libctf, next: introduce new class of easier-to-use iterators

The libctf machinery currently only provides one way to iterate over its
data structures: ctf_*_iter functions that take a callback and an arg
and repeatedly call it.

This *works*, but if you are doing a lot of iteration it is really quite
inconvenient: you have to package up your local variables into
structures over and over again and spawn lots of little functions even
if it would be clearer in a single run of code.  Look at ctf-string.c
for an extreme example of how unreadable this can get, with
three-line-long functions proliferating wildly.

The deduplicator takes this to the Nth level. It iterates over a whole
bunch of things: if we'd had to use _iter-class iterators for all of
them there would be twenty additional functions in the deduplicator
alone, for no other reason than that the iterator API requires it.

Let's do something better. strtok_r gives us half the design: generators
in a number of other languages give us the other half.

The *_next API allows you to iterate over CTF-like entities in a single
function using a normal while loop. e.g. here we are iterating over all
the types in a dict:

ctf_next_t *i = NULL;
int *hidden;
ctf_id_t id;

while ((id = ctf_type_next (fp, &i, &hidden, 1)) != CTF_ERR)
  {
    /* do something with 'hidden' and 'id' */
  }
if (ctf_errno (fp) != ECTF_NEXT_END)
    /* iteration error */

Here we are walking through the members of a struct with CTF ID
'struct_type':

ctf_next_t *i = NULL;
ssize_t offset;
const char *name;
ctf_id_t membtype;

while ((offset = ctf_member_next (fp, struct_type, &i, &name,
                                  &membtype)) >= 0
  {
    /* do something with offset, name, and membtype */
  }
if (ctf_errno (fp) != ECTF_NEXT_END)
    /* iteration error */

Like every other while loop, this means you have access to all the local
variables outside the loop while inside it, with no need to tiresomely
package things up in structures, move the body of the loop into a
separate function, etc, as you would with an iterator taking a callback.

ctf_*_next allocates 'i' for you on first entry (when it must be NULL),
and frees and NULLs it and returns a _next-dependent flag value when the
iteration is over: the fp errno is set to ECTF_NEXT_END when the
iteartion ends normally.  If you want to exit early, call
ctf_next_destroy on the iterator.  You can copy iterators using
ctf_next_copy, which copies their current iteration position so you can
remember loop positions and go back to them later (or ctf_next_destroy
them if you don't need them after all).

Each _next function returns an always-likely-to-be-useful property of
the thing being iterated over, and takes pointers to parameters for the
others: with very few exceptions all those parameters can be NULLs if
you're not interested in them, so e.g. you can iterate over only the
offsets of members of a structure this way:

while ((offset = ctf_member_next (fp, struct_id, &i, NULL, NULL)) >= 0)

If you pass an iterator in use by one iteration function to another one,
you get the new error ECTF_NEXT_WRONGFUN back; if you try to change
ctf_file_t in mid-iteration, you get ECTF_NEXT_WRONGFP back.

Internally the ctf_next_t remembers the iteration function in use,
various sizes and increments useful for almost all iterations, then
uses unions to overlap the actual entities being iterated over to keep
ctf_next_t size down.

Iterators available in the public API so far (all tested in actual use
in the deduplicator):

/* Iterate over the members of a STRUCT or UNION, returning each member's
   offset and optionally name and member type in turn.  On end-of-iteration,
   returns -1.  */
ssize_t
ctf_member_next (ctf_file_t *fp, ctf_id_t type, ctf_next_t **it,
                 const char **name, ctf_id_t *membtype);

/* Iterate over the members of an enum TYPE, returning each enumerand's
   NAME or NULL at end of iteration or error, and optionally passing
   back the enumerand's integer VALue.  */
const char *
ctf_enum_next (ctf_file_t *fp, ctf_id_t type, ctf_next_t **it,
              int *val);

/* Iterate over every type in the given CTF container (not including
   parents), optionally including non-user-visible types, returning
   each type ID and optionally the hidden flag in turn. Returns CTF_ERR
   on end of iteration or error.  */
ctf_id_t
ctf_type_next (ctf_file_t *fp, ctf_next_t **it, int *flag,
               int want_hidden);

/* Iterate over every variable in the given CTF container, in arbitrary
   order, returning the name and type of each variable in turn.  The
   NAME argument is not optional.  Returns CTF_ERR on end of iteration
   or error.  */
ctf_id_t
ctf_variable_next (ctf_file_t *fp, ctf_next_t **it, const char **name);

/* Iterate over all CTF files in an archive, returning each dict in turn as a
   ctf_file_t, and NULL on error or end of iteration.  It is the caller's
   responsibility to close it.  Parent dicts may be skipped.  Regardless of
   whether they are skipped or not, the caller must ctf_import the parent if
   need be.  */
ctf_file_t *
ctf_archive_next (const ctf_archive_t *wrapper, ctf_next_t **it,
                  const char **name, int skip_parent, int *errp);

ctf_label_next is prototyped but not implemented yet.

include/
	* ctf-api.h (ECTF_NEXT_END): New error.
	(ECTF_NEXT_WRONGFUN): Likewise.
	(ECTF_NEXT_WRONGFP): Likewise.
	(ECTF_NERR): Adjust.
	(ctf_next_t): New.
	(ctf_next_create): New prototype.
	(ctf_next_destroy): Likewise.
	(ctf_next_copy): Likewise.
	(ctf_member_next): Likewise.
	(ctf_enum_next): Likewise.
	(ctf_type_next): Likewise.
	(ctf_label_next): Likewise.
	(ctf_variable_next): Likewise.

libctf/
	* ctf-impl.h (ctf_next): New.
	(ctf_get_dict): New prototype.
	* ctf-lookup.c (ctf_get_dict): New, split out of...
	(ctf_lookup_by_id): ... here.
	* ctf-util.c (ctf_next_create): New.
	(ctf_next_destroy): New.
	(ctf_next_copy): New.
	* ctf-types.c (includes): Add <assert.h>.
	(ctf_member_next): New.
	(ctf_enum_next): New.
	(ctf_type_iter): Document the lack of iteration over parent
	types.
	(ctf_type_next): New.
	(ctf_variable_next): New.
	* ctf-archive.c (ctf_archive_next): New.
	* libctf.ver: Add new public functions.

