bpo-31338 (python#3374)

* Add Py_UNREACHABLE() as an alias to abort().
* Use Py_UNREACHABLE() instead of assert(0)
* Convert more unreachable code to use Py_UNREACHABLE()
* Document Py_UNREACHABLE() and a few other macros.

Doc/c-api/intro.rst

@@ -17,11 +17,11 @@ common use.  The second reason is to use Python as a component in a larger
 application; this technique is generally referred to as :dfn:`embedding` Python
 in an application.
 
-Writing an extension module is a relatively well-understood process,  where a
-"cookbook" approach works well.  There are several tools  that automate the
-process to some extent.  While people have embedded  Python in other
-applications since its early existence, the process of  embedding Python is less
-straightforward than writing an extension.
+Writing an extension module is a relatively well-understood process, where a
+"cookbook" approach works well.  There are several tools that automate the
+process to some extent.  While people have embedded Python in other
+applications since its early existence, the process of embedding Python is
+less straightforward than writing an extension.
 
 Many API functions are useful independent of whether you're embedding  or
 extending Python; moreover, most applications that embed Python  will need to
@@ -30,6 +30,16 @@ familiar with writing an extension before  attempting to embed Python in a real
 application.
 
 
+Coding standards
+================
+
+If you're writing C code for inclusion in CPython, you **must** follow the
+guidelines and standards defined in :PEP:`7`.  These guidelines apply
+regardless of the version of Python you are contributing to.  Following these
+conventions is not necessary for your own third party extension modules,
+unless you eventually expect to contribute them to Python.
+
+
 .. _api-includes:
 
 Include Files
@@ -81,6 +91,48 @@ header files do properly declare the entry points to be ``extern "C"``, so there
 is no need to do anything special to use the API from C++.
 
 
+Useful macros
+=============
+
+Several useful macros are defined in the Python header files.  Many are
+defined closer to where they are useful (e.g. :c:macro:`Py_RETURN_NONE`).
+Others of a more general utility are defined here.  This is not necessarily a
+complete listing.
+
+.. c:macro:: Py_UNREACHABLE()
+
+   Use this when you have a code path that you do not expect to be reached.
+   For example, in the ``default:`` clause in a ``switch`` statement for which
+   all possible values are covered in ``case`` statements.  Use this in places
+   where you might be tempted to put an ``assert(0)`` or ``abort()`` call.
+
+.. c:macro:: Py_ABS(x)
+
+   Return the absolute value of ``x``.
+
+.. c:macro:: Py_MIN(x, y)
+
+   Return the minimum value between ``x`` and ``y``.
+
+.. c:macro:: Py_MAX(x, y)
+
+   Return the maximum value between ``x`` and ``y``.
+
+.. c:macro:: Py_STRINGIFY(x)
+
+   Convert ``x`` to a C string.  E.g. ``Py_STRINGIFY(123)`` returns
+   ``"123"``.
+
+.. c:macro:: Py_MEMBER_SIZE(type, member)
+
+   Return the size of a structure (``type``) ``member`` in bytes.
+
+.. c:macro:: Py_CHARMASK(c)
+
+   Argument must be a character or an integer in the range [-128, 127] or [0,
+   255].  This macro returns ``c`` cast to an ``unsigned char``.
+
+
 .. _api-objects:
 
 Objects, Types and Reference Counts

Include/pymacro.h

@@ -95,4 +95,6 @@
 #define Py_UNUSED(name) _unused_ ## name
 #endif
 
+#define Py_UNREACHABLE() abort()
+
 #endif /* Py_PYMACRO_H */

Misc/NEWS.d/next/C API/2017-09-05-17-51-12.bpo-31338.LjA43Y.rst

@@ -0,0 +1,3 @@
+Added the ``Py_UNREACHABLE()`` macro for code paths which are never expected
+to be reached.  This and a few other useful macros are now documented in the
+C API manual.

Modules/_datetimemodule.c

@@ -1453,8 +1453,7 @@ diff_to_bool(int diff, int op)
         case Py_LT: istrue = diff < 0; break;
         case Py_GT: istrue = diff > 0; break;
         default:
-            assert(! "op unknown");
-            istrue = 0; /* To shut up compiler */
+            Py_UNREACHABLE();
     }
     result = istrue ? Py_True : Py_False;
     Py_INCREF(result);

Modules/_pickle.c

@@ -750,8 +750,7 @@ _PyMemoTable_Lookup(PyMemoTable *self, PyObject *key)
         if (entry->me_key == NULL || entry->me_key == key)
             return entry;
     }
-    assert(0);  /* Never reached */
-    return NULL;
+    Py_UNREACHABLE();
 }
 
