refs: introduce an iterator interface

Currently, the API for iterating over references is via a family of
for_each_ref()-type functions that invoke a callback function for each
selected reference. All of these eventually call do_for_each_ref(),
which knows how to do one thing: iterate in parallel through two
ref_caches, one for loose and one for packed refs, giving loose
references precedence over packed refs. This is rather complicated code,
and is quite specialized to the files backend. It also requires callers
to encapsulate their work into a callback function, which often means
that they have to define and use a "cb_data" struct to manage their
context.

The current design is already bursting at the seams, and will become
even more awkward in the upcoming world of multiple reference storage
backends:

* Per-worktree vs. shared references are currently handled via a kludge
  in git_path() rather than iterating over each part of the reference
  namespace separately and merging the results. This kludge will cease
  to work when we have multiple reference storage backends.

* The current scheme is inflexible. What if we sometimes want to bypass
  the ref_cache, or use it only for packed or only for loose refs? What
  if we want to store symbolic refs in one type of storage backend and
  non-symbolic ones in another?

In the future, each reference backend will need to define its own way of
iterating over references. The crux of the problem with the current
design is that it is impossible to compose for_each_ref()-style
iterations, because the flow of control is owned by the for_each_ref()
function. There is nothing that a caller can do but iterate through all
references in a single burst, so there is no way for it to interleave
references from multiple backends and present the result to the rest of
the world as a single compound backend.

This commit introduces a new iteration primitive for references: a
ref_iterator. A ref_iterator is a polymorphic object that a reference
storage backend can be asked to instantiate. There are three functions
that can be applied to a ref_iterator:

* ref_iterator_advance(): move to the next reference in the iteration
* ref_iterator_abort(): end the iteration before it is exhausted
* ref_iterator_peel(): peel the reference currently being looked at

Iterating using a ref_iterator leaves the flow of control in the hands
of the caller, which means that ref_iterators from multiple
sources (e.g., loose and packed refs) can be composed and presented to
the world as a single compound ref_iterator.

It also means that the backend code for implementing reference iteration
will sometimes be more complicated. For example, the
cache_ref_iterator (which iterates over a ref_cache) can't use the C
stack to recurse; instead, it must manage its own stack internally as
explicit data structures. There is also a lot of boilerplate connected
with object-oriented programming in C.

Eventually, end-user callers will be able to be written in a more
natural way—managing their own flow of control rather than having to
work via callbacks. Since there will only be a few reference backends
but there are many consumers of this API, this is a good tradeoff.

More importantly, we gain composability, and especially the possibility
of writing interchangeable parts that can work with any ref_iterator.

For example, merge_ref_iterator implements a generic way of merging the
contents of any two ref_iterators. It is used to merge loose + packed
refs as part of the implementation of the files_ref_iterator. But it
will also be possible to use it to merge other pairs of reference
sources (e.g., per-worktree vs. shared refs).

Another example is prefix_ref_iterator, which can be used to trim a
prefix off the front of reference names before presenting them to the
caller (e.g., "refs/heads/master" -> "master").

In this patch, we introduce the iterator abstraction and many utilities,
and implement a reference iterator for the files ref storage backend.
(I've written several other obvious utilities, for example a generic way
to filter references being iterated over. These will probably be useful
in the future. But they are not needed for this patch series, so I am
not including them at this time.)

In a moment we will rewrite do_for_each_ref() to work via reference
iterators (allowing some special-purpose code to be discarded), and do
something similar for reflogs. In future patch series, we will expose
the ref_iterator abstraction in the public refs API so that callers can
use it directly.

