libctf: add LIBCTF_WRITE_FOREIGN_ENDIAN debugging option

libctf has always handled endianness differences by detecting
foreign-endian CTF dicts on the input and endian-flipping them: dicts
are always written in native endianness.  This makes endian-awareness
very low overhead, but it means that the foreign-endian code paths
almost never get routinely tested, since "make check" usually reads in
dicts ld has just written out: only a few corrupted-CTF tests are
actually in fixed endianness, and even they only test the foreign-
endian code paths when you run make check on a big-endian machine.
(And the fix is surely not to add more .s-based tests like that, because
they are a nightmare to maintain compared to the C-code-based ones.)

To improve on this, add a new environment variable,
LIBCTF_WRITE_FOREIGN_ENDIAN, which causes libctf to unconditionally
endian-flip at ctf_write time, so the output is always in the wrong
endianness.  This then tests the foreign-endian read paths properly
at open time.

Make this easier by restructuring the writeout code in ctf-serialize.c,
which duplicates the maybe-gzip-and-write-out code three times (once
for ctf_write_mem, with thresholding, and once each for
ctf_compress_write and ctf_write just so those can avoid thresholding
and/or compression).  Instead, have the latter two call the former
with thresholds of 0 or (size_t) -1, respectively.

The endian-flipping code itself gains a bit of complexity, because
one single endian-flipper (flip_types) was assuming the input to be
in foreign-endian form and assuming it could pull things out of the
input once they had been flipped and make sense of them. At the
cost of a few lines of duplicated initializations, teach it to
read before flipping if we're flipping to foreign-endianness instead
of away from it.

libctf/
	* ctf-impl.h (ctf_flip_header): No longer static.
	(ctf_flip): Likewise.
	* ctf-open.c (flip_header): Rename to...
	(ctf_flip_header): ... this, now it is not private to one file.
	(flip_ctf): Rename...
	(ctf_flip): ... this too.  Add FOREIGN_ENDIAN arg.
	(flip_types): Likewise.  Use it.
	(ctf_bufopen_internal): Adjust calls.
	* ctf-serialize.c (ctf_write_mem): Add flip_endian path via
	a newly-allocated bounce buffer.
	(ctf_compress_write): Move below ctf_write_mem and reimplement
	in terms of it.
	(ctf_write): Likewise.
	(ctf_gzwrite): Note that this obscure writeout function does not
	support endian-flipping.

libctf/ctf-open.c

@@ -965,8 +965,8 @@ init_types (ctf_dict_t *fp, ctf_header_t *cth)
 
 /* Flip the endianness of the CTF header.  */
 
-static void
-flip_header (ctf_header_t *cth)
+void
+ctf_flip_header (ctf_header_t *cth)
 {
   swap_thing (cth->cth_preamble.ctp_magic);
   swap_thing (cth->cth_preamble.ctp_version);
@@ -1031,26 +1031,48 @@ flip_vars (void *start, size_t len)
    ctf_stype followed by variable data.  */
 
 static int
-flip_types (ctf_dict_t *fp, void *start, size_t len)
+flip_types (ctf_dict_t *fp, void *start, size_t len, int to_foreign)
 {
   ctf_type_t *t = start;
 
   while ((uintptr_t) t < ((uintptr_t) start) + len)
     {
+      uint32_t kind;
+      size_t size;
+      uint32_t vlen;
+      size_t vbytes;
+
+      if (to_foreign)
+	{
+	  kind = CTF_V2_INFO_KIND (t->ctt_info);
+	  size = t->ctt_size;
+	  vlen = CTF_V2_INFO_VLEN (t->ctt_info);
+	  vbytes = get_vbytes_v2 (fp, kind, size, vlen);
+	}
+
       swap_thing (t->ctt_name);
       swap_thing (t->ctt_info);
       swap_thing (t->ctt_size);
 
