diff_generate: refactor DIFF_FROM_ITERATORS macro of doom

While the `DIFF_FROM_ITERATORS` does make it shorter to implement the
various `git_diff_foo_to_bar` functions, it is a complex and unreadable
beast that implicitly assumes certain local variable names. This is not
something desirable to have at all and obstructs understanding and more
importantly debugging the code by quite a bit.

The `DIFF_FROM_ITERATORS` macro basically removed the burden of having
to derive the options for both iterators from a pair of iterator flags
and the diff options. This patch introduces a new function that does the
that exact and refactors all callers to manage the iterators by
themselves.

As we potentially need to allocate a shared prefix for the
iterator, we need to tell the caller to allocate that prefix as soon as
the options aren't required anymore. Thus, the function has a `char
**prefix` out pointer that will get set to the allocated string and
subsequently be free'd by the caller.

While this patch increases the line count, I personally deem this to an
acceptable tradeoff for increased readbiblity.

Author: pks-t

src/diff_generate.c

@@ -1262,29 +1262,30 @@ int git_diff__from_iterators(
 	return error;
 }
 
-#define DIFF_FROM_ITERATORS(MAKE_FIRST, FLAGS_FIRST, MAKE_SECOND, FLAGS_SECOND) do { \
-	git_iterator *a = NULL, *b = NULL; \
-	char *pfx = (opts && !(opts->flags & GIT_DIFF_DISABLE_PATHSPEC_MATCH)) ? \
-		git_pathspec_prefix(&opts->pathspec) : NULL; \
-	git_iterator_options a_opts = GIT_ITERATOR_OPTIONS_INIT, \
-		b_opts = GIT_ITERATOR_OPTIONS_INIT; \
-	a_opts.flags = FLAGS_FIRST; \
-	a_opts.start = pfx; \
-	a_opts.end = pfx; \
-	b_opts.flags = FLAGS_SECOND; \
-	b_opts.start = pfx; \
-	b_opts.end = pfx; \
-	GIT_ERROR_CHECK_VERSION(opts, GIT_DIFF_OPTIONS_VERSION, "git_diff_options"); \
-	if (opts && (opts->flags & GIT_DIFF_DISABLE_PATHSPEC_MATCH)) { \
-		a_opts.pathlist.strings = opts->pathspec.strings; \
-		a_opts.pathlist.count = opts->pathspec.count; \
-		b_opts.pathlist.strings = opts->pathspec.strings; \
-		b_opts.pathlist.count = opts->pathspec.count; \
-	} \
-	if (!error && !(error = MAKE_FIRST) && !(error = MAKE_SECOND)) \
-		error = git_diff__from_iterators(&diff, repo, a, b, opts); \
-	git__free(pfx); git_iterator_free(a); git_iterator_free(b); \
-} while (0)
+static int diff_prepare_iterator_opts(char **prefix, git_iterator_options *a, int aflags,
+		git_iterator_options *b, int bflags,
+		const git_diff_options *opts)
+{
+	GIT_ERROR_CHECK_VERSION(opts, GIT_DIFF_OPTIONS_VERSION, "git_diff_options");
+
+	*prefix = NULL;
+
+	if (opts && (opts->flags & GIT_DIFF_DISABLE_PATHSPEC_MATCH)) {
+		a->pathlist.strings = opts->pathspec.strings;
+		a->pathlist.count = opts->pathspec.count;
+		b->pathlist.strings = opts->pathspec.strings;
+		b->pathlist.count = opts->pathspec.count;
+	} else if (opts) {
+		*prefix = git_pathspec_prefix(&opts->pathspec);
+	}
+
+	a->flags = aflags;
+	b->flags = bflags;
+	a->start = b->start = *prefix;
+	a->end = b->end = *prefix;
+
+	return 0;
+}
 
 int git_diff_tree_to_tree(
 	git_diff **out,
@@ -1293,8 +1294,12 @@ int git_diff_tree_to_tree(
 	git_tree *new_tree,
 	const git_diff_options *opts)
 {
-	git_diff *diff = NULL;
 	git_iterator_flag_t iflag = GIT_ITERATOR_DONT_IGNORE_CASE;
+	git_iterator_options a_opts = GIT_ITERATOR_OPTIONS_INIT,
+		b_opts = GIT_ITERATOR_OPTIONS_INIT;
+	git_iterator *a = NULL, *b = NULL;
+	git_diff *diff = NULL;
+	char *prefix = NULL;
 	int error = 0;
 
 	assert(out && repo);
@@ -1308,13 +1313,19 @@ int git_diff_tree_to_tree(
 	if (opts && (opts->flags & GIT_DIFF_IGNORE_CASE) != 0)
 		iflag = GIT_ITERATOR_IGNORE_CASE;
 