Implementation note: I tried abstracting this a layer further to allow
generic iterators (over arbitrary types of objects) and generic
utilities like a generic merge_iterator. But the implementation in C was
very cumbersome, involving (in my opinion) too much boilerplate and too
much unsafe casting, some of which would have had to be done on the
caller side. However, I did put a few iterator-related constants in a
top-level header file, iterator.h, as they will be useful in a moment to
implement iteration over directory trees and possibly other types of
iterators in the future.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 830 insertions, 0 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 62280b5c7..395056b67 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1,6 +1,7 @@
 #include "../cache.h"
 #include "../refs.h"
 #include "refs-internal.h"
+#include "../iterator.h"
 #include "../lockfile.h"
 #include "../object.h"
 #include "../dir.h"
@@ -704,6 +705,153 @@ static void prime_ref_dir(struct ref_dir *dir)
 	}
 }
 
+/*
+ * A level in the reference hierarchy that is currently being iterated
+ * through.
+ */
+struct cache_ref_iterator_level {
+	/*
+	 * The ref_dir being iterated over at this level. The ref_dir
+	 * is sorted before being stored here.
+	 */
+	struct ref_dir *dir;
+
+	/*
+	 * The index of the current entry within dir (which might
+	 * itself be a directory). If index == -1, then the iteration
+	 * hasn't yet begun. If index == dir->nr, then the iteration
+	 * through this level is over.
+	 */
+	int index;
+};
+
+/*
+ * Represent an iteration through a ref_dir in the memory cache. The
+ * iteration recurses through subdirectories.
+ */
+struct cache_ref_iterator {
+	struct ref_iterator base;
+
+	/*
+	 * The number of levels currently on the stack. This is always
+	 * at least 1, because when it becomes zero the iteration is
+	 * ended and this struct is freed.
+	 */
+	size_t levels_nr;
+
+	/* The number of levels that have been allocated on the stack */
+	size_t levels_alloc;
+
+	/*
+	 * A stack of levels. levels[0] is the uppermost level that is
+	 * being iterated over in this iteration. (This is not
+	 * necessary the top level in the references hierarchy. If we
+	 * are iterating through a subtree, then levels[0] will hold
+	 * the ref_dir for that subtree, and subsequent levels will go
+	 * on from there.)
+	 */
+	struct cache_ref_iterator_level *levels;
+};
+
+static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct cache_ref_iterator *iter =
+		(struct cache_ref_iterator *)ref_iterator;
+
+	while (1) {
+		struct cache_ref_iterator_level *level =
+			&iter->levels[iter->levels_nr - 1];
+		struct ref_dir *dir = level->dir;
+		struct ref_entry *entry;
+
+		if (level->index == -1)
+			sort_ref_dir(dir);
+
+		if (++level->index == level->dir->nr) {
+			/* This level is exhausted; pop up a level */
+			if (--iter->levels_nr == 0)
+				return ref_iterator_abort(ref_iterator);
+
+			continue;
+		}
+
+		entry = dir->entries[level->index];
+
+		if (entry->flag & REF_DIR) {
+			/* push down a level */
+			ALLOC_GROW(iter->levels, iter->levels_nr + 1,
+				   iter->levels_alloc);
+
+			level = &iter->levels[iter->levels_nr++];
+			level->dir = get_ref_dir(entry);
+			level->index = -1;
+		} else {
+			iter->base.refname = entry->name;
+			iter->base.oid = &entry->u.value.oid;
+			iter->base.flags = entry->flag;
+			return ITER_OK;
+		}
+	}
+}
+
+static enum peel_status peel_entry(struct ref_entry *entry, int repeel);
+
+static int cache_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	struct cache_ref_iterator *iter =
+		(struct cache_ref_iterator *)ref_iterator;
+	struct cache_ref_iterator_level *level;
+	struct ref_entry *entry;
+
+	level = &iter->levels[iter->levels_nr - 1];
+
+	if (level->index == -1)
+		die("BUG: peel called before advance for cache iterator");
+
+	entry = level->dir->entries[level->index];
+
+	if (peel_entry(entry, 0))
+		return -1;
+	hashcpy(peeled->hash, entry->u.value.peeled.hash);
+	return 0;
+}
+
+static int cache_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct cache_ref_iterator *iter =
+		(struct cache_ref_iterator *)ref_iterator;
+
+	free(iter->levels);
+	base_ref_iterator_free(ref_iterator);
+	return ITER_DONE;
+}
+
+static struct ref_iterator_vtable cache_ref_iterator_vtable = {
+	cache_ref_iterator_advance,
+	cache_ref_iterator_peel,
+	cache_ref_iterator_abort
+};
+
+static struct ref_iterator *cache_ref_iterator_begin(struct ref_dir *dir)
+{
+	struct cache_ref_iterator *iter;
+	struct ref_iterator *ref_iterator;
+	struct cache_ref_iterator_level *level;
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+	base_ref_iterator_init(ref_iterator, &cache_ref_iterator_vtable);
+	ALLOC_GROW(iter->levels, 10, iter->levels_alloc);
+
+	iter->levels_nr = 1;
+	level = &iter->levels[0];
+	level->index = -1;
+	level->dir = dir;
+
+	return ref_iterator;
+}
+
 struct nonmatching_ref_data {
 	const struct string_list *skip;
 	const char *conflicting_refname;
@@ -1843,6 +1991,139 @@ int peel_ref(const char *refname, unsigned char *sha1)
 	return peel_object(base, sha1);
 }
 
