Add diff threading tests and attr file cache locks

This adds a basic test of doing simultaneous diffs on multiple
threads and adds basic locking for the attr file cache because
that was the immediate problem that arose from these tests.

src/attr.c

@@ -33,6 +33,7 @@ static int collect_attr_files(
 	const char *path,
 	git_vector *files);
 
+static void release_attr_files(git_vector *files);
 
 int git_attr_get(
 	const char **value,
@@ -76,7 +77,7 @@ int git_attr_get(
 	}
 
 cleanup:
-	git_vector_free(&files);
+	release_attr_files(&files);
 	git_attr_path__free(&path);
 
 	return error;
@@ -152,7 +153,7 @@ int git_attr_get_many(
 	}
 
 cleanup:
-	git_vector_free(&files);
+	release_attr_files(&files);
 	git_attr_path__free(&path);
 	git__free(info);
 
@@ -181,12 +182,10 @@ int git_attr_foreach(
 	if (git_attr_path__init(&path, pathname, git_repository_workdir(repo)) < 0)
 		return -1;
 
-	if ((error = collect_attr_files(repo, flags, pathname, &files)) < 0)
+	if ((error = collect_attr_files(repo, flags, pathname, &files)) < 0 ||
+		(error = git_strmap_alloc(&seen)) < 0)
 		goto cleanup;
 
-	seen = git_strmap_alloc();
-	GITERR_CHECK_ALLOC(seen);
-
 	git_vector_foreach(&files, i, file) {
 
 		git_attr_file__foreach_matching_rule(file, &path, j, rule) {
@@ -211,7 +210,7 @@ int git_attr_foreach(
 
 cleanup:
 	git_strmap_free(seen);
-	git_vector_free(&files);
+	release_attr_files(&files);
 	git_attr_path__free(&path);
 
 	return error;
@@ -350,12 +349,21 @@ static int load_attr_from_cache(
 	if (git_buf_printf(&cache_key, "%d#%s", (int)source, relative_path) < 0)
 		return -1;
 
-	cache_pos = git_strmap_lookup_index(cache->files, cache_key.ptr);
+	if (git_mutex_lock(&cache->lock) < 0) {
+		giterr_set(GITERR_OS, "Could not get cache attr lock");
+		git_buf_free(&cache_key);
+		return -1;
+	}
 
-	git_buf_free(&cache_key);
+	cache_pos = git_strmap_lookup_index(cache->files, cache_key.ptr);
 
-	if (git_strmap_valid_index(cache->files, cache_pos))
+	if (git_strmap_valid_index(cache->files, cache_pos)) {
 		*file = git_strmap_value_at(cache->files, cache_pos);
+		GIT_REFCOUNT_INC(*file);
+	}
+
+	git_mutex_unlock(&cache->lock);
+	git_buf_free(&cache_key);
 
 	return 0;
 }
@@ -367,20 +375,26 @@ int git_attr_cache__internal_file(
 {
 	int error = 0;
 	git_attr_cache *cache = git_repository_attr_cache(repo);
-	khiter_t cache_pos = git_strmap_lookup_index(cache->files, filename);
+	khiter_t cache_pos;
+
+	if (git_mutex_lock(&cache->lock) < 0) {
+		giterr_set(GITERR_OS, "Unable to get attr cache lock");
+		return -1;
+	}
+
+	cache_pos = git_strmap_lookup_index(cache->files, filename);
 
 	if (git_strmap_valid_index(cache->files, cache_pos)) {
 		*file = git_strmap_value_at(cache->files, cache_pos);
-		return 0;
 	}
+	else if (!(error = git_attr_file__new(file, 0, filename, &cache->pool))) {
 
-	if (git_attr_file__new(file, 0, filename, &cache->pool) < 0)
-		return -1;
-
-	git_strmap_insert(cache->files, (*file)->key + 2, *file, error);
-	if (error > 0)
-		error = 0;
+		git_strmap_insert(cache->files, (*file)->key + 2, *file, error);
+		if (error > 0)
+			error = 0;
+	}
 