-	DIFF_FROM_ITERATORS(
-		git_iterator_for_tree(&a, old_tree, &a_opts), iflag,
-		git_iterator_for_tree(&b, new_tree, &b_opts), iflag
-	);
+	if ((error = diff_prepare_iterator_opts(&prefix, &a_opts, iflag, &b_opts, iflag, opts)) < 0 ||
+	    (error = git_iterator_for_tree(&a, old_tree, &a_opts)) < 0 ||
+	    (error = git_iterator_for_tree(&b, new_tree, &b_opts)) < 0 ||
+	    (error = git_diff__from_iterators(&diff, repo, a, b, opts)) < 0)
+		goto out;
 
-	if (!error)
-		*out = diff;
+	*out = diff;
+	diff = NULL;
+out:
+	git_iterator_free(a);
+	git_iterator_free(b);
+	git_diff_free(diff);
+	git__free(prefix);
 
 	return error;
 }
@@ -1337,9 +1348,13 @@ int git_diff_tree_to_index(
 	git_index *index,
 	const git_diff_options *opts)
 {
-	git_diff *diff = NULL;
 	git_iterator_flag_t iflag = GIT_ITERATOR_DONT_IGNORE_CASE |
 		GIT_ITERATOR_INCLUDE_CONFLICTS;
+	git_iterator_options a_opts = GIT_ITERATOR_OPTIONS_INIT,
+		b_opts = GIT_ITERATOR_OPTIONS_INIT;
+	git_iterator *a = NULL, *b = NULL;
+	git_diff *diff = NULL;
+	char *prefix = NULL;
 	bool index_ignore_case = false;
 	int error = 0;
 
@@ -1352,17 +1367,23 @@ int git_diff_tree_to_index(
 
 	index_ignore_case = index->ignore_case;
 
-	DIFF_FROM_ITERATORS(
-		git_iterator_for_tree(&a, old_tree, &a_opts), iflag,
-		git_iterator_for_index(&b, repo, index, &b_opts), iflag
-	);
+	if ((error = diff_prepare_iterator_opts(&prefix, &a_opts, iflag, &b_opts, iflag, opts)) < 0 ||
+	    (error = git_iterator_for_tree(&a, old_tree, &a_opts)) < 0 ||
+	    (error = git_iterator_for_index(&b, repo, index, &b_opts)) < 0 ||
+	    (error = git_diff__from_iterators(&diff, repo, a, b, opts) < 0))
+		goto out;
 
 	/* if index is in case-insensitive order, re-sort deltas to match */
-	if (!error && index_ignore_case)
+	if (index_ignore_case)
 		git_diff__set_ignore_case(diff, true);
 
-	if (!error)
-		*out = diff;
+	*out = diff;
+	diff = NULL;
+out:
+	git_iterator_free(a);
+	git_iterator_free(b);
+	git_diff_free(diff);
+	git__free(prefix);
 
 	return error;
 }
@@ -1373,7 +1394,11 @@ int git_diff_index_to_workdir(
 	git_index *index,
 	const git_diff_options *opts)
 {
+	git_iterator_options a_opts = GIT_ITERATOR_OPTIONS_INIT,
+		b_opts = GIT_ITERATOR_OPTIONS_INIT;
+	git_iterator *a = NULL, *b = NULL;
 	git_diff *diff = NULL;
+	char *prefix = NULL;
 	int error = 0;
 
 	assert(out && repo);
@@ -1383,20 +1408,24 @@ int git_diff_index_to_workdir(
 	if (!index && (error = diff_load_index(&index, repo)) < 0)
 		return error;
 
-	DIFF_FROM_ITERATORS(
-		git_iterator_for_index(&a, repo, index, &a_opts),
-		GIT_ITERATOR_INCLUDE_CONFLICTS,
-
-		git_iterator_for_workdir(&b, repo, index, NULL, &b_opts),
-		GIT_ITERATOR_DONT_AUTOEXPAND
-	);
-
-	if (!error && (diff->opts.flags & GIT_DIFF_UPDATE_INDEX) != 0 &&
-		((git_diff_generated *)diff)->index_updated)
-		error = git_index_write(index);
-
-	if (!error)
-		*out = diff;
+	if ((error = diff_prepare_iterator_opts(&prefix, &a_opts, GIT_ITERATOR_INCLUDE_CONFLICTS,
+						&b_opts, GIT_ITERATOR_DONT_AUTOEXPAND, opts)) < 0 ||
+	    (error = git_iterator_for_index(&a, repo, index, &a_opts)) < 0 ||
+	    (error = git_iterator_for_workdir(&b, repo, index, NULL, &b_opts)) < 0 ||
+	    (error = git_diff__from_iterators(&diff, repo, a, b, opts)) < 0)
+		goto out;
+
+	if ((diff->opts.flags & GIT_DIFF_UPDATE_INDEX) && ((git_diff_generated *)diff)->index_updated)
+		if ((error = git_index_write(index)) < 0)
+			goto out;
+
+	*out = diff;
+	diff = NULL;
+out:
+	git_iterator_free(a);
+	git_iterator_free(b);
+	git_diff_free(diff);
+	git__free(prefix);
 
