issue with GDB python pretty-printers for libstdc++

[Paul Smith <paul at mad-scientist dot net>]

Hi all.  I was seeing a strange error in GDB (8.2.1) debugging some C++
code while trying to print a value.  The pretty printer was throwing Python
exceptions.

Debugging it I discovered the problem, which is here (from GCC 9.2):

libstdc++-v3/python/libstdcxx/v6/printers.py:
  # Starting with the type ORIG, search for the member type NAME.  This
  # handles searching upward through superclasses.  This is needed to
  # work around http://sourceware.org/bugzilla/show_bug.cgi?id=13615.
  def find_type(orig, name):
      typ = orig.strip_typedefs()
      while True:
          # Strip cv-qualifiers.  PR 67440.
-->       search = '%s::%s' % (typ.unqualified(), name)
          try:
              return gdb.lookup_type(search)
          except RuntimeError:
              pass
          # The type was not found, so try the superclass.  We only need
          # to check the first superclass, so we don't bother with
          # anything fancier here.
          field = typ.fields()[0]

(First that GDB bug was fixed in 2012 so I'm not sure if we still need this
method, but anyway...)

The issue is on the marked line above.  Here we are using the __str__()
method on a gdb.Type to obtain the string name of the type.  However, I've
discovered that (at least on my system) the __str__() representation of a
gdb.Type prepends the keyword "class " or "struct " (as appropriate) to the
output.  So the above will result in a string like:

  search = 'class std::unordered_map...::...'

The prepended "class " causes the code to break: it doesn't find the type,
then we try to use typ.fields()[0] which doesn't work as follows:

  Traceback (most recent call last):
      ...
    File "/cc/python/libstdcxx/v6/printers.py", line 97, in find_type
      field = typ.fields()[0]
  IndexError: list index out of range

I think that it's not correct for the Python macros here to be using the
gdb.Type.__str__() method to obtain the type name for anything other than
displaying to users.  They should always be using the gdb.Type.name data
member instead.  If I change the marked line above to be:

      search = '%s::%s' % (typ.unqualified().name, name)

then it all works as expected.

However, I took a quick look through the code and it _appears_ to me that
this type of thing (using the implied, or explicit, gdb.Type.__str__() to
obtain the type name) is done in a number of places.

This makes me wonder whether (a) for some reason no one noticed this
before, or (b) there's something bizarre about my GDB which is prepending
this "class" or "struct" where other peoples' don't do that?