libctf: deduplicate and sort the string table

ctf.h states:

> [...] the CTF string table does not contain any duplicated strings.

Unfortunately this is entirely untrue: libctf has before now made no
attempt whatsoever to deduplicate the string table. It computes the
string table's length on the fly as it adds new strings to the dynamic
CTF file, and ctf_update() just writes each string to the table and
notes the current write position as it traverses the dynamic CTF file's
data structures and builds the final CTF buffer.  There is no global
view of the strings and no deduplication.

Fix this by erasing the ctf_dtvstrlen dead-reckoning length, and adding
a new dynhash table ctf_str_atoms that maps unique strings to a list
of references to those strings: a reference is a simple uint32_t * to
some value somewhere in the under-construction CTF buffer that needs
updating to note the string offset when the strtab is laid out.

Adding a string is now a simple matter of calling ctf_str_add_ref(),
which adds a new atom to the atoms table, if one doesn't already exist,
and adding the location of the reference to this atom to the refs list
attached to the atom: this works reliably as long as one takes care to
only call ctf_str_add_ref() once the final location of the offset is
known (so you can't call it on a temporary structure and then memcpy()
that structure into place in the CTF buffer, because the ref will still
point to the old location: ctf_update() changes accordingly).

Generating the CTF string table is a matter of calling
ctf_str_write_strtab(), which counts the length and number of elements
in the atoms table using the ctf_dynhash_iter() function we just added,
populating an array of pointers into the atoms table and sorting it into
order (to help compressors), then traversing this table and emitting it,
updating the refs to each atom as we go.  The only complexity here is
arranging to keep the null string at offset zero, since a lot of code in
libctf depends on being able to leave strtab references at 0 to indicate
'no name'.  Once the table is constructed and the refs updated, we know
how long it is, so we can realloc() the partial CTF buffer we allocated
earlier and can copy the table on to the end of it (and purge the refs
because they're not needed any more and have been invalidated by the
realloc() call in any case).

The net effect of all this is a reduction in uncompressed strtab sizes
of about 30% (perhaps a quarter to a half of all strings across the
Linux kernel are eliminated as duplicates). Of course, duplicated
strings are highly redundant, so the space saving after compression is
only about 20%: when the other non-strtab sections are factored in, CTF
sizes shrink by about 10%.

No change in externally-visible API or file format (other than the
reduction in pointless redundancy).

libctf/
	* ctf-impl.h: (struct ctf_strs_writable): New, non-const version of
	struct ctf_strs.
	(struct ctf_dtdef): Note that dtd_data.ctt_name is unpopulated.
	(struct ctf_str_atom): New, disambiguated single string.
	(struct ctf_str_atom_ref): New, points to some other location that
	references this string's offset.
	(struct ctf_file): New members ctf_str_atoms and ctf_str_num_refs.
	Remove member ctf_dtvstrlen: we no longer track the total strlen
	as we add strings.
	(ctf_str_create_atoms): Declare new function in ctf-string.c.
	(ctf_str_free_atoms): Likewise.
	(ctf_str_add): Likewise.
	(ctf_str_add_ref): Likewise.
	(ctf_str_purge_refs): Likewise.
	(ctf_str_write_strtab): Likewise.
	(ctf_realloc): Declare new function in ctf-util.c.

	* ctf-open.c (ctf_bufopen): Create the atoms table.
	(ctf_file_close): Destroy it.
	* ctf-create.c (ctf_update): Copy-and-free it on update.  No longer
	special-case the position of the parname string.  Construct the
	strtab by calling ctf_str_add_ref and ctf_str_write_strtab after the
	rest of each buffer element is constructed, not via open-coding:
	realloc the CTF buffer and append the strtab to it.  No longer
	maintain ctf_dtvstrlen.  Sort the variable entry table later, after
	strtab construction.
	(ctf_copy_membnames): Remove: integrated into ctf_copy_{s,l,e}members.
	(ctf_copy_smembers): Drop the string offset: call ctf_str_add_ref
	after buffer element construction instead.
	(ctf_copy_lmembers): Likewise.
	(ctf_copy_emembers): Likewise.
	(ctf_create): No longer maintain the ctf_dtvstrlen.
	(ctf_dtd_delete): Likewise.
	(ctf_dvd_delete): Likewise.
	(ctf_add_generic): Likewise.
	(ctf_add_enumerator): Likewise.
	(ctf_add_member_offset): Likewise.
	(ctf_add_variable): Likewise.
	(membadd): Likewise.
	* ctf-util.c (ctf_realloc): New, wrapper around realloc that aborts
	if there are active ctf_str_num_refs.
	(ctf_strraw): Move to ctf-string.c.
	(ctf_strptr): Likewise.
	* ctf-string.c: New file, strtab manipulation.