-      uint32_t kind = CTF_V2_INFO_KIND (t->ctt_info);
-      size_t size = t->ctt_size;
-      uint32_t vlen = CTF_V2_INFO_VLEN (t->ctt_info);
-      size_t vbytes = get_vbytes_v2 (fp, kind, size, vlen);
+      if (!to_foreign)
+	{
+	  kind = CTF_V2_INFO_KIND (t->ctt_info);
+	  size = t->ctt_size;
+	  vlen = CTF_V2_INFO_VLEN (t->ctt_info);
+	  vbytes = get_vbytes_v2 (fp, kind, size, vlen);
+	}
 
       if (_libctf_unlikely_ (size == CTF_LSIZE_SENT))
 	{
+	  if (to_foreign)
+	    size = CTF_TYPE_LSIZE (t);
+
 	  swap_thing (t->ctt_lsizehi);
 	  swap_thing (t->ctt_lsizelo);
-	  size = CTF_TYPE_LSIZE (t);
+
+	  if (!to_foreign)
+	    size = CTF_TYPE_LSIZE (t);
+
 	  t = (ctf_type_t *) ((uintptr_t) t + sizeof (ctf_type_t));
 	}
       else
@@ -1182,22 +1204,27 @@ flip_types (ctf_dict_t *fp, void *start, size_t len)
 }
 
 /* Flip the endianness of BUF, given the offsets in the (already endian-
-   converted) CTH.
+   converted) CTH.  If TO_FOREIGN is set, flip to foreign-endianness; if not,
+   flip away.
 
    All of this stuff happens before the header is fully initialized, so the
    LCTF_*() macros cannot be used yet.  Since we do not try to endian-convert v1
    data, this is no real loss.  */
 
-static int
-flip_ctf (ctf_dict_t *fp, ctf_header_t *cth, unsigned char *buf)
+int
+ctf_flip (ctf_dict_t *fp, ctf_header_t *cth, unsigned char *buf,
+	  int to_foreign)
 {
+  ctf_dprintf("flipping endianness\n");
+
   flip_lbls (buf + cth->cth_lbloff, cth->cth_objtoff - cth->cth_lbloff);
   flip_objts (buf + cth->cth_objtoff, cth->cth_funcoff - cth->cth_objtoff);
   flip_objts (buf + cth->cth_funcoff, cth->cth_objtidxoff - cth->cth_funcoff);
   flip_objts (buf + cth->cth_objtidxoff, cth->cth_funcidxoff - cth->cth_objtidxoff);
   flip_objts (buf + cth->cth_funcidxoff, cth->cth_varoff - cth->cth_funcidxoff);
   flip_vars (buf + cth->cth_varoff, cth->cth_typeoff - cth->cth_varoff);
-  return flip_types (fp, buf + cth->cth_typeoff, cth->cth_stroff - cth->cth_typeoff);
+  return flip_types (fp, buf + cth->cth_typeoff,
+		     cth->cth_stroff - cth->cth_typeoff, to_foreign);
 }
 
 /* Set up the ctl hashes in a ctf_dict_t.  Called by both writable and
@@ -1404,7 +1431,7 @@ ctf_bufopen_internal (const ctf_sect_t *ctfsect, const ctf_sect_t *symsect,
     upgrade_header (hp);
 
   if (foreign_endian)
-    flip_header (hp);
+    ctf_flip_header (hp);
   fp->ctf_openflags = hp->cth_flags;
   fp->ctf_size = hp->cth_stroff + hp->cth_strlen;
 
@@ -1610,9 +1637,9 @@ ctf_bufopen_internal (const ctf_sect_t *ctfsect, const ctf_sect_t *symsect,
   fp->ctf_syn_ext_strtab = syn_strtab;
 
   if (foreign_endian &&
-      (err = flip_ctf (fp, hp, fp->ctf_buf)) != 0)
+      (err = ctf_flip (fp, hp, fp->ctf_buf, 0)) != 0)
     {
-      /* We can be certain that flip_ctf() will have endian-flipped everything
+      /* We can be certain that ctf_flip() will have endian-flipped everything
 	 other than the types table when we return.  In particular the header
 	 is fine, so set it, to allow freeing to use the usual code path.  */