 /* Returns -1 on failure, 0 on success. */

Modules/_tracemalloc.c

@@ -726,7 +726,7 @@ tracemalloc_realloc(void *ctx, void *ptr, size_t new_size)
 
                The GIL and the table lock ensures that only one thread is
                allocating memory. */
-            assert(0 && "should never happen");
+            Py_UNREACHABLE();
         }
         TABLES_UNLOCK();
     }

Modules/mathmodule.c

@@ -105,8 +105,7 @@ sinpi(double x)
         r = sin(pi*(y-2.0));
         break;
     default:
-        assert(0);  /* should never get here */
-        r = -1.23e200; /* silence gcc warning */
+        Py_UNREACHABLE();
     }
     return copysign(1.0, x)*r;
 }

Objects/abstract.c

@@ -1984,7 +1984,7 @@ _PySequence_IterSearch(PyObject *seq, PyObject *obj, int operation)
                 goto Done;
 
             default:
-                assert(!"unknown operation");
+                Py_UNREACHABLE();
             }
         }
 

Objects/bytesobject.c

@@ -868,7 +868,7 @@ _PyBytes_FormatEx(const char *format, Py_ssize_t format_len,
                     switch(c)
                     {
                         default:
-                            assert(0 && "'type' not in [diuoxX]");
+                            Py_UNREACHABLE();
                         case 'd':
                         case 'i':
                         case 'u':

Objects/dictobject.c

@@ -643,8 +643,7 @@ lookdict_index(PyDictKeysObject *k, Py_hash_t hash, Py_ssize_t index)
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
     }
-    assert(0);          /* NOT REACHED */
-    return DKIX_ERROR;
+    Py_UNREACHABLE();
 }
 
 /*
@@ -723,8 +722,7 @@ lookdict(PyDictObject *mp, PyObject *key,
         perturb >>= PERTURB_SHIFT;
         i = (i*5 + perturb + 1) & mask;
     }
-    assert(0);          /* NOT REACHED */
-    return 0;
+    Py_UNREACHABLE();
 }
 
 /* Specialized version for string-only keys */
@@ -766,9 +764,7 @@ lookdict_unicode(PyDictObject *mp, PyObject *key,
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
     }
-
-    assert(0);          /* NOT REACHED */
-    return 0;
+    Py_UNREACHABLE();
 }
 
 /* Faster version of lookdict_unicode when it is known that no <dummy> keys
@@ -810,8 +806,7 @@ lookdict_unicode_nodummy(PyDictObject *mp, PyObject *key,
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
     }
-    assert(0);          /* NOT REACHED */
-    return 0;
+    Py_UNREACHABLE();
 }
 
 /* Version of lookdict for split tables.
@@ -856,8 +851,7 @@ lookdict_split(PyDictObject *mp, PyObject *key,
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
     }
-    assert(0);          /* NOT REACHED */
-    return 0;
+    Py_UNREACHABLE();
 }
 
 int
@@ -3603,7 +3597,7 @@ dictiter_reduce(dictiterobject *di)
         else if (Py_TYPE(di) == &PyDictIterValue_Type)
             element = dictiter_iternextvalue(&tmp);
         else
-            assert(0);
+            Py_UNREACHABLE();
         if (element) {
             if (PyList_Append(list, element)) {
                 Py_DECREF(element);

Objects/longobject.c

@@ -1806,8 +1806,7 @@ long_format_binary(PyObject *aa, int base, int alternate,
         bits = 1;
         break;
     default:
-        assert(0); /* shouldn't ever get here */
-        bits = 0; /* to silence gcc warning */
+        Py_UNREACHABLE();
     }
 
     /* Compute exact length 'sz' of output string. */
@@ -2169,8 +2168,8 @@ PyLong_FromString(const char *str, char **pend, int base)
         }
     }
     if (str[0] == '_') {
-	    /* May not start with underscores. */
-	    goto onError;
+        /* May not start with underscores. */
+        goto onError;
     }
 
     start = str;

Objects/stringlib/eq.h

@@ -10,8 +10,7 @@ unicode_eq(PyObject *aa, PyObject *bb)
     PyUnicodeObject *b = (PyUnicodeObject *)bb;
 
     if (PyUnicode_READY(a) == -1 || PyUnicode_READY(b) == -1) {
-        assert(0 && "unicode_eq ready fail");
-        return 0;
+        Py_UNREACHABLE();
     }
 
     if (PyUnicode_GET_LENGTH(a) != PyUnicode_GET_LENGTH(b))