gh-158345: Don't grow an array when append, insert or extend overflows

ins1() checks the item with setitem(self, -1, v) before growing the
array, but since gh-156865 the 'e', 'f' and 'Zf' setters detect an
overflow only when they pack into the array, which that call skips.
The array then grew by one item before the real store failed.

Pack into a local buffer before the bounds check, so the check call
fails too, and copy the result into the array only for a real index.

Author: fedonman

Lib/test/test_array.py

@@ -1612,13 +1612,26 @@ def test_overflows(self):
         self.assertRaises(OverflowError, array.array, self.typecode, [123456])
         # Overflows also float type:
         self.assertRaises(OverflowError, array.array, self.typecode, [1e300])
+        # A failed append, insert or extend leaves the array unchanged:
+        for x in 123456, 1e300:
+            a = array.array(self.typecode, [1])
+            self.assertRaises(OverflowError, a.append, x)
+            self.assertRaises(OverflowError, a.insert, 0, x)
+            self.assertRaises(OverflowError, a.extend, [x])
+            self.assertEqual(a, array.array(self.typecode, [1]))
 
 class FloatTest(FPTest, unittest.TestCase):
     typecode = 'f'
     minitemsize = 4
 
     def test_overflows(self):
         self.assertRaises(OverflowError, array.array, self.typecode, [1e300])
+        # A failed append, insert or extend leaves the array unchanged:
+        a = array.array(self.typecode, [1])
+        self.assertRaises(OverflowError, a.append, 1e300)
+        self.assertRaises(OverflowError, a.insert, 0, 1e300)
+        self.assertRaises(OverflowError, a.extend, [1e300])
+        self.assertEqual(a, array.array(self.typecode, [1]))
 
 class DoubleTest(FPTest, unittest.TestCase):
     typecode = 'd'
@@ -1649,6 +1662,13 @@ class ComplexFloatTest(CFPTest, unittest.TestCase):
     def test_overflows(self):
         self.assertRaises(OverflowError, array.array, self.typecode, [1e300])
         self.assertRaises(OverflowError, array.array, self.typecode, [1e300j])
+        # A failed append, insert or extend leaves the array unchanged:
+        for x in 1e300, 1e300j:
+            a = array.array(self.typecode, [1])
+            self.assertRaises(OverflowError, a.append, x)
+            self.assertRaises(OverflowError, a.insert, 0, x)
+            self.assertRaises(OverflowError, a.extend, [x])
+            self.assertEqual(a, array.array(self.typecode, [1]))
 
 class ComplexDoubleTest(CFPTest, unittest.TestCase):
     typecode = 'Zd'

Misc/NEWS.d/next/Library/2026-09-28-13-52-36.gh-issue-158345.GNlDCF.rst

@@ -0,0 +1,3 @@
+Fix :meth:`array.array.append`, :meth:`~array.array.insert` and
+:meth:`~array.array.extend` leaving an extra item in the array when they
+raise :exc:`OverflowError` for the ``'e'``, ``'f'`` or ``'Zf'`` type code.

Modules/arraymodule.c

@@ -585,15 +585,18 @@ static int
 e_setitem(arrayobject *ap, Py_ssize_t i, PyObject *v)
 {
     double x;
+    char buf[sizeof(short)];
     if (!PyArg_Parse(v, "d;array item must be float", &x)) {
         return -1;
     }
+    if (PyFloat_Pack2(x, buf, PY_LITTLE_ENDIAN) < 0) {
+        return -1;
+    }
 
     CHECK_ARRAY_BOUNDS(ap, i);
 
     if (i >= 0) {
-        return PyFloat_Pack2(x, ap->ob_item + sizeof(short)*i,
-                             PY_LITTLE_ENDIAN);
+        memcpy(ap->ob_item + sizeof(short)*i, buf, sizeof(buf));
     }
     return 0;
 }
@@ -608,14 +611,17 @@ static int
 f_setitem(arrayobject *ap, Py_ssize_t i, PyObject *v)
 {
     double x;
+    char buf[sizeof(float)];
     if (!PyArg_Parse(v, "d;array item must be float", &x))
         return -1;
+    if (PyFloat_Pack4(x, buf, PY_LITTLE_ENDIAN) < 0) {
+        return -1;
+    }
 
     CHECK_ARRAY_BOUNDS(ap, i);
 
     if (i >= 0) {
-        return PyFloat_Pack4(x, ap->ob_item + sizeof(float)*i,
-                             PY_LITTLE_ENDIAN);
+        memcpy(ap->ob_item + sizeof(float)*i, buf, sizeof(buf));
     }
     return 0;
 }
@@ -653,25 +659,21 @@ static int
 cf_setitem(arrayobject *ap, Py_ssize_t i, PyObject *v)
 {
     Py_complex x;
+    char f[8];
 
     if (!PyArg_Parse(v, "D;array item must be complex", &x)) {
         return -1;
     }
+    if (PyFloat_Pack4(x.real, f, PY_LITTLE_ENDIAN) < 0
+        || PyFloat_Pack4(x.imag, f + sizeof(float), PY_LITTLE_ENDIAN) < 0)
+    {
+        return -1;
+    }
 
     CHECK_ARRAY_BOUNDS(ap, i);
 
     if (i >= 0) {
-        char f[8];
-        int ret = PyFloat_Pack4(x.real, f, PY_LITTLE_ENDIAN);
-
-        if (ret) {
-            return ret;
-        }
-        ret = PyFloat_Pack4(x.imag, f + sizeof(float), PY_LITTLE_ENDIAN);
-        if (!ret) {
-            memcpy(ap->ob_item + i*sizeof(f), &f, sizeof(f));
-        }
-        return ret;
+        memcpy(ap->ob_item + i*sizeof(f), &f, sizeof(f));
     }
     return 0;
 }