+struct files_ref_iterator {
+	struct ref_iterator base;
+
+	struct packed_ref_cache *packed_ref_cache;
+	struct ref_iterator *iter0;
+	unsigned int flags;
+};
+
+static int files_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct files_ref_iterator *iter =
+		(struct files_ref_iterator *)ref_iterator;
+	int ok;
+
+	while ((ok = ref_iterator_advance(iter->iter0)) == ITER_OK) {
+		if (!(iter->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
+		    !ref_resolves_to_object(iter->iter0->refname,
+					    iter->iter0->oid,
+					    iter->iter0->flags))
+			continue;
+
+		iter->base.refname = iter->iter0->refname;
+		iter->base.oid = iter->iter0->oid;
+		iter->base.flags = iter->iter0->flags;
+		return ITER_OK;
+	}
+
+	iter->iter0 = NULL;
+	if (ref_iterator_abort(ref_iterator) != ITER_DONE)
+		ok = ITER_ERROR;
+
+	return ok;
+}
+
+static int files_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	struct files_ref_iterator *iter =
+		(struct files_ref_iterator *)ref_iterator;
+
+	return ref_iterator_peel(iter->iter0, peeled);
+}
+
+static int files_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct files_ref_iterator *iter =
+		(struct files_ref_iterator *)ref_iterator;
+	int ok = ITER_DONE;
+
+	if (iter->iter0)
+		ok = ref_iterator_abort(iter->iter0);
+
+	release_packed_ref_cache(iter->packed_ref_cache);
+	base_ref_iterator_free(ref_iterator);
+	return ok;
+}
+
+static struct ref_iterator_vtable files_ref_iterator_vtable = {
+	files_ref_iterator_advance,
+	files_ref_iterator_peel,
+	files_ref_iterator_abort
+};
+
+struct ref_iterator *files_ref_iterator_begin(
+		const char *submodule,
+		const char *prefix, unsigned int flags)
+{
+	struct ref_cache *refs = get_ref_cache(submodule);
+	struct ref_dir *loose_dir, *packed_dir;
+	struct ref_iterator *loose_iter, *packed_iter;
+	struct files_ref_iterator *iter;
+	struct ref_iterator *ref_iterator;
+
+	if (!refs)
+		return empty_ref_iterator_begin();
+
+	if (ref_paranoia < 0)
+		ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
+	if (ref_paranoia)
+		flags |= DO_FOR_EACH_INCLUDE_BROKEN;
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+	base_ref_iterator_init(ref_iterator, &files_ref_iterator_vtable);
+
+	/*
+	 * We must make sure that all loose refs are read before
+	 * accessing the packed-refs file; this avoids a race
+	 * condition if loose refs are migrated to the packed-refs
+	 * file by a simultaneous process, but our in-memory view is
+	 * from before the migration. We ensure this as follows:
+	 * First, we call prime_ref_dir(), which pre-reads the loose
+	 * references for the subtree into the cache. (If they've
+	 * already been read, that's OK; we only need to guarantee
+	 * that they're read before the packed refs, not *how much*
+	 * before.) After that, we call get_packed_ref_cache(), which
+	 * internally checks whether the packed-ref cache is up to
+	 * date with what is on disk, and re-reads it if not.
+	 */
+
+	loose_dir = get_loose_refs(refs);
+
+	if (prefix && *prefix)
+		loose_dir = find_containing_dir(loose_dir, prefix, 0);
+
+	if (loose_dir) {
+		prime_ref_dir(loose_dir);
+		loose_iter = cache_ref_iterator_begin(loose_dir);
+	} else {
+		/* There's nothing to iterate over. */
+		loose_iter = empty_ref_iterator_begin();
+	}
+
+	iter->packed_ref_cache = get_packed_ref_cache(refs);
