libctf: fix lookups of pointers by name in parent dicts

When you look up a type by name using ctf_lookup_by_name, in most cases
libctf can just strip off any qualifiers and look for the name, but for
pointer types this doesn't work, since the caller will want the pointer
type itself.  But pointer types are nameless, and while they cite the
types they point to, looking up a type by name requires a link going the
*other way*, from the type pointed to to the pointer type that points to
it.

libctf has always built this up at open time: ctf_ptrtab is an array of
type indexes pointing from the index of every type to the index of the
type that points to it.  But because it is built up at open time (and
because it uses type indexes and not type IDs) it is restricted to
working within a single dict and ignoring parent/child
relationships. This is normally invisible, unless you manage to get a
dict with a type in the parent but the only pointer to it in a child.
The ctf_ptrtab will not track this relationship, so lookups of this
pointer type by name will fail.  Since which type is in the parent and
which in the child is largely opaque to the user (which goes where is up
to the deduplicator, and it can and does reshuffle things to save
space), this leads to a very bad user experience, with an
obviously-visible pointer type which ctf_lookup_by_name claims doesn't
exist.

The fix is to have another array, ctf_pptrtab, which is populated in
child dicts: like the parent's ctf_ptrtab, it has one element per type
in the parent, but is all zeroes except for those types which are
pointed to by types in the child: so it maps parent dict indices to
child dict indices.  The array is grown, and new child types scanned,
whenever a lookup happens and new types have been added to the child
since the last time a lookup happened that might need the pptrtab.
(So for non-writable dicts, this only happens once, since new types
cannot be added to non-writable dicts at all.)

Since this introduces new complexity (involving updating only part of
the ctf_pptrtab) which is only seen when a writable dict is in use, we
introduce a new libctf-writable testsuite that contains lookup tests
with no corresponding CTF-containing .c files (which can thus be run
even on platforms with no .ctf-section support in the linker yet), and
add a test to check that creation of pointers in children to types in
parents and a following lookup by name works as expected.  The non-
writable case is tested in a new libctf-regression testsuite which is
used to track now-fixed outright bugs in libctf.

libctf/ChangeLog
2021-01-05  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-impl.h (ctf_dict_t) <ctf_pptrtab>: New.
	<ctf_pptrtab_len>: New.
	<ctf_pptrtab_typemax>: New.
	* ctf-create.c (ctf_serialize): Update accordingly.
	(ctf_add_reftype): Note that we don't need to update pptrtab here,
	despite updating ptrtab.
	* ctf-open.c (ctf_dict_close): Destroy the pptrtab.
	(ctf_import): Likewise.
	(ctf_import_unref): Likewise.
	* ctf-lookup.c (grow_pptrtab): New.
	(refresh_pptrtab): New, update a pptrtab.
	(ctf_lookup_by_name): Turn into a wrapper around (and rename to)...
	(ctf_lookup_by_name_internal): ... this: construct the pptrtab, and
	use it in addition to the parent's ptrtab when parent dicts are
	searched.
	* testsuite/libctf-regression/regression.exp: New testsuite for
	regression tests.
	* testsuite/libctf-regression/pptrtab*: New test.
	* testsuite/libctf-writable/writable.exp: New testsuite for tests of
	writable CTF dicts.
	* testsuite/libctf-writable/pptrtab*: New test.

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