1 files changed, 29 insertions, 4 deletions

diff --git a/include/ctf-api.h b/include/ctf-api.h
index faa7b72..47a1f73 100644
--- a/include/ctf-api.h
+++ b/include/ctf-api.h
@@ -204,10 +204,13 @@ enum
    ECTF_DUMPSECTCHANGED, /* Section changed in middle of dump.  */
    ECTF_NOTYET,		/* Feature not yet implemented.  */
    ECTF_INTERNAL,	/* Internal error in link.  */
-   ECTF_NONREPRESENTABLE /* Type not representable in CTF.  */
+   ECTF_NONREPRESENTABLE, /* Type not representable in CTF.  */
+   ECTF_NEXT_END,	/* End of iteration.  */
+   ECTF_NEXT_WRONGFUN,	/* Wrong iteration function called.  */
+   ECTF_NEXT_WRONGFP	/* Iteration entity changed in mid-iterate.  */
   };
 
-#define ECTF_NERR (ECTF_NONREPRESENTABLE - ECTF_BASE + 1)	/* Count of CTF errors.  */
+#define ECTF_NERR (ECTF_NEXT_WRONGFP - ECTF_BASE + 1)	/* Count of CTF errors.  */
 
 /* The CTF data model is inferred to be the caller's data model or the data
    model of the given object, unless ctf_setmodel() is explicitly called.  */
@@ -227,8 +230,9 @@ enum
 #define	CTF_ADD_NONROOT	0	/* Type only visible in nested scope.  */
 #define	CTF_ADD_ROOT	1	/* Type visible at top-level scope.  */
 
-/* These typedefs are used to define the signature for callback functions
-   that can be used with the iteration and visit functions below.  */
+/* These typedefs are used to define the signature for callback functions that
+   can be used with the iteration and visit functions below.  There is also a
+   family of iteration functions that do not require callbacks.  */
 
 typedef int ctf_visit_f (const char *name, ctf_id_t type, unsigned long offset,
 			 int depth, void *arg);
@@ -248,6 +252,15 @@ typedef char *ctf_dump_decorate_f (ctf_sect_names_t sect,
 
 typedef struct ctf_dump_state ctf_dump_state_t;
 
+/* Iteration state for the _next() functions, and allocators/copiers/freers for
+   it.  (None of these are needed for the simple case of iterating to the end:
+   the _next() function allocate and free the iterators for you.)  */
+
+typedef struct ctf_next ctf_next_t;
+extern ctf_next_t *ctf_next_create (void);
+extern void ctf_next_destroy (ctf_next_t *);
+extern ctf_next_t *ctf_next_copy (ctf_next_t *);
+
 /* Opening.  These mostly return an abstraction over both CTF files and CTF
    archives: so they can be used to open both.  CTF files will appear to be an
    archive with one member named '.ctf'.  The low-level functions
@@ -353,13 +366,25 @@ extern int ctf_label_info (ctf_file_t *, const char *, ctf_lblinfo_t *);
 
 extern int ctf_member_count (ctf_file_t *, ctf_id_t);
 extern int ctf_member_iter (ctf_file_t *, ctf_id_t, ctf_member_f *, void *);
+extern ssize_t ctf_member_next (ctf_file_t *, ctf_id_t, ctf_next_t **,
+				const char **name, ctf_id_t *membtype);
 extern int ctf_enum_iter (ctf_file_t *, ctf_id_t, ctf_enum_f *, void *);
+extern const char *ctf_enum_next (ctf_file_t *, ctf_id_t, ctf_next_t **,
+				  int *);
 extern int ctf_type_iter (ctf_file_t *, ctf_type_f *, void *);
 extern int ctf_type_iter_all (ctf_file_t *, ctf_type_all_f *, void *);
+extern ctf_id_t ctf_type_next (ctf_file_t *, ctf_next_t **,
+			       int *flag, int want_hidden);
 extern int ctf_label_iter (ctf_file_t *, ctf_label_f *, void *);
+extern int ctf_label_next (ctf_file_t *, ctf_next_t **, const char **); /* TBD */
 extern int ctf_variable_iter (ctf_file_t *, ctf_variable_f *, void *);
+extern ctf_id_t ctf_variable_next (ctf_file_t *, ctf_next_t **,
+				   const char **);
 extern int ctf_archive_iter (const ctf_archive_t *, ctf_archive_member_f *,
 			     void *);
+extern ctf_file_t *ctf_archive_next (const ctf_archive_t *, ctf_next_t **,
+				     const char **, int skip_parent, int *errp);
+
 /* This function alone does not currently operate on CTF files masquerading
    as archives, and returns -EINVAL: the raw data is no longer available.  It is
    expected to be used only by archiving tools, in any case, which have no need