bug#38912: 27.0.60; PDumper meets segmentation fault when evil is loaded

[Stefan Monnier]

The problem is simply that `sxhash` doesn't use the same "rules" about
which objects are compared by identity and which objects are compared
by contents.

In `src/fns.c`, when we compare `internal_equal` and `sxhash`, we see
that `sxhash` only looks at the contents of vectorlikes if they are:

      BIGNUMP, VECTORP, RECORDP, or BOOL_VECTOR_P

whereas `internal_equal` looks inside many more vectorlikes:

        if (BIGNUMP (o1))
          return mpz_cmp (*xbignum_val (o1), *xbignum_val (o2)) == 0;
        if (OVERLAYP (o1))
          {
            if (!internal_equal (OVERLAY_START (o1), OVERLAY_START (o2),
                                 equal_kind, depth + 1, ht)
                || !internal_equal (OVERLAY_END (o1), OVERLAY_END (o2),
                                    equal_kind, depth + 1, ht))
              return false;
            o1 = XOVERLAY (o1)->plist;
            o2 = XOVERLAY (o2)->plist;
            depth++;
            goto tail_recurse;
          }
        if (MARKERP (o1))
          {
            return (XMARKER (o1)->buffer == XMARKER (o2)->buffer
                    && (XMARKER (o1)->buffer == 0
                        || XMARKER (o1)->bytepos == XMARKER (o2)->bytepos));
          }
        /* Boolvectors are compared much like strings.  */
        if (BOOL_VECTOR_P (o1))
          {
            EMACS_INT size = bool_vector_size (o1);
            if (size != bool_vector_size (o2))
              return false;
            if (memcmp (bool_vector_data (o1), bool_vector_data (o2),
                        bool_vector_bytes (size)))
              return false;
            return true;
          }
        if (WINDOW_CONFIGURATIONP (o1))
          {
            eassert (equal_kind != EQUAL_NO_QUIT);
            return compare_window_configurations (o1, o2, false);
          }

        /* Aside from them, only true vectors, char-tables, compiled
           functions, and fonts (font-spec, font-entity, font-object)
           are sensible to compare, so eliminate the others now.  */
        if (size & PSEUDOVECTOR_FLAG)
          {
            if (((size & PVEC_TYPE_MASK) >> PSEUDOVECTOR_AREA_BITS)
                < PVEC_COMPILED)
              return false;
            size &= PSEUDOVECTOR_SIZE_MASK;
          }
        for (ptrdiff_t i = 0; i < size; i++)
          {
            Lisp_Object v1, v2;
            v1 = AREF (o1, i);
            v2 = AREF (o2, i);
            if (!internal_equal (v1, v2, equal_kind, depth + 1, ht))
              return false;
          }
        return true;
      }
      break;

so the problem doesn't affect only byte-compiled objects but also
overlays, markers, windowconfigs, chartables, and fonts, AFAICT.

The fix should be to make `sxhash` follow the same rules as `internal_equal`.

This is a fairly long-standing problem, so unless it is newly triggered
in "normal" circumstances in Emacs-27, the fix is probably best on
`master`.


        Stefan