gh-143050: Correct PyLong_FromString() to use _PyLong_Negate() (#145901)

The long_from_string_base() might return a small integer, when the
_pylong.py is used to do conversion.  Hence, we must be careful here to
not smash it "small int" bit by using the _PyLong_FlipSign().

Co-authored-by: Victor Stinner <vstinner@python.org>

Include/internal/pycore_long.h

diff --git a/Include/internal/pycore_long.h b/Include/internal/pycore_long.h
index 4386e8b..5ef9cc4 100644
--- a/Include/internal/pycore_long.h
+++ b/Include/internal/pycore_long.h
@@ -232,6 +232,20 @@ _PyLong_IsPositive(const PyLongObject *op)
     return (op->long_value.lv_tag & SIGN_MASK) == 0;
 }
 
+/* Return true if the argument is a small int */
+static inline bool
+_PyLong_IsSmallInt(const PyLongObject *op)
+{
+    assert(PyLong_Check(op));
+    bool is_small_int = (op->long_value.lv_tag & IMMORTALITY_BIT_MASK) != 0;
+    assert(PyLong_CheckExact(op) || (!is_small_int));
+    assert(_Py_IsImmortal(op) || (!is_small_int));
+    assert((_PyLong_IsCompact(op)
+            && _PY_IS_SMALL_INT(_PyLong_CompactValue(op)))
+           || (!is_small_int));
+    return is_small_int;
+}
+
 static inline Py_ssize_t
 _PyLong_DigitCount(const PyLongObject *op)
 {
@@ -293,7 +307,9 @@ _PyLong_SetDigitCount(PyLongObject *op, Py_ssize_t size)
 #define NON_SIZE_MASK ~(uintptr_t)((1 << NON_SIZE_BITS) - 1)
 
 static inline void
-_PyLong_FlipSign(PyLongObject *op) {
+_PyLong_FlipSign(PyLongObject *op)
+{
+    assert(!_PyLong_IsSmallInt(op));
     unsigned int flipped_sign = 2 - (op->long_value.lv_tag & SIGN_MASK);
     op->long_value.lv_tag &= NON_SIZE_MASK;
     op->long_value.lv_tag |= flipped_sign;

Lib/test/test_capi/test_long.py

diff --git a/Lib/test/test_capi/test_long.py b/Lib/test/test_capi/test_long.py
index d315664..fc0454b 100644
--- a/Lib/test/test_capi/test_long.py
+++ b/Lib/test/test_capi/test_long.py
@@ -803,6 +803,16 @@ def to_digits(num):
                 self.assertEqual(pylongwriter_create(negative, digits), num,
                                  (negative, digits))
 
+    def test_bug_143050(self):
+        with support.adjust_int_max_str_digits(0):
+            # Bug coming from using _pylong.int_from_string(), that
+            # currently requires > 6000 decimal digits.
+            int('-' + '0' * 7000, 10)
+            _testcapi.test_immortal_small_ints()
+            # Test also nonzero small int
+            int('-' + '0' * 7000 + '123', 10)
+            _testcapi.test_immortal_small_ints()
+
 
 if __name__ == "__main__":
     unittest.main()

Modules/_testcapi/immortal.c

diff --git a/Modules/_testcapi/immortal.c b/Modules/_testcapi/immortal.c
index af510ca..1c87025 100644
--- a/Modules/_testcapi/immortal.c
+++ b/Modules/_testcapi/immortal.c
@@ -31,13 +31,13 @@ test_immortal_small_ints(PyObject *self, PyObject *Py_UNUSED(ignored))
     for (int i = -5; i <= 1024; i++) {
         PyObject *obj = PyLong_FromLong(i);
         assert(verify_immortality(obj));
-        int has_int_immortal_bit = ((PyLongObject *)obj)->long_value.lv_tag & IMMORTALITY_BIT_MASK;
+        int has_int_immortal_bit = _PyLong_IsSmallInt((PyLongObject *)obj);
         assert(has_int_immortal_bit);
     }
     for (int i = 1025; i <= 1030; i++) {
         PyObject *obj = PyLong_FromLong(i);
         assert(obj);
-        int has_int_immortal_bit = ((PyLongObject *)obj)->long_value.lv_tag & IMMORTALITY_BIT_MASK;
+        int has_int_immortal_bit = _PyLong_IsSmallInt((PyLongObject *)obj);
         assert(!has_int_immortal_bit);
         Py_DECREF(obj);
     }

Objects/longobject.c

diff --git a/Objects/longobject.c b/Objects/longobject.c
index 0d3ea9b..d416fc1 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -3119,11 +3119,11 @@ PyLong_FromString(const char *str, char **pend, int base)
     }
 
     /* Set sign and normalize */
-    if (sign < 0) {
-        _PyLong_FlipSign(z);
-    }
     long_normalize(z);
     z = maybe_small_long(z);
+    if (sign < 0) {
+        _PyLong_Negate(&z);
+    }
 
     if (pend != NULL) {
         *pend = (char *)str;
@@ -3623,21 +3623,11 @@ long_richcompare(PyObject *self, PyObject *other, int op)
     Py_RETURN_RICHCOMPARE(result, 0, op);
 }
 
-static inline int
-/// Return 1 if the object is one of the immortal small ints
-_long_is_small_int(PyObject *op)
-{
-    PyLongObject *long_object = (PyLongObject *)op;
-    int is_small_int = (long_object->long_value.lv_tag & IMMORTALITY_BIT_MASK) != 0;
-    assert((!is_small_int) || PyLong_CheckExact(op));
-    return is_small_int;
-}
-
 void
 _PyLong_ExactDealloc(PyObject *self)
 {
     assert(PyLong_CheckExact(self));
-    if (_long_is_small_int(self)) {
+    if (_PyLong_IsSmallInt((PyLongObject *)self)) {
         // See PEP 683, section Accidental De-Immortalizing for details
         _Py_SetImmortal(self);
         return;
@@ -3652,7 +3642,7 @@ _PyLong_ExactDealloc(PyObject *self)
 static void
 long_dealloc(PyObject *self)
 {
-    if (_long_is_small_int(self)) {
+    if (_PyLong_IsSmallInt((PyLongObject *)self)) {
         /* This should never get called, but we also don't want to SEGV if
          * we accidentally decref small Ints out of existence. Instead,
          * since small Ints are immortal, re-set the reference count.