From 5d19ab0d8e9b734e3452c6e1210a7f2501709397 Mon Sep 17 00:00:00 2001
From: Trim21 <trim21.me@gmail.com>
Date: Mon, 19 Aug 2024 00:13:21 +0800
Subject: [PATCH] fix: keep lock file eol (#9468)
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Co-authored-by: Randy Döring <30527984+radoering@users.noreply.github.com>
---
 src/poetry/packages/locker.py | 29 +++++++++++++++++++++++++++--
 tests/packages/test_locker.py | 17 +++++++++++++++++
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/src/poetry/packages/locker.py b/src/poetry/packages/locker.py
index 420cd392d10..80177f16dd3 100644
--- a/src/poetry/packages/locker.py
+++ b/src/poetry/packages/locker.py
@@ -290,8 +290,33 @@ def _should_write(self, lock: TOMLDocument) -> bool:
         return do_write
 
     def _write_lock_data(self, data: TOMLDocument) -> None:
-        lockfile = TOMLFile(self.lock)
-        lockfile.write(data)
+        if self.lock.exists():
+            # The following code is roughly equivalent to
+            # • lockfile = TOMLFile(self.lock)
+            # • lockfile.read()
+            # • lockfile.write(data)
+            # However, lockfile.read() takes more than half a second even
+            # for a modestly sized project like Poetry itself and the only reason
+            # for reading the lockfile is to determine the line endings. Thus,
+            # we do that part for ourselves here, which only takes about 10 ms.
+
+            # get original line endings
+            with open(self.lock, encoding="utf-8", newline="") as f:
+                line = f.readline()
+            linesep = "\r\n" if line.endswith("\r\n") else "\n"
+
+            # enforce original line endings
+            content = data.as_string()
+            if linesep == "\n":
+                content = content.replace("\r\n", "\n")
+            elif linesep == "\r\n":
+                content = re.sub(r"(?<!\r)\n", "\r\n", content)
+            with open(self.lock, "w", encoding="utf-8", newline="") as f:
+                f.write(content)
+
+        else:
+            lockfile = TOMLFile(self.lock)
+            lockfile.write(data)
 
         self._lock_data = None
 
diff --git a/tests/packages/test_locker.py b/tests/packages/test_locker.py
index fd11eab8dec..0c3203b0b1d 100644
--- a/tests/packages/test_locker.py
+++ b/tests/packages/test_locker.py
@@ -1231,3 +1231,20 @@ def test_lockfile_is_not_rewritten_if_only_poetry_version_changed(
         content = f.read()
 
     assert content == old_content
+
+
+def test_lockfile_keep_eol(locker: Locker, root: ProjectPackage) -> None:
+    sep = "\n" if os.linesep == "\r\n" else "\r\n"
+
+    with open(locker.lock, "wb") as f:
+        f.write((sep * 10).encode())
+
+    assert locker.set_lock_data(root, [Package("test", version="0.0.1")])
+
+    with locker.lock.open(encoding="utf-8", newline="") as f:
+        line, *_ = f.read().splitlines(keepends=True)
+
+    if sep == "\r\n":
+        assert line.endswith("\r\n")
+    else:
+        assert not line.endswith("\r\n")