diff: add a notify callback to `git_diff__from_iterators`

The callback will be called for each file, just before the `git_delta_t` gets inserted into the diff list.

When the callback:
- returns < 0, the diff process will be aborted
- returns > 0, the delta will not be inserted into the diff list, but the diff process continues
- returns 0, the delta is inserted into the diff list, and the diff process continues

4 files changed, 260 insertions, 44 deletions

diff --git a/include/git2/diff.h b/include/git2/diff.h
index 81c41df04..3a88902ad 100644
--- a/include/git2/diff.h
+++ b/include/git2/diff.h
@@ -123,40 +123,6 @@ typedef enum {
 } git_diff_option_t;
 
 /**
- * Structure describing options about how the diff should be executed.
- *
- * Setting all values of the structure to zero will yield the default
- * values.  Similarly, passing NULL for the options structure will
- * give the defaults.  The default values are marked below.
- *
- * - `flags` is a combination of the `git_diff_option_t` values above
- * - `context_lines` is the number of unchanged lines that define the
- *    boundary of a hunk (and to display before and after)
- * - `interhunk_lines` is the maximum number of unchanged lines between
- *    hunk boundaries before the hunks will be merged into a one.
- * - `old_prefix` is the virtual "directory" to prefix to old file names
- *   in hunk headers (default "a")
- * - `new_prefix` is the virtual "directory" to prefix to new file names
- *   in hunk headers (default "b")
- * - `pathspec` is an array of paths / fnmatch patterns to constrain diff
- * - `max_size` is a file size (in bytes) above which a blob will be marked
- *   as binary automatically; pass a negative value to disable.
- */
-typedef struct {
-	unsigned int version;      /**< version for the struct */
-	uint32_t flags;            /**< defaults to GIT_DIFF_NORMAL */
-	uint16_t context_lines;    /**< defaults to 3 */
-	uint16_t interhunk_lines;  /**< defaults to 0 */
-	const char *old_prefix;    /**< defaults to "a" */
-	const char *new_prefix;    /**< defaults to "b" */
-	git_strarray pathspec;     /**< defaults to include all paths */
-	git_off_t max_size;        /**< defaults to 512MB */
-} git_diff_options;
-
-#define GIT_DIFF_OPTIONS_VERSION 1
-#define GIT_DIFF_OPTIONS_INIT {GIT_DIFF_OPTIONS_VERSION}
-
-/**
  * The diff list object that contains all individual file deltas.
  *
  * This is an opaque structure which will be allocated by one of the diff
@@ -266,6 +232,64 @@ typedef struct {
 } git_diff_delta;
 