+	acquire_packed_ref_cache(iter->packed_ref_cache);
+	packed_dir = get_packed_ref_dir(iter->packed_ref_cache);
+
+	if (prefix && *prefix)
+		packed_dir = find_containing_dir(packed_dir, prefix, 0);
+
+	if (packed_dir) {
+		packed_iter = cache_ref_iterator_begin(packed_dir);
+	} else {
+		/* There's nothing to iterate over. */
+		packed_iter = empty_ref_iterator_begin();
+	}
+
+	iter->iter0 = overlay_ref_iterator_begin(loose_iter, packed_iter);
+	iter->flags = flags;
+
+	return ref_iterator;
+}
+
 /*
  * Call fn for each reference in the specified ref_cache, omitting
  * references not in the containing_dir of prefix. Call fn for all
diff --git a/refs/iterator.c b/refs/iterator.c
new file mode 100644
index 000000000..93ba472f9
--- /dev/null
+++ b/refs/iterator.c
@@ -0,0 +1,355 @@
+/*
+ * Generic reference iterator infrastructure. See refs-internal.h for
+ * documentation about the design and use of reference iterators.
+ */
+
+#include "cache.h"
+#include "refs.h"
+#include "refs/refs-internal.h"
+#include "iterator.h"
+
+int ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	return ref_iterator->vtable->advance(ref_iterator);
+}
+
+int ref_iterator_peel(struct ref_iterator *ref_iterator,
+		      struct object_id *peeled)
+{
+	return ref_iterator->vtable->peel(ref_iterator, peeled);
+}
+
+int ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	return ref_iterator->vtable->abort(ref_iterator);
+}
+
+void base_ref_iterator_init(struct ref_iterator *iter,
+			    struct ref_iterator_vtable *vtable)
+{
+	iter->vtable = vtable;
+	iter->refname = NULL;
+	iter->oid = NULL;
+	iter->flags = 0;
+}
+
+void base_ref_iterator_free(struct ref_iterator *iter)
+{
+	/* Help make use-after-free bugs fail quickly: */
+	iter->vtable = NULL;
+	free(iter);
+}
+
+struct empty_ref_iterator {
+	struct ref_iterator base;
+};
+
+static int empty_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	return ref_iterator_abort(ref_iterator);
+}
+
+static int empty_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	die("BUG: peel called for empty iterator");
+}
+
+static int empty_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	base_ref_iterator_free(ref_iterator);
+	return ITER_DONE;
+}
+
+static struct ref_iterator_vtable empty_ref_iterator_vtable = {
+	empty_ref_iterator_advance,
+	empty_ref_iterator_peel,
+	empty_ref_iterator_abort
+};
+
+struct ref_iterator *empty_ref_iterator_begin(void)
+{
+	struct empty_ref_iterator *iter = xcalloc(1, sizeof(*iter));
+	struct ref_iterator *ref_iterator = &iter->base;
+
+	base_ref_iterator_init(ref_iterator, &empty_ref_iterator_vtable);
+	return ref_iterator;
+}
+
+int is_empty_ref_iterator(struct ref_iterator *ref_iterator)
+{
+	return ref_iterator->vtable == &empty_ref_iterator_vtable;
+}
+
+struct merge_ref_iterator {
+	struct ref_iterator base;
+
+	struct ref_iterator *iter0, *iter1;
+
+	ref_iterator_select_fn *select;
+	void *cb_data;
+
+	/*
+	 * A pointer to iter0 or iter1 (whichever is supplying the
+	 * current value), or NULL if advance has not yet been called.
+	 */
+	struct ref_iterator **current;
+};
+
+static int merge_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct merge_ref_iterator *iter =
+		(struct merge_ref_iterator *)ref_iterator;
+	int ok;
+
+	if (!iter->current) {
+		/* Initialize: advance both iterators to their first entries */
