[clang] [clang] Auto-detect which newline style to use for cxx_dr_status.html (PR #132045)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll created https://github.com/llvm/llvm-project/pull/132045

Aaron reported that `make_cxx_dr_status` replaces all newlines in `cxx_dr_status.html`, which makes for a huge diff. On Windows, we can't be compatible with all `autocrlf` modes at once, so this patch adds autodetection of newline style using the existing file, if one is present (which should be the case for all reasonable use cases).

>From 55069b8e9bc38d5ac6d2f0325baa63176e3cc881 Mon Sep 17 00:00:00 2001
From: Vlad Serebrennikov <serebrennikov.vladislav at gmail.com>
Date: Wed, 19 Mar 2025 17:57:52 +0300
Subject: [PATCH] [clang] Auto-detect which newline style to use for
 `cxx_dr_status.html`

Aaron reported that `make_cxx_dr_status` replaces all newlines in `cxx_dr_status.html`, which makes for a huge diff. On Windows, we can't be compatible with all `autocrlf` modes at once, so this patch adds autodetection of newline style using the existing file, if one is present (which should be the case for all reasonable use cases).
---
 clang/www/make_cxx_dr_status | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/clang/www/make_cxx_dr_status b/clang/www/make_cxx_dr_status
index c787bbaff0a36..a4b87b32fc5d8 100755
--- a/clang/www/make_cxx_dr_status
+++ b/clang/www/make_cxx_dr_status
@@ -318,6 +318,22 @@ out_html.append('''\
 </html>
 ''')
 
-out_file = open(output, 'w')
+# Make an effort to remain consistent with the existing file.
+# We can't pick one newline style and use it on Windows,
+# because we can't be compatible with all 'autocrlf' modes at once.
+def detect_newline_style(file_path):
+  if not os.path.exists(file_path):
+    return '\n'
+  f = open(file_path)
+  f.readline()
+  if f.newlines is None:
+    return '\n'
+  if isinstance(f.newlines, str):
+    return f.newlines
+  newline = f.newlines[0]
+  print(f"Existing '{file_path}' has inconsistent newlines; picking '{newline.encode('unicode_escape').decode('utf-8')}'")
+  return newline
+    
+out_file = open(output, 'w', newline=detect_newline_style(output))
 out_file.write(''.join(out_html))
 out_file.close()