Fix crash when using PYTHONMALLOC=debug (PR python/24742)

Fix crash when using PYTHONMALLOC=debug (PR python/24742)

This bug was originally reported against Fedora GDB:

  https://bugzilla.redhat.com/show_bug.cgi?id=1723564

The problem is that GDB will crash in the following scenario:

- PYTHONMALLOC=debug or PYTHONDEVMODE=1 is set.

- The Python debuginfo is installed.

- GDB is used to debug Python.

The crash looks like this:

  $ PYTHONMALLOC=debug gdb -args python3 -c pass
  GNU gdb (GDB) Fedora 8.3-3.fc30
  Reading symbols from python3...
  Reading symbols from /usr/lib/debug/usr/bin/python3.7m-3.7.3-3.fc30.x86_64.debug...
  (gdb) run
  Starting program: /usr/bin/python3 -c pass
  Missing separate debuginfos, use: dnf debuginfo-install glibc-2.29-9.fc30.x86_64
  Debug memory block at address p=0x5603977bf330: API ''
      8098648152243306496 bytes originally requested
      The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfb):
	  at p-7: 0x03 *** OUCH
	  at p-6: 0x00 *** OUCH
	  at p-5: 0x00 *** OUCH
	  at p-4: 0x00 *** OUCH
	  at p-3: 0x00 *** OUCH
	  at p-2: 0x00 *** OUCH
	  at p-1: 0x00 *** OUCH
      Because memory is corrupted at the start, the count of bytes requested
	 may be bogus, and checking the trailing pad bytes may segfault.
      The 8 pad bytes at tail=0x706483999ad1f330 are Segmentation fault (core dumped)

It's hard to determine what happens, but after doing some
investigation and talking to Victor Stinner I found that GDB should
not use the Python memory allocation functions before the Python
interpreter is initialized (which makes sense).  However, we do just
that on python/python.c:do_start_initialization:

  ...
  progsize = strlen (progname.get ());
  progname_copy = (wchar_t *) PyMem_Malloc ((progsize + 1) * sizeof (wchar_t));
  ...
  /* Note that Py_SetProgramName expects the string it is passed to
     remain alive for the duration of the program's execution, so
     it is not freed after this call.  */
  Py_SetProgramName (progname_copy);
  ...
  Py_Initialize ();
  PyEval_InitThreads ();

Upon reading the Python 3 C API documentation, I
found (https://docs.python.org/3.5/c-api/memory.html):

  To avoid memory corruption, extension writers should never try to
  operate on Python objects with the functions exported by the C
  library: malloc(), calloc(), realloc() and free(). This will result in
  mixed calls between the C allocator and the Python memory manager with
  fatal consequences, because they implement different algorithms and
  operate on different heaps. However, one may safely allocate and
  release memory blocks with the C library allocator for individual
  purposes[...]

And Py_SetProgramName seems like a very simple call that doesn't need
a Python-allocated memory to work on.  So I'm proposing this patch,
which simply replaces PyMem_Malloc by xmalloc.

Testing this is more complicated.  First, the crash is completely
non-deterministic; I was able to reproduce it 10 times in a row, and
then I wasn't able to reproduce it anymore.  I found that if you
completely remove your build directory and rebuild GDB from scratch,
you can reproduce it again confidently.  And with my patch, I
confirmed that the bug doesn't manifest even in this situation.

No regressions found.

OK to apply?

gdb/ChangeLog:
2019-06-28  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR python/24742
	https://bugzilla.redhat.com/show_bug.cgi?id=1723564
	* python/python.c (do_start_initialization): Use 'xmalloc'
	instead of 'PyMem_Malloc'.