 /**
+ * Diff notification callback function.
+ *
+ * The callback will be called for each file, just before the `git_delta_t`
+ * gets inserted into the diff list.
+ *
+ * When the callback:
+ * - returns < 0, the diff process will be aborted.
+ * - returns > 0, the delta will not be inserted into the diff list, but the
+ *		diff process continues.
+ * - returns 0, the delta is inserted into the diff list, and the diff process
+ *		continues.
+ */
+typedef int (*git_diff_notify_cb)(
+	const git_diff_list *diff_so_far,
+	const git_diff_delta *delta_to_add,
+	const char *matched_pathspec,
+	void *payload);
+
+/**
+ * Structure describing options about how the diff should be executed.
+ *
+ * Setting all values of the structure to zero will yield the default
+ * values.  Similarly, passing NULL for the options structure will
+ * give the defaults.  The default values are marked below.
+ *
+ * - `flags` is a combination of the `git_diff_option_t` values above
+ * - `context_lines` is the number of unchanged lines that define the
+ *    boundary of a hunk (and to display before and after)
+ * - `interhunk_lines` is the maximum number of unchanged lines between
+ *    hunk boundaries before the hunks will be merged into a one.
+ * - `old_prefix` is the virtual "directory" to prefix to old file names
+ *   in hunk headers (default "a")
+ * - `new_prefix` is the virtual "directory" to prefix to new file names
+ *   in hunk headers (default "b")
+ * - `pathspec` is an array of paths / fnmatch patterns to constrain diff
+ * - `max_size` is a file size (in bytes) above which a blob will be marked
+ *   as binary automatically; pass a negative value to disable.
+ * - `notify_cb` is an optional callback function, notifying the consumer of
+ *   which files are being examined as the diff is generated
+ * - `notify_payload` is the payload data to pass to the `notify_cb` function
+ */
+typedef struct {
+	unsigned int version;      /**< version for the struct */
+	uint32_t flags;            /**< defaults to GIT_DIFF_NORMAL */
+	uint16_t context_lines;    /**< defaults to 3 */
+	uint16_t interhunk_lines;  /**< defaults to 0 */
+	const char *old_prefix;    /**< defaults to "a" */
+	const char *new_prefix;    /**< defaults to "b" */
+	git_strarray pathspec;     /**< defaults to include all paths */
+	git_off_t max_size;        /**< defaults to 512MB */
+	git_diff_notify_cb notify_cb;
+	void *notify_payload;
+} git_diff_options;
+
+#define GIT_DIFF_OPTIONS_VERSION 1
+#define GIT_DIFF_OPTIONS_INIT {GIT_DIFF_OPTIONS_VERSION}
+
+/**
  * When iterating over a diff, callback that will be made per file.
  *
  * @param delta A pointer to the delta data for the file
diff --git a/src/diff.c b/src/diff.c
index 3e8028551..d9bc32a37 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -41,6 +41,18 @@ static git_diff_delta *diff_delta__alloc(
 	return delta;
 }
 
+static int diff_notify(
+	const git_diff_list *diff,
+	const git_diff_delta *delta,
+	const char *matched_pathspec)
+{
+	if (!diff->opts.notify_cb)
+		return 0;
+
+	return diff->opts.notify_cb(
+		diff, delta, matched_pathspec, diff->opts.notify_payload);
+}
+
 static int diff_delta__from_one(
 	git_diff_list *diff,
 	git_delta_t   status,
@@ -48,6 +60,7 @@ static int diff_delta__from_one(
 {
 	git_diff_delta *delta;
 	const char *matched_pathspec;
+	int notify_res;
 
 	if (status == GIT_DELTA_IGNORED &&
 		(diff->opts.flags & GIT_DIFF_INCLUDE_IGNORED) == 0)
@@ -85,12 +98,16 @@ static int diff_delta__from_one(
 		!git_oid_iszero(&delta->new_file.oid))
 		delta->new_file.flags |= GIT_DIFF_FILE_VALID_OID;
 
-	if (git_vector_insert(&diff->deltas, delta) < 0) {
+	notify_res = diff_notify(diff, delta, matched_pathspec);
+
+	if (notify_res)
+		git__free(delta);
+	else if (git_vector_insert(&diff->deltas, delta) < 0) {
 		git__free(delta);
 		return -1;
 	}
 
-	return 0;
+	return notify_res < 0 ? GIT_EUSER : 0;
 }
 
 static int diff_delta__from_two(
@@ -100,9 +117,11 @@ static int diff_delta__from_two(
 	uint32_t old_mode,
 	const git_index_entry *new_entry,
 	uint32_t new_mode,
-	git_oid *new_oid)
+	git_oid *new_oid,
+	const char *matched_pathspec)
 {
 	git_diff_delta *delta;
+	int notify_res;
 
 	if (status == GIT_DELTA_UNMODIFIED &&
 		(diff->opts.flags & GIT_DIFF_INCLUDE_UNMODIFIED) == 0)
@@ -139,12 +158,16 @@ static int diff_delta__from_two(
 	if (new_oid || !git_oid_iszero(&new_entry->oid))
 		delta->new_file.flags |= GIT_DIFF_FILE_VALID_OID;
 
-	if (git_vector_insert(&diff->deltas, delta) < 0) {
+	notify_res = diff_notify(diff, delta, matched_pathspec);
+
+	if (notify_res)
+		git__free(delta);
+	else if (git_vector_insert(&diff->deltas, delta) < 0) {
 		git__free(delta);
 		return -1;
 	}
 