	* Makefile.am (libctf_a_SOURCES): Add it.
	* Makefile.in: Regenerate.

1 files changed, 330 insertions, 0 deletions

diff --git a/libctf/ctf-string.c b/libctf/ctf-string.c
new file mode 100644
index 0000000..27bd7c2
--- /dev/null
+++ b/libctf/ctf-string.c
@@ -0,0 +1,330 @@
+/* CTF string table management.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of libctf.
+
+   libctf is free software; you can redistribute it and/or modify it under
+   the terms of the GNU General Public License as published by the Free
+   Software Foundation; either version 3, or (at your option) any later
+   version.
+
+   This program is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+   See the GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; see the file COPYING.  If not see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <ctf-impl.h>
+#include <string.h>
+
+/* Convert an encoded CTF string name into a pointer to a C string by looking
+  up the appropriate string table buffer and then adding the offset.  */
+const char *
+ctf_strraw (ctf_file_t *fp, uint32_t name)
+{
+  ctf_strs_t *ctsp = &fp->ctf_str[CTF_NAME_STID (name)];
+
+  if (ctsp->cts_strs != NULL && CTF_NAME_OFFSET (name) < ctsp->cts_len)
+    return (ctsp->cts_strs + CTF_NAME_OFFSET (name));
+
+  /* String table not loaded or corrupt offset.  */
+  return NULL;
+}
+
+/* Return a guaranteed-non-NULL pointer to the string with the given CTF
+   name.  */
+const char *
+ctf_strptr (ctf_file_t *fp, uint32_t name)
+{
+  const char *s = ctf_strraw (fp, name);
+  return (s != NULL ? s : "(?)");
+}
+
+/* Remove all refs to a given atom.  */
+static void
+ctf_str_purge_atom_refs (ctf_str_atom_t *atom)
+{
+  ctf_str_atom_ref_t *ref, *next;
+
+  for (ref = ctf_list_next (&atom->csa_refs); ref != NULL; ref = next)
+    {
+      next = ctf_list_next (ref);
+      ctf_list_delete (&atom->csa_refs, ref);
+      ctf_free (ref);
+    }
+}
+
+/* Free an atom (only called on ctf_close().)  */
+static void
+ctf_str_free_atom (void *a)
+{
+  ctf_str_atom_t *atom = a;
+
+  ctf_str_purge_atom_refs (atom);
+  ctf_free (atom);
+}
+
+/* Create the atoms table.  There is always at least one atom in it, the null
+   string.  */
+int
+ctf_str_create_atoms (ctf_file_t *fp)
+{
+  fp->ctf_str_atoms = ctf_dynhash_create (ctf_hash_string, ctf_hash_eq_string,
+					  ctf_free, ctf_str_free_atom);
+  if (fp->ctf_str_atoms == NULL)
+    return -ENOMEM;
+
+  ctf_str_add (fp, "");
+  return 0;
+}
+
+/* Destroy the atoms table.  */
+void
+ctf_str_free_atoms (ctf_file_t *fp)
+{
+  ctf_dynhash_destroy (fp->ctf_str_atoms);
+}
+
+/* Add a string to the atoms table and return it, or return an existing string
+   if present, copying the passed-in string.  Returns NULL only when out of
+   memory (and do not touch the passed-in string in that case).  Possibly
+   augment the ref list with the passed-in ref.  */
+static const char *
+ctf_str_add_ref_internal (ctf_file_t *fp, const char *str,
+			  int add_ref, uint32_t *ref)
+{
+  char *newstr = NULL;
+  ctf_str_atom_t *atom = NULL;
+  ctf_str_atom_ref_t *aref = NULL;
+
+  atom = ctf_dynhash_lookup (fp->ctf_str_atoms, str);
