gdb: Don't fault for 'maint print psymbols' when using an index

gdb: Don't fault for 'maint print psymbols' when using an index

I found that these tests:

  make check-gdb RUNTESTFLAGS="--target_board=cc-with-gdb-index gdb.base/maint.exp"
  make check-gdb RUNTESTFLAGS="--target_board=cc-with-debug-names gdb.base/maint.exp"

were causing GDB to segfault.  It turns out that this test runs this
command:

  maint print psymbols -pc main /path/to/some/file

which tries to lookup the partial_symtab for 'main'.  The problem is
that there is no partial_symtab for 'main' as we are using the
.gdb_index or .debug_names instead of partial_symtabs.

What happens is that maintenance_print_symbols calls
find_pc_sect_psymtab, which looks for the partial_symtab in the
objfile's objfile->partial_symtabs->psymtabs_addrmap.

This is a problem because when we are using the indexes
psymtabs_addrmap is reused to hold things other than partial_symtabs,
this can be seen in dwarf2read.c in create_addrmap_from_index and
create_addrmap_from_aranges.  If we then lookup in psymtabs_addrmap we
end up returning a pointer to something that isn't really a
partial_symtab, after which everything goes wrong.

Initially I simply added a check at the start of find_pc_sect_psymtab
that the objfile had some partial_symtabs, like:

  if (objfile->partial_symtabs->psymtabs == NULL)
    return NULL;

Figuring that if there were no partial_symtabs at all then this
function should always return NULL, however, this caused a failure in
the test gdb.python/py-event.exp which I didn't dig into too deeply,
but seems to be that in this tests there are initially no psymtabs,
but the second part of find_pc_sect_psymtab does manage to read some
in from somewhere, with the check I added the test fails as we
returned NULL here and this caused GDB to load in the full symtabs
earlier than was expected.

Instead I chose to guard only the access to psymtabs_addrmap with a
check that the function has some psymtabs.  This allows my original
tests to pass, and the py-event.exp test to pass too.

Now, a good argument can be made that we simply should never call
find_pc_sect_psymtab on an objfile that is using indexes instead of
partial_symtabs.  I did consider this approach, we could easily add an
assert into find_pc_sect_psymtab that if we find a partial_symtab in
psymtabs_addrmap then the psymtabs pointer must be non-null.  The
responsibility would then be on the user of find_pc_sect_psymtab to
ensure that the objfile being checked is suitable.  In the end I
didn't take this approach as the check in find_pc_sect_psymtab is
cheap and this ensures that any future miss-uses of the function will
not cause problems.

I also extended the comment on psymtabs_addrmap to indicate that it
holds more than just partial_symtabs as this was not at all clear from
the original comment, and caused me some confusion when I was
initially debugging this problem.

gdb/ChangeLog:

	* psymtab.c (find_pc_sect_psymtab): Move baseaddr local into more
	inner scope, add check that the objfile has psymtabs before
	checking psymtabs_addrmap.
	* psymtab.h (psymtab_storage) <psymtabs_addrmap>: Extend comment.