-	return 0;
+	return notify_res < 0 ? GIT_EUSER : 0;
 }
 
 static git_diff_delta *diff_delta__last_for_item(
@@ -528,7 +551,7 @@ static int maybe_modified(
 	}
 
 	return diff_delta__from_two(
-		diff, status, oitem, omode, nitem, nmode, use_noid);
+		diff, status, oitem, omode, nitem, nmode, use_noid, matched_pathspec);
 }
 
 static bool entry_is_prefixed(
@@ -749,10 +772,11 @@ int git_diff__from_iterators(
 		else {
 			assert(oitem && nitem && cmp == 0);
 
-			if (maybe_modified(old_iter, oitem, new_iter, nitem, diff) < 0 ||
-				git_iterator_advance(old_iter, &oitem) < 0 ||
-				git_iterator_advance(new_iter, &nitem) < 0)
-				goto fail;
+			if (maybe_modified(
+				old_iter, oitem, new_iter, nitem, diff) < 0 ||
+					git_iterator_advance(old_iter, &oitem) < 0 ||
+					git_iterator_advance(new_iter, &nitem) < 0)
+					goto fail;
 		}
 	}
 
diff --git a/tests-clar/diff/diff_helpers.h b/tests-clar/diff/diff_helpers.h
index 49c265285..a43847b79 100644
--- a/tests-clar/diff/diff_helpers.h
+++ b/tests-clar/diff/diff_helpers.h
@@ -20,6 +20,11 @@ typedef struct {
 	int line_dels;
 } diff_expects;
 
+typedef struct {
+	const char *path;
+	const char *matched_pathspec;
+} notify_expected;
+
 extern int diff_file_cb(
 	const git_diff_delta *delta,
 	float progress,
diff --git a/tests-clar/diff/workdir.c b/tests-clar/diff/workdir.c
index 21da63954..7e8915c4b 100644
--- a/tests-clar/diff/workdir.c
+++ b/tests-clar/diff/workdir.c
@@ -307,6 +307,169 @@ void test_diff_workdir__to_index_with_pathspec(void)
 	git_diff_list_free(diff);
 }
 
