dir-iterator: refactor state machine model

dir_iterator_advance() is a large function with two nested loops. Let's
improve its readability factoring out three functions and simplifying
its mechanics. The refactored model will no longer depend on
level.initialized and level.dir_state to keep track of the iteration
state and will perform on a single loop.

Also, dir_iterator_begin() currently does not check if the given string
represents a valid directory path. Since the refactored model will have
to stat() the given path at initialization, let's also check for this
kind of error and make dir_iterator_begin() return NULL, on failures,
with errno appropriately set. And add tests for this new behavior.

Improve documentation at dir-iteration.h and code comments at
dir-iterator.c to reflect the changes and eliminate possible
ambiguities.

Finally, adjust refs/files-backend.c to check for now possible
dir_iterator_begin() failures.

Original-patch-by: Daniel Ferreira <bnmvco@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 13 insertions, 4 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 63e55e6773..7ed81046d4 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -2143,13 +2143,22 @@ static struct ref_iterator_vtable files_reflog_iterator_vtable = {
 static struct ref_iterator *reflog_iterator_begin(struct ref_store *ref_store,
 						  const char *gitdir)
 {
-	struct files_reflog_iterator *iter = xcalloc(1, sizeof(*iter));
-	struct ref_iterator *ref_iterator = &iter->base;
+	struct dir_iterator *diter;
+	struct files_reflog_iterator *iter;
+	struct ref_iterator *ref_iterator;
 	struct strbuf sb = STRBUF_INIT;
 
-	base_ref_iterator_init(ref_iterator, &files_reflog_iterator_vtable, 0);
 	strbuf_addf(&sb, "%s/logs", gitdir);
-	iter->dir_iterator = dir_iterator_begin(sb.buf);
+
+	diter = dir_iterator_begin(sb.buf);
+	if(!diter)
+		return empty_ref_iterator_begin();
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+
+	base_ref_iterator_init(ref_iterator, &files_reflog_iterator_vtable, 0);
+	iter->dir_iterator = diter;
 	iter->ref_store = ref_store;
 	strbuf_release(&sb);