 	return error;
 }
@@ -1407,24 +1436,33 @@ int git_diff_tree_to_workdir(
 	git_tree *old_tree,
 	const git_diff_options *opts)
 {
+	git_iterator_options a_opts = GIT_ITERATOR_OPTIONS_INIT,
+		b_opts = GIT_ITERATOR_OPTIONS_INIT;
+	git_iterator *a = NULL, *b = NULL;
 	git_diff *diff = NULL;
+	char *prefix = NULL;
 	git_index *index;
-	int error = 0;
+	int error;
 
 	assert(out && repo);
 
 	*out = NULL;
 
-	if ((error = git_repository_index__weakptr(&index, repo)))
-		return error;
-
-	DIFF_FROM_ITERATORS(
-		git_iterator_for_tree(&a, old_tree, &a_opts), 0,
-		git_iterator_for_workdir(&b, repo, index, old_tree, &b_opts), GIT_ITERATOR_DONT_AUTOEXPAND
-	);
-
-	if (!error)
-		*out = diff;
+	if ((error = diff_prepare_iterator_opts(&prefix, &a_opts, 0,
+						&b_opts, GIT_ITERATOR_DONT_AUTOEXPAND, opts) < 0) ||
+	    (error = git_repository_index__weakptr(&index, repo)) < 0 ||
+	    (error = git_iterator_for_tree(&a, old_tree, &a_opts)) < 0 ||
+	    (error = git_iterator_for_workdir(&b, repo, index, old_tree, &b_opts)) < 0 ||
+	    (error = git_diff__from_iterators(&diff, repo, a, b, opts)) < 0)
+		goto out;
+
+	*out = diff;
+	diff = NULL;
+out:
+	git_iterator_free(a);
+	git_iterator_free(b);
+	git_diff_free(diff);
+	git__free(prefix);
 
 	return error;
 }
@@ -1468,24 +1506,35 @@ int git_diff_index_to_index(
 	git_index *new_index,
 	const git_diff_options *opts)
 {
-	git_diff *diff;
-	int error = 0;
+	git_iterator_options a_opts = GIT_ITERATOR_OPTIONS_INIT,
+		b_opts = GIT_ITERATOR_OPTIONS_INIT;
+	git_iterator *a = NULL, *b = NULL;
+	git_diff *diff = NULL;
+	char *prefix = NULL;
+	int error;
 
 	assert(out && old_index && new_index);
 
 	*out = NULL;
 
-	DIFF_FROM_ITERATORS(
-		git_iterator_for_index(&a, repo, old_index, &a_opts), GIT_ITERATOR_DONT_IGNORE_CASE,
-		git_iterator_for_index(&b, repo, new_index, &b_opts), GIT_ITERATOR_DONT_IGNORE_CASE
-	);
+	if ((error = diff_prepare_iterator_opts(&prefix, &a_opts, GIT_ITERATOR_DONT_IGNORE_CASE,
+						&b_opts, GIT_ITERATOR_DONT_IGNORE_CASE, opts) < 0) ||
+	    (error = git_iterator_for_index(&a, repo, old_index, &a_opts)) < 0 ||
+	    (error = git_iterator_for_index(&b, repo, new_index, &b_opts)) < 0 ||
+	    (error = git_diff__from_iterators(&diff, repo, a, b, opts)) < 0)
+		goto out;
 
 	/* if index is in case-insensitive order, re-sort deltas to match */
-	if (!error && (old_index->ignore_case || new_index->ignore_case))
+	if (old_index->ignore_case || new_index->ignore_case)
 		git_diff__set_ignore_case(diff, true);
 
-	if (!error)
-		*out = diff;
+	*out = diff;
+	diff = NULL;
+out:
+	git_iterator_free(a);
+	git_iterator_free(b);
+	git_diff_free(diff);
+	git__free(prefix);
 
 	return error;
 }