+		if ((ok = ref_iterator_advance(iter->iter0)) != ITER_OK) {
+			iter->iter0 = NULL;
+			if (ok == ITER_ERROR)
+				goto error;
+		}
+		if ((ok = ref_iterator_advance(iter->iter1)) != ITER_OK) {
+			iter->iter1 = NULL;
+			if (ok == ITER_ERROR)
+				goto error;
+		}
+	} else {
+		/*
+		 * Advance the current iterator past the just-used
+		 * entry:
+		 */
+		if ((ok = ref_iterator_advance(*iter->current)) != ITER_OK) {
+			*iter->current = NULL;
+			if (ok == ITER_ERROR)
+				goto error;
+		}
+	}
+
+	/* Loop until we find an entry that we can yield. */
+	while (1) {
+		struct ref_iterator **secondary;
+		enum iterator_selection selection =
+			iter->select(iter->iter0, iter->iter1, iter->cb_data);
+
+		if (selection == ITER_SELECT_DONE) {
+			return ref_iterator_abort(ref_iterator);
+		} else if (selection == ITER_SELECT_ERROR) {
+			ref_iterator_abort(ref_iterator);
+			return ITER_ERROR;
+		}
+
+		if ((selection & ITER_CURRENT_SELECTION_MASK) == 0) {
+			iter->current = &iter->iter0;
+			secondary = &iter->iter1;
+		} else {
+			iter->current = &iter->iter1;
+			secondary = &iter->iter0;
+		}
+
+		if (selection & ITER_SKIP_SECONDARY) {
+			if ((ok = ref_iterator_advance(*secondary)) != ITER_OK) {
+				*secondary = NULL;
+				if (ok == ITER_ERROR)
+					goto error;
+			}
+		}
+
+		if (selection & ITER_YIELD_CURRENT) {
+			iter->base.refname = (*iter->current)->refname;
+			iter->base.oid = (*iter->current)->oid;
+			iter->base.flags = (*iter->current)->flags;
+			return ITER_OK;
+		}
+	}
+
+error:
+	ref_iterator_abort(ref_iterator);
+	return ITER_ERROR;
+}
+
+static int merge_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	struct merge_ref_iterator *iter =
+		(struct merge_ref_iterator *)ref_iterator;
+
+	if (!iter->current) {
+		die("BUG: peel called before advance for merge iterator");
+	}
+	return ref_iterator_peel(*iter->current, peeled);
+}
+
+static int merge_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct merge_ref_iterator *iter =
+		(struct merge_ref_iterator *)ref_iterator;
+	int ok = ITER_DONE;
+
+	if (iter->iter0) {
+		if (ref_iterator_abort(iter->iter0) != ITER_DONE)
+			ok = ITER_ERROR;
+	}
+	if (iter->iter1) {
+		if (ref_iterator_abort(iter->iter1) != ITER_DONE)
+			ok = ITER_ERROR;
+	}
+	base_ref_iterator_free(ref_iterator);
+	return ok;
+}
+
+static struct ref_iterator_vtable merge_ref_iterator_vtable = {
+	merge_ref_iterator_advance,
+	merge_ref_iterator_peel,
+	merge_ref_iterator_abort
+};
+
+struct ref_iterator *merge_ref_iterator_begin(
+		struct ref_iterator *iter0, struct ref_iterator *iter1,
+		ref_iterator_select_fn *select, void *cb_data)
+{
+	struct merge_ref_iterator *iter = xcalloc(1, sizeof(*iter));
+	struct ref_iterator *ref_iterator = &iter->base;
+
+	/*
+	 * We can't do the same kind of is_empty_ref_iterator()-style
+	 * optimization here as overlay_ref_iterator_begin() does,
+	 * because we don't know the semantics of the select function.
+	 * It might, for example, implement "intersect" by passing
+	 * references through only if they exist in both iterators.
+	 */
+
+	base_ref_iterator_init(ref_iterator, &merge_ref_iterator_vtable);
+	iter->iter0 = iter0;
+	iter->iter1 = iter1;
+	iter->select = select;
+	iter->cb_data = cb_data;
+	iter->current = NULL;
+	return ref_iterator;
+}
+
+/*
