bpo-29564:_PyMem_DumpTraceback() suggests enabling tracemalloc (GH-10510)

If tracemalloc is not tracing Python memory allocations,
_PyMem_DumpTraceback() now suggests to enable tracemalloc
to get the traceback where the memory block has been allocated.

Lib/test/test_capi.py

@@ -486,6 +486,8 @@ class PyMemDebugTests(unittest.TestCase):
                  r"    The block was made by call #[0-9]+ to debug malloc/realloc.\n"
                  r"    Data at p: cb cb cb .*\n"
                  r"\n"
+                 r"Enable tracemalloc to get the memory block allocation traceback\n"
+                 r"\n"
                  r"Fatal Python error: bad trailing pad byte")
         regex = regex.format(ptr=self.PTR_REGEX)
         regex = re.compile(regex, flags=re.DOTALL)
@@ -500,6 +502,8 @@ class PyMemDebugTests(unittest.TestCase):
                  r"    The block was made by call #[0-9]+ to debug malloc/realloc.\n"
                  r"    Data at p: cb cb cb .*\n"
                  r"\n"
+                 r"Enable tracemalloc to get the memory block allocation traceback\n"
+                 r"\n"
                  r"Fatal Python error: bad ID: Allocated using API 'm', verified using API 'r'\n")
         regex = regex.format(ptr=self.PTR_REGEX)
         self.assertRegex(out, regex)

Modules/_tracemalloc.c

@@ -1471,6 +1471,12 @@ _PyMem_DumpTraceback(int fd, const void *ptr)
     traceback_t *traceback;
     int i;
 
+    if (!_Py_tracemalloc_config.tracing) {
+        PUTS(fd, "Enable tracemalloc to get the memory block "
+                 "allocation traceback\n\n");
+        return;
+    }
+
     traceback = tracemalloc_get_traceback(DEFAULT_DOMAIN, (uintptr_t)ptr);
     if (traceback == NULL)
         return;