fix: Crash when update replaces file with symlink (or vice versa)

Previously if an update would try to replace a file with a symlink
copier would crash. If an update tried to replace a symlink with a file
it would just overwrite the symlink target. Now both cases work as
expected.

copier/main.py

@@ -373,9 +373,10 @@ def _render_allowed(
         dst_abspath = Path(self.subproject.local_abspath, dst_relpath)
         if dst_relpath != Path(".") and self.match_exclude(dst_relpath):
             return False
+        previous_is_symlink = dst_abspath.is_symlink()
         try:
             previous_content: Union[bytes, Path]
-            if is_symlink:
+            if previous_is_symlink:
                 previous_content = readlink(dst_abspath)
             else:
                 previous_content = dst_abspath.read_bytes()
@@ -394,7 +395,9 @@ def _render_allowed(
                 raise
         except IsADirectoryError:
             assert is_dir
-        if is_dir or previous_content == expected_contents:
+        if is_dir or (
+            previous_content == expected_contents and previous_is_symlink == is_symlink
+        ):
             printf(
                 "identical",
                 dst_relpath,
@@ -573,6 +576,10 @@ def _render_file(self, src_abspath: Path) -> None:
             return
         if not self.pretend:
             dst_abspath.parent.mkdir(parents=True, exist_ok=True)
+            if dst_abspath.is_symlink():
+                # Writing to a symlink just writes to its target, so if we want to
+                # replace a symlink with a file we have to unlink it first
+                dst_abspath.unlink()
             dst_abspath.write_bytes(new_content)
             dst_abspath.chmod(src_mode)
 

tests/test_symlinks.py

@@ -293,6 +293,67 @@ def test_update_symlink(tmp_path_factory: pytest.TempPathFactory) -> None:
     assert readlink(dst / "symlink.txt") == Path("bbbb.txt")
 
 
+def test_update_file_to_symlink(tmp_path_factory: pytest.TempPathFactory) -> None:
+    src, dst = map(tmp_path_factory.mktemp, ("src", "dst"))
+
+    build_file_tree(
+        {
+            src
+            / ".copier-answers.yml.jinja": """\
+                # Changes here will be overwritten by Copier
+                {{ _copier_answers|to_nice_yaml }}
+            """,
+            src
+            / "copier.yml": """\
+                _preserve_symlinks: true
+            """,
+            src
+            / "aaaa.txt": """
+                Lorem ipsum
+            """,
+            src
+            / "bbbb.txt": """
+                dolor sit amet
+            """,
+            src / "cccc.txt": Path("./aaaa.txt"),
+        }
+    )
+
+    with local.cwd(src):
+        git("init")
+        git("add", "-A")
+        git("commit", "-m", "first commit on src")
+
+    run_copy(str(src), dst, defaults=True, overwrite=True)
+
+    with local.cwd(src):
+        # test updating a symlink
+        os.remove("bbbb.txt")
+        os.symlink("aaaa.txt", "bbbb.txt")
+        os.remove("cccc.txt")
+        with open("cccc.txt", "w+") as f:
+            f.write("Lorem ipsum")
+
+    # dst must be vcs-tracked to use run_update
+    with local.cwd(dst):
+        git("init")
+        git("add", "-A")
+        git("commit", "-m", "first commit on dst")
+
+    # make sure changes have not yet propagated
+    assert not (dst / "bbbb.txt").is_symlink()
+    assert (dst / "cccc.txt").is_symlink()
+
+    with pytest.warns(DirtyLocalWarning):
+        run_update(dst, defaults=True, overwrite=True)
+
+    # make sure changes propagate after update
+    assert (dst / "bbbb.txt").is_symlink()
+    assert readlink(dst / "bbbb.txt") == Path("aaaa.txt")
+    assert not (dst / "cccc.txt").is_symlink()
+    assert (dst / "cccc.txt").read_text() == "Lorem ipsum"
+
+
 def test_exclude_symlink(tmp_path_factory: pytest.TempPathFactory) -> None:
     src, dst = map(tmp_path_factory.mktemp, ("src", "dst"))
     # Prepare repo bundle