Fix generate_system_pyproject newline issues

src/poetry/console/commands/self/self_command.py

@@ -77,7 +77,8 @@ def generate_system_pyproject(self) -> None:
         for key in preserved:
             content["tool"]["poetry"][key] = preserved[key]  # type: ignore[index]
 
-        self.system_pyproject.write_text(content.as_string(), encoding="utf-8")
+        pyproject = PyProjectTOML(self.system_pyproject)
+        pyproject.file.write(content)
 
     def reset_poetry(self) -> None:
         with directory(self.system_pyproject.parent):

src/poetry/factory.py

@@ -290,7 +290,6 @@ def create_pyproject_from_package(
             content["extras"] = extras_section
 
         pyproject = cast("TOMLDocument", pyproject)
-        pyproject.add(tomlkit.nl())
 
         if path:
             path.joinpath("pyproject.toml").write_text(

tests/console/commands/self/test_system_pyproject.py

@@ -0,0 +1,53 @@
+from __future__ import annotations
+
+import pytest
+
+from poetry.core.packages.dependency import Dependency
+from poetry.core.packages.package import Package
+from poetry.core.packages.project_package import ProjectPackage
+
+from poetry.__version__ import __version__
+from poetry.console.commands.self.self_command import SelfCommand
+from poetry.factory import Factory
+
+
+@pytest.fixture
+def example_system_pyproject():
+    package = ProjectPackage("poetry-instance", __version__)
+    plugin = Package("poetry-plugin", "1.2.3")
+
+    package.add_dependency(
+        Dependency(plugin.name, "^1.2.3", groups=[SelfCommand.ADDITIONAL_PACKAGE_GROUP])
+    )
+    content = Factory.create_pyproject_from_package(package)
+    return content.as_string().strip("\n")
+
+
+@pytest.mark.parametrize("existing_newlines", [0, 2])
+def test_generate_system_pyproject_trailing_newline(
+    existing_newlines: int,
+    example_system_pyproject: str,
+):
+    cmd = SelfCommand()
+    cmd.system_pyproject.write_text(example_system_pyproject + "\n" * existing_newlines)
+    cmd.generate_system_pyproject()
+    generated = cmd.system_pyproject.read_text()
+
+    for _i, c in enumerate(generated[::-1]):
+        if c != "\n":
+            break
+
+    assert _i == existing_newlines
+
+
+def test_generate_system_pyproject_carraige_returns(
+    example_system_pyproject: str,
+):
+    cmd = SelfCommand()
+    cmd.system_pyproject.write_text(example_system_pyproject + "\n")
+    cmd.generate_system_pyproject()
+
+    with open(cmd.system_pyproject, newline="") as f:  # do not translate newlines
+        generated = f.read()
+
+    assert "\r\r" not in generated