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.

Author: JohannesGezachew

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):

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.

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;
             }