thodg/libgit2

diff --git a/include/git2/apply.h b/include/git2/apply.h
index 94bac1f..b248eaa 100644
--- a/include/git2/apply.h
+++ b/include/git2/apply.h
@@ -53,6 +53,15 @@ typedef int GIT_CALLBACK(git_apply_hunk_cb)(
 	const git_diff_hunk *hunk,
 	void *payload);
 
+/** Flags controlling the behavior of git_apply */
+typedef enum {
+	/**
+	 * Don't actually make changes, just test that the patch applies.
+	 * This is the equivalent of `git apply --check`.
+	 */
+	GIT_APPLY_CHECK = (1 << 0),
+} git_apply_flags_t;
+
 /**
  * Apply options structure
  *
@@ -72,6 +81,9 @@ typedef struct {
 
 	/** Payload passed to both delta_cb & hunk_cb. */
 	void *payload;
+
+	/** Bitmask of git_apply_flags_t */
+	unsigned int flags;
 } git_apply_options;
 
 #define GIT_APPLY_OPTIONS_VERSION 1
diff --git a/src/apply.c b/src/apply.c
index 0424262..65b057c 100644
--- a/src/apply.c
+++ b/src/apply.c
@@ -845,13 +845,17 @@ int git_apply(
 	    (error = git_reader_for_index(&post_reader, repo, postimage)) < 0)
 		goto done;
 
-	if ((error = git_repository_index(&index, repo)) < 0 ||
-	    (error = git_indexwriter_init(&indexwriter, index)) < 0)
-		goto done;
+	if (!(opts.flags & GIT_APPLY_CHECK))
+		if ((error = git_repository_index(&index, repo)) < 0 ||
+		    (error = git_indexwriter_init(&indexwriter, index)) < 0)
+			goto done;
 
 	if ((error = apply_deltas(repo, pre_reader, preimage, post_reader, postimage, diff, &opts)) < 0)
 		goto done;
 
+	if ((opts.flags & GIT_APPLY_CHECK))
+		goto done;
+
 	switch (location) {
 	case GIT_APPLY_LOCATION_BOTH:
 		error = git_apply__to_workdir(repo, diff, preimage, postimage, location, &opts);
diff --git a/tests/apply/apply_helpers.h b/tests/apply/apply_helpers.h
index 364daf4..2d0019a 100644
--- a/tests/apply/apply_helpers.h
+++ b/tests/apply/apply_helpers.h
@@ -21,6 +21,27 @@
 	"-longer. take out the slices of ham, and skim off the grease if any\n" \
 	"+longer; take out the slices of ham, and skim off the grease if any\n"
 
