-Wwrite-strings: Add a PyArg_ParseTupleAndKeywords "const char *" overload

-Wwrite-strings flags code like:

   static char *keywords[] = {"command", "from_tty", "to_string", NULL };

as needing "(char *)" casts, because string literals are "const char []".

We can get rid of the casts by changing the array type like this:

 -  static char *keywords[] = {"command", "from_tty", "to_string", NULL };
 +  static const char *keywords[] = {"command", "from_tty", "to_string", NULL };

However, passing the such array to PyArg_ParseTupleAndKeywords no longer
works OOTB, because PyArg_ParseTupleAndKeywords expects a "char **":

  PyArg_ParseTupleAndKeywords(PyObject *args, PyObject *kw,
                              const char *format,
			      char *keywords[], ...);

and "const char **" is not implicitly convertible to "char **".  C++
is more tolerant that C here WRT aliasing, and a const_cast<char **>
is fine.  However, to avoid having all callers do the cast themselves,
this commit defines a gdb_PyArg_ParseTupleAndKeywords function here
with a corresponding 'keywords' parameter type that does the cast in a
single place.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* python/python-internal.h (gdb_PyArg_ParseTupleAndKeywords): New
	static inline function.
	* python/py-arch.c (archpy_disassemble): Constify 'keywords'
	array and use gdb_PyArg_ParseTupleAndKeywords.
	* python/py-cmd.c (cmdpy_init): Likewise.
	* python/py-finishbreakpoint.c (bpfinishpy_init): Likewise.
	* python/py-inferior.c (infpy_read_memory, infpy_write_memory)
	(infpy_search_memory): Likewise.
	* python/py-objfile.c (objfpy_add_separate_debug_file)
	(gdbpy_lookup_objfile): Likewise.
	* python/py-symbol.c (gdbpy_lookup_symbol)
	(gdbpy_lookup_global_symbol): Likewise.
	* python/py-type.c (gdbpy_lookup_type): Likewise.
	* python/py-value.c (valpy_lazy_string, valpy_string): Likewise.
	* python/python.c (execute_gdb_command, gdbpy_write, gdbpy_flush):
	Likewise.

1 files changed, 28 insertions, 0 deletions

diff --git a/gdb/python/python-internal.h b/gdb/python/python-internal.h
index 027faa5..e84c8d2 100644
--- a/gdb/python/python-internal.h
+++ b/gdb/python/python-internal.h
@@ -316,6 +316,34 @@ struct gdb_PyGetSetDef : PyGetSetDef
   {}
 };
 
+/* The 'keywords' parameter of PyArg_ParseTupleAndKeywords has type
+   'char **'.  However, string literals are const in C++, and so to
+   avoid casting at every keyword array definition, we'll need to make
+   the keywords array an array of 'const char *'.  To avoid having all
+   callers add a 'const_cast<char **>' themselves when passing such an
+   array through 'char **', we define our own version of
+   PyArg_ParseTupleAndKeywords here with a corresponding 'keywords'
+   parameter type that does the cast in a single place.  (This is not
+   an overload of PyArg_ParseTupleAndKeywords in order to make it
+   clearer that we're calling our own function instead of a function
+   that exists in some newer Python version.)  */
+
+static inline int
+gdb_PyArg_ParseTupleAndKeywords (PyObject *args, PyObject *kw,
+				 const char *format, const char **keywords, ...)
+{
+  va_list ap;
+  int res;
+
+  va_start (ap, keywords);
+  res = PyArg_VaParseTupleAndKeywords (args, kw, format,
+				       const_cast<char **> (keywords),
+				       ap);
+  va_end (ap);
+
+  return res;
+}
+
 /* In order to be able to parse symtab_and_line_to_sal_object function
    a real symtab_and_line structure is needed.  */
 #include "symtab.h"