+static int assert_called_notifications(
+	const git_diff_list *diff_so_far,
+	const git_diff_delta *delta_to_add,
+	const char *matched_pathspec,
+	void *payload)
+{
+	bool found = false;
+	notify_expected *exp = (notify_expected*)payload;
+	notify_expected *e;;
+
+	GIT_UNUSED(diff_so_far);
+
+	for (e = exp; e->path != NULL; e++) {
+		if (strcmp(e->path, delta_to_add->new_file.path))
+			continue;
+
+		cl_assert_equal_s(e->matched_pathspec, matched_pathspec);
+
+		found = true;
+		break;
+	}
+
+	cl_assert(found);
+	return 0;
+}
+
+void test_diff_workdir__to_index_notify(void)
+{
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	git_diff_list *diff = NULL;
+	diff_expects exp;
+
+	char *searched_pathspecs_solo[] = {
+		"*_deleted",
+	};
+	notify_expected expected_matched_pathspecs_solo[] = {
+		{ "file_deleted", "*_deleted" },
+		{ "staged_changes_file_deleted", "*_deleted" },
+		{ NULL, NULL }
+	};
+
+	char *searched_pathspecs_multiple[] = {
+		"staged_changes_cant_find_me",
+		"subdir/modified_cant_find_me",
+		"subdir/*",
+		"staged*"
+	};
+	notify_expected expected_matched_pathspecs_multiple[] = {
+		{ "staged_changes_file_deleted", "staged*" },
+		{ "staged_changes_modified_file", "staged*" },
+		{ "staged_delete_modified_file", "staged*" },
+		{ "staged_new_file_deleted_file", "staged*" },
+		{ "staged_new_file_modified_file", "staged*" },
+		{ "subdir/deleted_file", "subdir/*" },
+		{ "subdir/modified_file", "subdir/*" },
+		{ "subdir/new_file", "subdir/*" },
+		{ NULL, NULL }
+	};
+
+	g_repo = cl_git_sandbox_init("status");
+
+	opts.flags |= GIT_DIFF_INCLUDE_IGNORED | GIT_DIFF_INCLUDE_UNTRACKED;
+	opts.notify_cb = assert_called_notifications;
+	opts.pathspec.strings = searched_pathspecs_solo;
+	opts.pathspec.count   = 1;
+
+	opts.notify_payload = &expected_matched_pathspecs_solo;
+	memset(&exp, 0, sizeof(exp));
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, NULL, &opts));
+	cl_git_pass(git_diff_foreach(diff, diff_file_cb, NULL, NULL, &exp));
+
+	cl_assert_equal_i(2, exp.files);
+
+	git_diff_list_free(diff);
+
+	opts.pathspec.strings = searched_pathspecs_multiple;
+	opts.pathspec.count   = 4;
+	opts.notify_payload = &expected_matched_pathspecs_multiple;
+	memset(&exp, 0, sizeof(exp));
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, NULL, &opts));
+	cl_git_pass(git_diff_foreach(diff, diff_file_cb, NULL, NULL, &exp));
+
+	cl_assert_equal_i(8, exp.files);
+
+	git_diff_list_free(diff);
+}
+
+static int abort_diff(
+	const git_diff_list *diff_so_far,
+	const git_diff_delta *delta_to_add,
+	const char *matched_pathspec,
+	void *payload)
+{
+	GIT_UNUSED(diff_so_far);
+	GIT_UNUSED(delta_to_add);
+	GIT_UNUSED(matched_pathspec);
+	GIT_UNUSED(payload);
+
+	return -42;
+}
+
+void test_diff_workdir__to_index_notify_can_be_aborted_by_callback(void)
+{
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	git_diff_list *diff = NULL;
+	char *pathspec = NULL;
+
+	g_repo = cl_git_sandbox_init("status");
+
+	opts.flags |= GIT_DIFF_INCLUDE_IGNORED | GIT_DIFF_INCLUDE_UNTRACKED;
+	opts.notify_cb = abort_diff;
+	opts.pathspec.strings = &pathspec;
+	opts.pathspec.count   = 1;
+
+	pathspec = "file_deleted";
+	cl_git_fail(git_diff_index_to_workdir(&diff, g_repo, NULL, &opts));
+
+	pathspec = "staged_changes_modified_file";
+	cl_git_fail(git_diff_index_to_workdir(&diff, g_repo, NULL, &opts));
+}
+
+static int filter_all(
+	const git_diff_list *diff_so_far,
+	const git_diff_delta *delta_to_add,
+	const char *matched_pathspec,
+	void *payload)
+{
+	GIT_UNUSED(diff_so_far);
+	GIT_UNUSED(delta_to_add);
+	GIT_UNUSED(matched_pathspec);
+	GIT_UNUSED(payload);
+
+	return 42;
+}
+
+void test_diff_workdir__to_index_notify_can_be_used_as_filtering_function(void)
+{
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	git_diff_list *diff = NULL;
+	char *pathspec = NULL;
+	diff_expects exp;
+
+	g_repo = cl_git_sandbox_init("status");
+
+	opts.flags |= GIT_DIFF_INCLUDE_IGNORED | GIT_DIFF_INCLUDE_UNTRACKED;
+	opts.notify_cb = filter_all;
+	opts.pathspec.strings = &pathspec;
+	opts.pathspec.count   = 1;
+
+	pathspec = "*_deleted";
+	memset(&exp, 0, sizeof(exp));
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, NULL, &opts));
+	cl_git_pass(git_diff_foreach(diff, diff_file_cb, NULL, NULL, &exp));
+
+	cl_assert_equal_i(0, exp.files);
+
+	git_diff_list_free(diff);
+}
+
+
 void test_diff_workdir__filemode_changes(void)
 {
 	git_config *cfg;