+ * A ref_iterator_select_fn that overlays the items from front on top
+ * of those from back (like loose refs over packed refs). See
+ * overlay_ref_iterator_begin().
+ */
+static enum iterator_selection overlay_iterator_select(
+		struct ref_iterator *front, struct ref_iterator *back,
+		void *cb_data)
+{
+	int cmp;
+
+	if (!back)
+		return front ? ITER_SELECT_0 : ITER_SELECT_DONE;
+	else if (!front)
+		return ITER_SELECT_1;
+
+	cmp = strcmp(front->refname, back->refname);
+
+	if (cmp < 0)
+		return ITER_SELECT_0;
+	else if (cmp > 0)
+		return ITER_SELECT_1;
+	else
+		return ITER_SELECT_0_SKIP_1;
+}
+
+struct ref_iterator *overlay_ref_iterator_begin(
+		struct ref_iterator *front, struct ref_iterator *back)
+{
+	/*
+	 * Optimization: if one of the iterators is empty, return the
+	 * other one rather than incurring the overhead of wrapping
+	 * them.
+	 */
+	if (is_empty_ref_iterator(front)) {
+		ref_iterator_abort(front);
+		return back;
+	} else if (is_empty_ref_iterator(back)) {
+		ref_iterator_abort(back);
+		return front;
+	}
+
+	return merge_ref_iterator_begin(front, back,
+					overlay_iterator_select, NULL);
+}
+
+struct prefix_ref_iterator {
+	struct ref_iterator base;
+
+	struct ref_iterator *iter0;
+	char *prefix;
+	int trim;
+};
+
+static int prefix_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct prefix_ref_iterator *iter =
+		(struct prefix_ref_iterator *)ref_iterator;
+	int ok;
+
+	while ((ok = ref_iterator_advance(iter->iter0)) == ITER_OK) {
+		if (!starts_with(iter->iter0->refname, iter->prefix))
+			continue;
+
+		iter->base.refname = iter->iter0->refname + iter->trim;
+		iter->base.oid = iter->iter0->oid;
+		iter->base.flags = iter->iter0->flags;
+		return ITER_OK;
+	}
+
+	iter->iter0 = NULL;
+	if (ref_iterator_abort(ref_iterator) != ITER_DONE)
+		return ITER_ERROR;
+	return ok;
+}
+
+static int prefix_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				    struct object_id *peeled)
+{
+	struct prefix_ref_iterator *iter =
+		(struct prefix_ref_iterator *)ref_iterator;
+
+	return ref_iterator_peel(iter->iter0, peeled);
+}
+
+static int prefix_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct prefix_ref_iterator *iter =
+		(struct prefix_ref_iterator *)ref_iterator;
+	int ok = ITER_DONE;
+
+	if (iter->iter0)
+		ok = ref_iterator_abort(iter->iter0);
+	free(iter->prefix);
+	base_ref_iterator_free(ref_iterator);
+	return ok;
+}
+
+static struct ref_iterator_vtable prefix_ref_iterator_vtable = {
+	prefix_ref_iterator_advance,
+	prefix_ref_iterator_peel,
+	prefix_ref_iterator_abort
+};
+
+struct ref_iterator *prefix_ref_iterator_begin(struct ref_iterator *iter0,
+					       const char *prefix,
+					       int trim)
+{
+	struct prefix_ref_iterator *iter;
+	struct ref_iterator *ref_iterator;
+
+	if (!*prefix && !trim)
+		return iter0; /* optimization: no need to wrap iterator */
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+
+	base_ref_iterator_init(ref_iterator, &prefix_ref_iterator_vtable);
+
+	iter->iter0 = iter0;
+	iter->prefix = xstrdup(prefix);
+	iter->trim = trim;
+
+	return ref_iterator;
+}
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 8ad02d84d..fc2088ba5 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -249,6 +249,200 @@ int rename_ref_available(const char *oldname, const char *newname);
 #define DO_FOR_EACH_INCLUDE_BROKEN 0x01
 
