Determine endianness at a time when it doesn't inadvertantly clear gdb's wrap_buffer via gdb.execute.

[saugustine]

Summary

I haven't managed a small reproduction for this bug; it involves
complicated and deeply nested data structures with a wide variety
of pretty printers. But in general, we shouldn't be combining
gdb's command line interface (via gdb.execute) with pretty-printers.

libcxx/utils/gdb/libcxx/printers.py

 # Therefore all self.FIELDs must be set in the pretty-printer's
 # __init__ function.
 
 _void_pointer_type = gdb.lookup_type("void").pointer()
 
 
 _long_int_type = gdb.lookup_type("unsigned long long")
 
+_libcpp_big_endian = False
 
 def addr_as_long(addr):
     return int(addr.cast(_long_int_type))
 
 
 # The size of a pointer in bytes.
 _pointer_size = _void_pointer_type.sizeof
 
class StdStringPrinter(object):
     def _get_short_size(self, short_field, short_size):
         """Short size depends on both endianness and a compile-time define."""
 
         # If the padding field is present after all this indirection, then string
         # was compiled with _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT defined.
         field = short_field.type.fields()[1].type.fields()[0]
         libcpp_abi_alternate_string_layout = field.name and "__padding" in field.name
 
-        # Strictly, this only tells us the current mode, not how libcxx was
-        # compiled.
-        libcpp_big_endian = "big endian" in gdb.execute("show endian",
-                                                        to_string=True)
-
         # This logical structure closely follows the original code (which is clearer
         # in C++).  Keep them parallel to make them easier to compare.
         if libcpp_abi_alternate_string_layout:
-            if libcpp_big_endian:
+            if _libcpp_big_endian:
                 return short_size >> 1
             else:
                 return short_size
-        elif libcpp_big_endian:
+        elif _libcpp_big_endian:
             return short_size
         else:
             return short_size >> 1
 
     def __init__(self, val):
         self.val = val
 
     def to_string(self):

 
 _libcxx_printer_name = "libcxx_pretty_printer"
 
 
 # These are called for every binary object file, which could be thousands in
 # certain pathological cases. Limit our pretty printers to the progspace.
 def _register_libcxx_printers(event):
     progspace = event.new_objfile.progspace
+    # It would be ideal to get the endianness at print time, but
+    # gdb.execute clears gdb's internal wrap buffer, removing any values
+    # already generated as part of a larger data structure, and there is
+    # no python api to get the endianness. Mixed-endianness debugging
+    # rare enough that this workaround should be adequate.
+    _libcpp_big_endian = "big endian" in gdb.execute("show endian",
+                                                     to_string=True)
+
     if not getattr(progspace, _libcxx_printer_name, False):
         print("Loading libc++ pretty-printers.")
         gdb.printing.register_pretty_printer(
             progspace, LibcxxPrettyPrinter(_libcxx_printer_name))
         setattr(progspace, _libcxx_printer_name, True)
 
 
 def _unregister_libcxx_printers(event):