+
+  if (add_ref)
+    {
+      if ((aref = ctf_alloc (sizeof (struct ctf_str_atom_ref))) == NULL)
+	return NULL;
+      aref->caf_ref = ref;
+    }
+
+  if (atom)
+    {
+      if (add_ref)
+	{
+	  ctf_list_append (&atom->csa_refs, aref);
+	  fp->ctf_str_num_refs++;
+	}
+      return atom->csa_str;
+    }
+
+  if ((atom = ctf_alloc (sizeof (struct ctf_str_atom))) == NULL)
+    goto oom;
+  memset (atom, 0, sizeof (struct ctf_str_atom));
+
+  if ((newstr = ctf_strdup (str)) == NULL)
+    goto oom;
+
+  if (ctf_dynhash_insert (fp->ctf_str_atoms, newstr, atom) < 0)
+    goto oom;
+
+  atom->csa_str = newstr;
+  atom->csa_snapshot_id = fp->ctf_snapshots;
+  if (add_ref)
+    {
+      ctf_list_append (&atom->csa_refs, aref);
+      fp->ctf_str_num_refs++;
+    }
+  return newstr;
+
+ oom:
+  ctf_free (atom);
+  ctf_free (aref);
+  ctf_free (newstr);
+  return NULL;
+}
+
+/* Add a string to the atoms table and return it, without augmenting the ref
+   list for this string.  */
+const char *
+ctf_str_add (ctf_file_t *fp, const char *str)
+{
+  if (str)
+    return ctf_str_add_ref_internal (fp, str, FALSE, 0);
+  return NULL;
+}
+
+/* A ctf_dynhash_iter_remove() callback that removes atoms later than a given
+   snapshot ID.  */
+static int
+ctf_str_rollback_atom (void *key _libctf_unused_, void *value, void *arg)
+{
+  ctf_str_atom_t *atom = (ctf_str_atom_t *) value;
+  ctf_snapshot_id_t *id = (ctf_snapshot_id_t *) arg;
+
+  return (atom->csa_snapshot_id > id->snapshot_id);
+}
+
+/* Roll back, deleting all atoms created after a particular ID.  */
+void
+ctf_str_rollback (ctf_file_t *fp, ctf_snapshot_id_t id)
+{
+  ctf_dynhash_iter_remove (fp->ctf_str_atoms, ctf_str_rollback_atom, &id);
+}
+
+/* Like ctf_str_add(), but additionally augment the atom's refs list with the
+   passed-in ref, whether or not the string is already present.  There is no
+   attempt to deduplicate the refs list (but duplicates are harmless).  */
+const char *
+ctf_str_add_ref (ctf_file_t *fp, const char *str, uint32_t *ref)
+{
+  if (str)
+    return ctf_str_add_ref_internal (fp, str, TRUE, ref);
+  return NULL;
+}
+
+/* An adaptor around ctf_purge_atom_refs.  */
+static void
+ctf_str_purge_one_atom_refs (void *key _libctf_unused_, void *value,
+			     void *arg _libctf_unused_)
+{
+  ctf_str_atom_t *atom = (ctf_str_atom_t *) value;
+  ctf_str_purge_atom_refs (atom);
+}
+
+/* Remove all the recorded refs from the atoms table.  */
+void
+ctf_str_purge_refs (ctf_file_t *fp)
+{
+  if (fp->ctf_str_num_refs > 0)
+    ctf_dynhash_iter (fp->ctf_str_atoms, ctf_str_purge_one_atom_refs, NULL);
+  fp->ctf_str_num_refs = 0;
+}
+
+/* Update a list of refs to the specified value. */
+static void
+ctf_str_update_refs (ctf_str_atom_t *refs, uint32_t value)
+{
+  ctf_str_atom_ref_t *ref;
+
+  for (ref = ctf_list_next (&refs->csa_refs); ref != NULL;
+       ref = ctf_list_next (ref))
+      *(ref->caf_ref) = value;
+}
+
+/* State shared across the strtab write process.  */
+typedef struct ctf_strtab_write_state
+{
+  /* Strtab we are writing, and the number of strings in it.  */
+  ctf_strs_writable_t *strtab;
