Fix python-interactive with Python 3.6

New in v2:

 - Define PyMem_RawMalloc as PyMem_Malloc for Python < 3.4 and use
   PyMem_RawMalloc in the code.

Since Python 3.4, the callback installed in PyOS_ReadlineFunctionPointer
should return a value allocated with PyMem_RawMalloc instead of
PyMem_Malloc.  The reason is that PyMem_Malloc must be called with the
Python Global Interpreter Lock (GIL) held, which is not the case in the
context where this function is called.  PyMem_RawMalloc was introduced
for cases like this.

In Python 3.6, it looks like they added an assert to verify that
PyMem_Malloc was not called without the GIL.  The consequence is that
typing anything in the python-interactive mode of gdb crashes the
process.  The same behavior was observed with the official package on
Arch Linux as well as with a manual Python build on Ubuntu 14.04.

This is what is shown with a debug build of Python 3.6 (the error with a
non-debug build is far less clear):

  (gdb) pi
  >>> print(1)
  Fatal Python error: Python memory allocator called without holding the GIL

  Current thread 0x00007f1459af8780 (most recent call first):
  [1]    21326 abort      ./gdb

and the backtrace:

  #0  0x00007ffff618bc37 in raise () from /lib/x86_64-linux-gnu/libc.so.6
  #1  0x00007ffff618f028 in abort () from /lib/x86_64-linux-gnu/libc.so.6
  #2  0x00007ffff6b104d6 in Py_FatalError (msg=msg@entry=0x7ffff6ba15b8 "Python memory allocator called without holding the GIL") at Python/pylifecycle.c:1457
  #3  0x00007ffff6a37a68 in _PyMem_DebugCheckGIL () at Objects/obmalloc.c:1972
  #4  0x00007ffff6a3804e in _PyMem_DebugFree (ctx=0x7ffff6e65290 <_PyMem_Debug+48>, ptr=0x24f8830) at Objects/obmalloc.c:1994
  #5  0x00007ffff6a38e1d in PyMem_Free (ptr=<optimized out>) at Objects/obmalloc.c:442
  #6  0x00007ffff6b866c6 in _PyFaulthandler_Fini () at ./Modules/faulthandler.c:1369
  #7  0x00007ffff6b104bd in Py_FatalError (msg=msg@entry=0x7ffff6ba15b8 "Python memory allocator called without holding the GIL") at Python/pylifecycle.c:1431
  #8  0x00007ffff6a37a68 in _PyMem_DebugCheckGIL () at Objects/obmalloc.c:1972
  #9  0x00007ffff6a37aa3 in _PyMem_DebugMalloc (ctx=0x7ffff6e65290 <_PyMem_Debug+48>, nbytes=5) at Objects/obmalloc.c:1980
  #10 0x00007ffff6a38d91 in PyMem_Malloc (size=<optimized out>) at Objects/obmalloc.c:418
  #11 0x000000000064dbe2 in gdbpy_readline_wrapper (sys_stdin=0x7ffff6514640 <_IO_2_1_stdin_>, sys_stdout=0x7ffff6514400 <_IO_2_1_stdout_>, prompt=0x7ffff4d4f7d0 ">>> ")
    at /home/emaisin/src/binutils-gdb/gdb/python/py-gdb-readline.c:75

The documentation is very clear about it [1] and it was also mentioned
in the "What's New In Python 3.4" page [2].

[1] https://docs.python.org/3/c-api/veryhigh.html#c.PyOS_ReadlineFunctionPointer
[2] https://docs.python.org/3/whatsnew/3.4.html#changes-in-the-c-api

gdb/ChangeLog:

	* python/python-internal.h (PyMem_RawMalloc): Define for
	Python < 3.4.
	* python/py-gdb-readline.c (gdbpy_readline_wrapper): Use
	PyMem_RawMalloc instead of PyMem_Malloc.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2017-01-20  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* python/python-internal.h (PyMem_RawMalloc): Define for
+	Python < 3.4.
+	* python/py-gdb-readline.c (gdbpy_readline_wrapper): Use
+	PyMem_RawMalloc instead of PyMem_Malloc.
+
 2017-01-20  Mike Wrighton  <mike_wrighton@codesourcery.com>
 	    Luis Machado  <lgustavo@codesourcery.com>
 

gdb/python/py-gdb-readline.c

@@ -21,6 +21,7 @@
 #include "python-internal.h"
 #include "top.h"
 #include "cli/cli-utils.h"
+
 /* Readline function suitable for PyOS_ReadlineFunctionPointer, which
    is used for Python's interactive parser and raw_input.  In both
    cases, sys_stdin and sys_stdout are always stdin and stdout
@@ -63,7 +64,7 @@ gdbpy_readline_wrapper (FILE *sys_stdin, FILE *sys_stdout,
   /* Detect EOF (Ctrl-D).  */
   if (p == NULL)
     {
-      q = (char *) PyMem_Malloc (1);
+      q = (char *) PyMem_RawMalloc (1);
       if (q != NULL)
 	q[0] = '\0';
       return q;
@@ -72,7 +73,7 @@ gdbpy_readline_wrapper (FILE *sys_stdin, FILE *sys_stdout,
   n = strlen (p);
 
   /* Copy the line to Python and return.  */
-  q = (char *) PyMem_Malloc (n + 2);
+  q = (char *) PyMem_RawMalloc (n + 2);
   if (q != NULL)
     {
       strncpy (q, p, n);

gdb/python/python-internal.h

@@ -172,6 +172,13 @@ typedef unsigned long gdb_py_ulongest;
 typedef long Py_hash_t;
 #endif
 
+/* PyMem_RawMalloc appeared in Python 3.4.  For earlier versions, we can just
+   fall back to PyMem_Malloc.  */
+
+#if PY_VERSION_HEX < 0x03040000
+#define PyMem_RawMalloc PyMem_Malloc
+#endif
+
 /* Python 2.6 did not wrap Py_DECREF in 'do {...} while (0)', leading
    to 'suggest explicit braces to avoid ambiguous ‘else’' gcc errors.
    Wrap it ourselves, so that callers don't need to care.  */