From 2eb60c1934f47671e6b3c9b90b6d9f1912d829a0 Mon Sep 17 00:00:00 2001
From: Chris Markiewicz <effigies@gmail.com>
Date: Thu, 24 Aug 2023 04:23:01 -0400
Subject: [PATCH] gh-108111: Flush gzip write buffer before seeking, fixing bad
 writes (#108341)
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Co-authored-by: Łukasz Langa <lukasz@langa.pl>
---
 Lib/gzip.py                                          |  3 +++
 Lib/test/test_gzip.py                                | 12 ++++++++++++
 Misc/ACKS                                            |  1 +
 .../2023-08-22-17-27-12.gh-issue-108111.N7a4u_.rst   |  2 ++
 4 files changed, 18 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2023-08-22-17-27-12.gh-issue-108111.N7a4u_.rst

diff --git a/Lib/gzip.py b/Lib/gzip.py
index cf8b675064ce89..177f9080dc5af8 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -401,6 +401,9 @@ def seekable(self):
 
     def seek(self, offset, whence=io.SEEK_SET):
         if self.mode == WRITE:
+            self._check_not_closed()
+            # Flush buffer to ensure validity of self.offset
+            self._buffer.flush()
             if whence != io.SEEK_SET:
                 if whence == io.SEEK_CUR:
                     offset = self.offset + offset
diff --git a/Lib/test/test_gzip.py b/Lib/test/test_gzip.py
index b06b3b09411d62..128f933787a3f6 100644
--- a/Lib/test/test_gzip.py
+++ b/Lib/test/test_gzip.py
@@ -665,6 +665,18 @@ def flush(self, mode=-1):
         ]
         self.assertEqual(fc.modes, expected_modes)
 
+    def test_write_seek_write(self):
+        # Make sure that offset is up-to-date before seeking
+        # See issue GH-108111
+        b = io.BytesIO()
+        message = b"important message here."
+        with gzip.GzipFile(fileobj=b, mode='w') as f:
+            f.write(message)
+            f.seek(len(message))
+            f.write(message)
+        data = b.getvalue()
+        self.assertEqual(gzip.decompress(data), message * 2)
+
 
 class TestOpen(BaseTest):
     def test_binary_modes(self):
diff --git a/Misc/ACKS b/Misc/ACKS
index 475c6ec3dab73b..70c4ea37adda75 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1150,6 +1150,7 @@ Colin Marc
 Vincent Marchetti
 David Marek
 Doug Marien
+Chris Markiewicz
 Sven Marnach
 John Marshall
 Alex Martelli
diff --git a/Misc/NEWS.d/next/Library/2023-08-22-17-27-12.gh-issue-108111.N7a4u_.rst b/Misc/NEWS.d/next/Library/2023-08-22-17-27-12.gh-issue-108111.N7a4u_.rst
new file mode 100644
index 00000000000000..8eafa6cfbbf8cf
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-08-22-17-27-12.gh-issue-108111.N7a4u_.rst
@@ -0,0 +1,2 @@
+Fix a regression introduced in GH-101251 for 3.12, resulting in an incorrect
+offset calculation in :meth:`gzip.GzipFile.seek`.