 /*
+ * Reference iterators
+ *
+ * A reference iterator encapsulates the state of an in-progress
+ * iteration over references. Create an instance of `struct
+ * ref_iterator` via one of the functions in this module.
+ *
+ * A freshly-created ref_iterator doesn't yet point at a reference. To
+ * advance the iterator, call ref_iterator_advance(). If successful,
+ * this sets the iterator's refname, oid, and flags fields to describe
+ * the next reference and returns ITER_OK. The data pointed at by
+ * refname and oid belong to the iterator; if you want to retain them
+ * after calling ref_iterator_advance() again or calling
+ * ref_iterator_abort(), you must make a copy. When the iteration has
+ * been exhausted, ref_iterator_advance() releases any resources
+ * assocated with the iteration, frees the ref_iterator object, and
+ * returns ITER_DONE. If you want to abort the iteration early, call
+ * ref_iterator_abort(), which also frees the ref_iterator object and
+ * any associated resources. If there was an internal error advancing
+ * to the next entry, ref_iterator_advance() aborts the iteration,
+ * frees the ref_iterator, and returns ITER_ERROR.
+ *
+ * The reference currently being looked at can be peeled by calling
+ * ref_iterator_peel(). This function is often faster than peel_ref(),
+ * so it should be preferred when iterating over references.
+ *
+ * Putting it all together, a typical iteration looks like this:
+ *
+ *     int ok;
+ *     struct ref_iterator *iter = ...;
+ *
+ *     while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
+ *             if (want_to_stop_iteration()) {
+ *                     ok = ref_iterator_abort(iter);
+ *                     break;
+ *             }
+ *
+ *             // Access information about the current reference:
+ *             if (!(iter->flags & REF_ISSYMREF))
+ *                     printf("%s is %s\n", iter->refname, oid_to_hex(&iter->oid));
+ *
+ *             // If you need to peel the reference:
+ *             ref_iterator_peel(iter, &oid);
+ *     }
+ *
+ *     if (ok != ITER_DONE)
+ *             handle_error();
+ */
+struct ref_iterator {
+	struct ref_iterator_vtable *vtable;
+	const char *refname;
+	const struct object_id *oid;
+	unsigned int flags;
+};
+
+/*
+ * Advance the iterator to the first or next item and return ITER_OK.
+ * If the iteration is exhausted, free the resources associated with
+ * the ref_iterator and return ITER_DONE. On errors, free the iterator
+ * resources and return ITER_ERROR. It is a bug to use ref_iterator or
+ * call this function again after it has returned ITER_DONE or
+ * ITER_ERROR.
+ */
+int ref_iterator_advance(struct ref_iterator *ref_iterator);
+
+/*
+ * If possible, peel the reference currently being viewed by the
+ * iterator. Return 0 on success.
+ */
+int ref_iterator_peel(struct ref_iterator *ref_iterator,
+		      struct object_id *peeled);
+
+/*
+ * End the iteration before it has been exhausted, freeing the
+ * reference iterator and any associated resources and returning
+ * ITER_DONE. If the abort itself failed, return ITER_ERROR.
+ */
+int ref_iterator_abort(struct ref_iterator *ref_iterator);
+
+/*
+ * An iterator over nothing (its first ref_iterator_advance() call
+ * returns ITER_DONE).
+ */
+struct ref_iterator *empty_ref_iterator_begin(void);
+
+/*
+ * Return true iff ref_iterator is an empty_ref_iterator.
+ */
+int is_empty_ref_iterator(struct ref_iterator *ref_iterator);
+
+/*
+ * A callback function used to instruct merge_ref_iterator how to
+ * interleave the entries from iter0 and iter1. The function should
+ * return one of the constants defined in enum iterator_selection. It
+ * must not advance either of the iterators itself.
+ *
+ * The function must be prepared to handle the case that iter0 and/or
+ * iter1 is NULL, which indicates that the corresponding sub-iterator
