From 85262187a1023fec2e0b14554c2c0bff30ae67ba Mon Sep 17 00:00:00 2001
From: Johannes Gezachew <yohannis995@gmail.com>
Date: Mon, 14 Sep 2026 21:17:24 +0300
Subject: [PATCH] gh-157379: Fix crash in csv.reader with re-entrant iterator
 at end of input

The gh-145105 guard only runs after the input iterator returns a line.
When a re-entrant call leaves a quoted field open and the outer call then
reaches the end of input, parse_save_field() appended to the NULL fields
list. Raise csv.Error in that branch too.
---
 Lib/test/test_csv.py                          | 24 +++++++++++++++++++
 ...-09-14-18-15-54.gh-issue-157379.Tq4vRw.rst |  2 ++
 Modules/_csv.c                                |  4 ++++
 3 files changed, 30 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-09-14-18-15-54.gh-issue-157379.Tq4vRw.rst

diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 36fa7e3572a5ac2..1925d655ae4fb09 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -598,6 +598,30 @@ def __next__(self):
         with self.assertRaises(csv.Error):
             next(reader)
 
+    def test_reader_reentrant_iterator_eof_in_quoted_field(self):
+        # gh-157379: the re-entrant call leaves an open quoted field, then
+        # the outer call reaches the end of input.
+        class ReentrantIter:
+            def __init__(self):
+                self.reader = None
+                self.n = 0
+            def __iter__(self):
+                return self
+            def __next__(self):
+                self.n += 1
+                if self.n == 1:
+                    next(self.reader)
+                    raise StopIteration
+                if self.n == 2:
+                    return '"x'
+                raise StopIteration
+
+        it = ReentrantIter()
+        reader = csv.reader(it)
+        it.reader = reader
+        with self.assertRaises(csv.Error):
+            next(reader)
+
 
 class TestDialectRegistry(unittest.TestCase):
     def test_registry_badargs(self):
diff --git a/Misc/NEWS.d/next/Library/2026-09-14-18-15-54.gh-issue-157379.Tq4vRw.rst b/Misc/NEWS.d/next/Library/2026-09-14-18-15-54.gh-issue-157379.Tq4vRw.rst
new file mode 100644
index 000000000000000..be20c3f7ba044b9
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-09-14-18-15-54.gh-issue-157379.Tq4vRw.rst
@@ -0,0 +1,2 @@
+Fix a crash in :mod:`csv` reader when a re-entrant iterator reaches the end
+of input while a quoted field is still open.
diff --git a/Modules/_csv.c b/Modules/_csv.c
index 6af66c3f09a03b1..8bf32e4d4454f4e 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -1029,6 +1029,10 @@ Reader_iternext_lock_held(PyObject *op)
                 if (self->dialect->strict)
                     PyErr_SetString(module_state->error_obj,
                                     "unexpected end of data");
+                else if (self->fields == NULL)
+                    PyErr_SetString(module_state->error_obj,
+                                    "iterator has already advanced "
+                                    "the reader");
                 else if (parse_save_field(self) >= 0)
                     break;
             }