+/* This is the binary equivalent of DIFF_MODIFY_TWO_FILES */
+#define DIFF_MODIFY_TWO_FILES_BINARY \
+   "diff --git a/asparagus.txt b/asparagus.txt\n" \
+   "index f51658077d85f2264fa179b4d0848268cb3475c3..ffb36e513f5fdf8a6ba850a20142676a2ac4807d 100644\n" \
+   "GIT binary patch\n" \
+   "delta 24\n" \
+   "fcmX@ja+-zTF*v|6$k9DCSRvRyG(c}7zYP-rT_OhP\n" \
+   "\n" \
+   "delta 24\n" \
+   "fcmX@ja+-zTF*v|6$k9DCSRvRyG(d49zYP-rT;T@W\n" \
+   "\n" \
+   "diff --git a/veal.txt b/veal.txt\n" \
+   "index 94d2c01087f48213bd157222d54edfefd77c9bba..a7b066537e6be7109abfe4ff97b675d4e077da20 100644\n" \
+   "GIT binary patch\n" \
+   "delta 26\n" \
+   "hcmX@kah!uI%+=9HA=p1OKyM?L03)OIW@$zpW&mXg25bNT\n" \
+   "\n" \
+   "delta 26\n" \
+   "hcmX@kah!uI%+=9HA=p1OKyf3N03)N`W@$zpW&mU#22ub3\n" \
+   "\n"
+
 #define DIFF_DELETE_FILE \
 	"diff --git a/gravy.txt b/gravy.txt\n" \
 	"deleted file mode 100644\n" \
diff --git a/tests/apply/check.c b/tests/apply/check.c
new file mode 100644
index 0000000..9e42365
--- /dev/null
+++ b/tests/apply/check.c
@@ -0,0 +1,121 @@
+#include "clar_libgit2.h"
+#include "apply_helpers.h"
+
+static git_repository *repo;
+
+#define TEST_REPO_PATH "merge-recursive"
+
+void test_apply_check__initialize(void)
+{
+	git_oid oid;
+	git_commit *commit;
+
+	repo = cl_git_sandbox_init(TEST_REPO_PATH);
+
+	git_oid_fromstr(&oid, "539bd011c4822c560c1d17cab095006b7a10f707");
+	cl_git_pass(git_commit_lookup(&commit, repo, &oid));
+	cl_git_pass(git_reset(repo, (git_object *)commit, GIT_RESET_HARD, NULL));
+	git_commit_free(commit);
+}
+
+void test_apply_check__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+void test_apply_check__generate_diff(void)
+{
+	git_oid a_oid, b_oid;
+	git_commit *a_commit, *b_commit;
+	git_tree *a_tree, *b_tree;
+	git_diff *diff;
+	git_diff_options diff_opts = GIT_DIFF_OPTIONS_INIT;
+	git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
+
+	cl_git_pass(git_oid_fromstr(&a_oid, "539bd011c4822c560c1d17cab095006b7a10f707"));
+	cl_git_pass(git_oid_fromstr(&b_oid, "7c7bf85e978f1d18c0566f702d2cb7766b9c8d4f"));
+	cl_git_pass(git_commit_lookup(&a_commit, repo, &a_oid));
+	cl_git_pass(git_commit_lookup(&b_commit, repo, &b_oid));
+
+	cl_git_pass(git_commit_tree(&a_tree, a_commit));
+	cl_git_pass(git_commit_tree(&b_tree, b_commit));
+
+	opts.flags |= GIT_APPLY_CHECK;
+	cl_git_pass(git_diff_tree_to_tree(&diff, repo, a_tree, b_tree, &diff_opts));
+	cl_git_pass(git_apply(repo, diff, GIT_APPLY_LOCATION_BOTH, &opts));
+
+	validate_index_unchanged(repo);
+	validate_workdir_unchanged(repo);
+
+	git_diff_free(diff);
+	git_tree_free(a_tree);
+	git_tree_free(b_tree);
+	git_commit_free(a_commit);
+	git_commit_free(b_commit);
+}
+
+void test_apply_check__parsed_diff(void)
+{
+	git_diff *diff;
+	git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
+
+	opts.flags |= GIT_APPLY_CHECK;
+	cl_git_pass(git_diff_from_buffer(&diff,
+		DIFF_MODIFY_TWO_FILES, strlen(DIFF_MODIFY_TWO_FILES)));
+	cl_git_pass(git_apply(repo, diff, GIT_APPLY_LOCATION_INDEX, &opts));
+
+	validate_index_unchanged(repo);
+	validate_workdir_unchanged(repo);
+
+	git_diff_free(diff);
+}
+
+void test_apply_check__binary(void)
+{
+   git_diff *diff;
+   git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
+
+   opts.flags |= GIT_APPLY_CHECK;
+   cl_git_pass(git_diff_from_buffer(&diff,
+       DIFF_MODIFY_TWO_FILES_BINARY,
+       strlen(DIFF_MODIFY_TWO_FILES_BINARY)));
+   cl_git_pass(git_apply(repo, diff, GIT_APPLY_LOCATION_INDEX, &opts));
+
+   validate_index_unchanged(repo);
+   validate_workdir_unchanged(repo);
+
+   git_diff_free(diff);
+}
+
+void test_apply_check__does_not_apply(void)
+{
+	git_diff *diff;
+	git_index *index;
+	git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
+
+	const char *diff_file = DIFF_MODIFY_TWO_FILES;
+
+	struct merge_index_entry index_expected[] = {
+		{ 0100644, "f51658077d85f2264fa179b4d0848268cb3475c3", 0, "asparagus.txt" },
+		{ 0100644, "68f6182f4c85d39e1309d97c7e456156dc9c0096", 0, "beef.txt" },
+		{ 0100644, "4b7c5650008b2e747fe1809eeb5a1dde0e80850a", 0, "bouilli.txt" },
+		{ 0100644, "c4e6cca3ec6ae0148ed231f97257df8c311e015f", 0, "gravy.txt" },
+		{ 0100644, "68af1fc7407fd9addf1701a87eb1c95c7494c598", 0, "oyster.txt" },
+	};
+	size_t index_expected_cnt = sizeof(index_expected) /
+		sizeof(struct merge_index_entry);
+
+	/* mutate the index */
+	cl_git_pass(git_repository_index(&index, repo));
+	cl_git_pass(git_index_remove(index, "veal.txt", 0));
+	cl_git_pass(git_index_write(index));
+	git_index_free(index);
+
+	opts.flags |= GIT_APPLY_CHECK;
+	cl_git_pass(git_diff_from_buffer(&diff, diff_file, strlen(diff_file)));
+	cl_git_fail_with(GIT_EAPPLYFAIL, git_apply(repo, diff, GIT_APPLY_LOCATION_INDEX, &opts));
+
+	validate_apply_index(repo, index_expected, index_expected_cnt);
+
+	git_diff_free(diff);
+}