+ * has been exhausted. Its return value must be consistent with the
+ * current states of the iterators; e.g., it must not return
+ * ITER_SKIP_1 if iter1 has already been exhausted.
+ */
+typedef enum iterator_selection ref_iterator_select_fn(
+		struct ref_iterator *iter0, struct ref_iterator *iter1,
+		void *cb_data);
+
+/*
+ * Iterate over the entries from iter0 and iter1, with the values
+ * interleaved as directed by the select function. The iterator takes
+ * ownership of iter0 and iter1 and frees them when the iteration is
+ * over.
+ */
+struct ref_iterator *merge_ref_iterator_begin(
+		struct ref_iterator *iter0, struct ref_iterator *iter1,
+		ref_iterator_select_fn *select, void *cb_data);
+
+/*
+ * An iterator consisting of the union of the entries from front and
+ * back. If there are entries common to the two sub-iterators, use the
+ * one from front. Each iterator must iterate over its entries in
+ * strcmp() order by refname for this to work.
+ *
+ * The new iterator takes ownership of its arguments and frees them
+ * when the iteration is over. As a convenience to callers, if front
+ * or back is an empty_ref_iterator, then abort that one immediately
+ * and return the other iterator directly, without wrapping it.
+ */
+struct ref_iterator *overlay_ref_iterator_begin(
+		struct ref_iterator *front, struct ref_iterator *back);
+
+/*
+ * Wrap iter0, only letting through the references whose names start
+ * with prefix. If trim is set, set iter->refname to the name of the
+ * reference with that many characters trimmed off the front;
+ * otherwise set it to the full refname. The new iterator takes over
+ * ownership of iter0 and frees it when iteration is over. It makes
+ * its own copy of prefix.
+ *
+ * As an convenience to callers, if prefix is the empty string and
+ * trim is zero, this function returns iter0 directly, without
+ * wrapping it.
+ */
+struct ref_iterator *prefix_ref_iterator_begin(struct ref_iterator *iter0,
+					       const char *prefix,
+					       int trim);
+
+/*
+ * Iterate over the packed and loose references in the specified
+ * submodule that are within find_containing_dir(prefix). If prefix is
+ * NULL or the empty string, iterate over all references in the
+ * submodule.
+ */
+struct ref_iterator *files_ref_iterator_begin(const char *submodule,
+					      const char *prefix,
+					      unsigned int flags);
+
+/* Internal implementation of reference iteration: */
+
+/*
+ * Base class constructor for ref_iterators. Initialize the
+ * ref_iterator part of iter, setting its vtable pointer as specified.
+ * This is meant to be called only by the initializers of derived
+ * classes.
+ */
+void base_ref_iterator_init(struct ref_iterator *iter,
+			    struct ref_iterator_vtable *vtable);
+
+/*
+ * Base class destructor for ref_iterators. Destroy the ref_iterator
+ * part of iter and shallow-free the object. This is meant to be
+ * called only by the destructors of derived classes.
+ */
+void base_ref_iterator_free(struct ref_iterator *iter);
+
+/* Virtual function declarations for ref_iterators: */
+
+typedef int ref_iterator_advance_fn(struct ref_iterator *ref_iterator);
+
+typedef int ref_iterator_peel_fn(struct ref_iterator *ref_iterator,
+				 struct object_id *peeled);
+
+/*
+ * Implementations of this function should free any resources specific
+ * to the derived class, then call base_ref_iterator_free() to clean
+ * up and free the ref_iterator object.
+ */
+typedef int ref_iterator_abort_fn(struct ref_iterator *ref_iterator);
+
+struct ref_iterator_vtable {
+	ref_iterator_advance_fn *advance;
+	ref_iterator_peel_fn *peel;
+	ref_iterator_abort_fn *abort;
+};
+
+/*
  * Call fn for each reference in the specified submodule for which the
  * refname begins with prefix. If trim is non-zero, then trim that
  * many characters off the beginning of each refname before passing