-    os.rename(src, dst)
 
 
 @system_path_filter

lib/spack/spack/test/llnl/util/filesystem.py

@@ -9,6 +9,7 @@
 import shutil
 import stat
 import sys
+from contextlib import contextmanager
 
 import pytest
 
@@ -908,3 +909,93 @@ def test_find_first_file(tmpdir, bfs_depth):
 
     # Should find first dir
     assert os.path.samefile(fs.find_first(root, "a", bfs_depth=bfs_depth), os.path.join(root, "a"))
+
+
+def test_rename_dest_exists(tmpdir):
+
+    @contextmanager
+    def setup_test_files():
+        a = tmpdir.join("a", "file1")
+        b = tmpdir.join("a", "file2")
+        fs.touchp(a)
+        fs.touchp(b)
+        with open(a, "w") as oa, open(b, "w") as ob:
+            oa.write("I am A")
+            ob.write("I am B")
+        yield a, b
+        shutil.rmtree(tmpdir.join("a"))
+
+    @contextmanager
+    def setup_test_dirs():
+        a = tmpdir.join("d", "a")
+        b = tmpdir.join("d", "b")
+        fs.mkdirp(a)
+        fs.mkdirp(b)
+        yield a, b
+        shutil.rmtree(tmpdir.join("d"))
+
+    # test standard behavior of rename
+    # smoke test
+    with setup_test_files() as files:
+        a, b = files
+        fs.rename(str(a), str(b))
+        assert os.path.exists(b)
+        assert not os.path.exists(a)
+        with open(b, "r") as ob:
+            content = ob.read()
+        assert content == "I am A"
+
+    # test relatitve paths
+    # another sanity check/smoke test
+    with setup_test_files() as files:
+        a, b = files
+        with fs.working_dir(str(tmpdir)):
+            fs.rename(os.path.join("a", "file1"), os.path.join("a", "file2"))
+            assert os.path.exists(b)
+            assert not os.path.exists(a)
+            with open(b, "r") as ob:
+                content = ob.read()
+            assert content == "I am A"
+
+    # Test rename symlinks to same file
+    c = tmpdir.join("a", "file1")
+    a = tmpdir.join("a", "link1")
+    b = tmpdir.join("a", "link2")
+    fs.touchp(c)
+    symlink(c, a)
+    symlink(c, b)
+    fs.rename(str(a), str(b))
+    assert os.path.exists(b)
+    assert not os.path.exists(a)
+    assert os.path.realpath(b) == c
+    shutil.rmtree(tmpdir.join("a"))
+
+    # test rename onto itself
+    a = tmpdir.join("a", "file1")
+    b = a
+    fs.touchp(a)
+    with open(a, "w") as oa:
+        oa.write("I am A")
+    fs.rename(str(a), str(b))
+    # check a, or b, doesn't matter, same file
+    assert os.path.exists(a)
+    # ensure original file was not duplicated
+    assert len(os.listdir(tmpdir.join("a"))) == 1
+    with open(a, "r") as oa:
+        assert oa.read()
+    shutil.rmtree(tmpdir.join("a"))
+
+    # test rename onto symlink
+    # to directory from symlink to directory
+    # (this is something spack does when regenerating views)
+    with setup_test_dirs() as dirs:
+        a, b = dirs
+        link1 = tmpdir.join("f", "link1")
+        link2 = tmpdir.join("f", "link2")
+        fs.mkdirp(tmpdir.join("f"))
+        symlink(a, link1)
+        symlink(b, link2)
+        fs.rename(str(link1), str(link2))
+        assert os.path.exists(link2)
+        assert os.path.realpath(link2) == a
+        shutil.rmtree(tmpdir.join("f"))