+  size_t strtab_count;
+
+  /* Pointers to (existing) atoms in the atoms table, for qsorting.  */
+  ctf_str_atom_t **sorttab;
+
+  /* Loop counter for sorttab population.  */
+  size_t i;
+
+  /* The null-string atom (skipped during population).  */
+  ctf_str_atom_t *nullstr;
+} ctf_strtab_write_state_t;
+
+/* Count the number of entries in the strtab, and its length.  */
+static void
+ctf_str_count_strtab (void *key _libctf_unused_, void *value,
+	      void *arg)
+{
+  ctf_str_atom_t *atom = (ctf_str_atom_t *) value;
+  ctf_strtab_write_state_t *s = (ctf_strtab_write_state_t *) arg;
+
+  s->strtab->cts_len += strlen (atom->csa_str) + 1;
+  s->strtab_count++;
+}
+
+/* Populate the sorttab with pointers to the strtab atoms.  */
+static void
+ctf_str_populate_sorttab (void *key _libctf_unused_, void *value,
+		  void *arg)
+{
+  ctf_str_atom_t *atom = (ctf_str_atom_t *) value;
+  ctf_strtab_write_state_t *s = (ctf_strtab_write_state_t *) arg;
+
+  /* Skip the null string.  */
+  if (s->nullstr == atom)
+    return;
+
+  s->sorttab[s->i++] = atom;
+}
+
+/* Sort the strtab.  */
+static int
+ctf_str_sort_strtab (const void *a, const void *b)
+{
+  ctf_str_atom_t **one = (ctf_str_atom_t **) a;
+  ctf_str_atom_t **two = (ctf_str_atom_t **) b;
+
+  return (strcmp ((*one)->csa_str, (*two)->csa_str));
+}
+
+/* Write out and return a strtab containing all strings with recorded refs,
+   adjusting the refs to refer to the corresponding string.  The returned
+   strtab may be NULL on error.  */
+ctf_strs_writable_t
+ctf_str_write_strtab (ctf_file_t *fp)
+{
+  ctf_strs_writable_t strtab;
+  ctf_str_atom_t *nullstr;
+  uint32_t cur_stroff = 0;
+  ctf_strtab_write_state_t s;
+  ctf_str_atom_t **sorttab;
+  size_t i;
+
+  memset (&strtab, 0, sizeof (struct ctf_strs_writable));
+  memset (&s, 0, sizeof (struct ctf_strtab_write_state));
+  s.strtab = &strtab;
+
+  nullstr = ctf_dynhash_lookup (fp->ctf_str_atoms, "");
+  if (!nullstr)
+    {
+      ctf_dprintf ("Internal error: null string not found in strtab.\n");
+      strtab.cts_strs = NULL;
+      return strtab;
+    }
+
+  ctf_dynhash_iter (fp->ctf_str_atoms, ctf_str_count_strtab, &s);
+
+  ctf_dprintf ("%lu bytes of strings in strtab.\n",
+	       (unsigned long) strtab.cts_len);
+
+  /* Sort the strtab.  Force the null string to be first.  */
+  sorttab = calloc (s.strtab_count, sizeof (ctf_str_atom_t *));
+  if (!sorttab)
+      return strtab;
+
+  sorttab[0] = nullstr;
+  s.i = 1;
+  s.sorttab = sorttab;
+  s.nullstr = nullstr;
+  ctf_dynhash_iter (fp->ctf_str_atoms, ctf_str_populate_sorttab, &s);
+
+  qsort (&sorttab[1], s.strtab_count - 1, sizeof (ctf_str_atom_t *),
+	 ctf_str_sort_strtab);
+
+  if ((strtab.cts_strs = ctf_alloc (strtab.cts_len)) == NULL)
+    {
+      free (sorttab);
+      return strtab;
+    }
+
+  /* Update the strtab, and all refs.  */
+  for (i = 0; i < s.strtab_count; i++)
+    {
+      strcpy (&strtab.cts_strs[cur_stroff], sorttab[i]->csa_str);
+      ctf_str_update_refs (sorttab[i], cur_stroff);
+      cur_stroff += strlen (sorttab[i]->csa_str) + 1;
+    }
+  free (sorttab);
+
+  return strtab;
+}