+	git_mutex_unlock(&cache->lock);
 	return error;
 }
 
@@ -452,9 +466,17 @@ int git_attr_cache__push_file(
 	if (parse && (error = parse(repo, parsedata, content, file)) < 0)
 		goto finish;
 
-	git_strmap_insert(cache->files, file->key, file, error); //-V595
-	if (error > 0)
-		error = 0;
+	if (git_mutex_lock(&cache->lock) < 0) {
+		giterr_set(GITERR_OS, "Unable to get attr cache lock");
+		error = -1;
+	} else {
+		git_strmap_insert(cache->files, file->key, file, error); /* -V595 */
+		if (error > 0) { /* > 0 means inserting for the first time */
+			error = 0;
+			GIT_REFCOUNT_INC(file);
+		}
+		git_mutex_unlock(&cache->lock);
+	}
 
 	/* remember "cache buster" file signature */
 	if (blob)
@@ -481,7 +503,8 @@ finish:
 }
 
 #define push_attr_file(R,S,B,F) \
-	git_attr_cache__push_file((R),(B),(F),GIT_ATTR_FILE_FROM_FILE,git_attr_file__parse_buffer,NULL,(S))
+	git_attr_cache__push_file											\
+	((R),(B),(F),GIT_ATTR_FILE_FROM_FILE,git_attr_file__parse_buffer,NULL,(S))
 
 typedef struct {
 	git_repository *repo;
@@ -535,6 +558,18 @@ static int push_one_attr(void *ref, git_buf *path)
 	return error;
 }
 
+static void release_attr_files(git_vector *files)
+{
+	size_t i;
+	git_attr_file *file;
+
+	git_vector_foreach(files, i, file) {
+		git_attr_file__free(file);
+		files->contents[i] = NULL;
+	}
+	git_vector_free(files);
+}
+
 static int collect_attr_files(
 	git_repository *repo,
 	uint32_t flags,
@@ -600,7 +635,7 @@ static int collect_attr_files(
 
  cleanup:
 	if (error < 0)
-		git_vector_free(files);
+		release_attr_files(files);
 	git_buf_free(&dir);
 
 	return error;
@@ -637,61 +672,11 @@ static int attr_cache__lookup_path(
 	return error;
 }
 
-int git_attr_cache__init(git_repository *repo)
-{
-	int ret;
-	git_attr_cache *cache = git_repository_attr_cache(repo);
-	git_config *cfg;
-
-	if (cache->initialized)
-		return 0;
-
-	/* cache config settings for attributes and ignores */
-	if (git_repository_config__weakptr(&cfg, repo) < 0)
-		return -1;
-
-	ret = attr_cache__lookup_path(
-		&cache->cfg_attr_file, cfg, GIT_ATTR_CONFIG, GIT_ATTR_FILE_XDG);
-	if (ret < 0)
-		return ret;
-
-	ret = attr_cache__lookup_path(
-		&cache->cfg_excl_file, cfg, GIT_IGNORE_CONFIG, GIT_IGNORE_FILE_XDG);
-	if (ret < 0)
-		return ret;
-
-	/* allocate hashtable for attribute and ignore file contents */
-	if (cache->files == NULL) {
-		cache->files = git_strmap_alloc();
-		GITERR_CHECK_ALLOC(cache->files);
-	}
-
-	/* allocate hashtable for attribute macros */
-	if (cache->macros == NULL) {
-		cache->macros = git_strmap_alloc();
-		GITERR_CHECK_ALLOC(cache->macros);
-	}
-
-	/* allocate string pool */
-	if (git_pool_init(&cache->pool, 1, 0) < 0)
-		return -1;
-
-	cache->initialized = 1;
-
-	/* insert default macros */
-	return git_attr_add_macro(repo, "binary", "-diff -crlf -text");
-}
-
-void git_attr_cache_flush(
-	git_repository *repo)
+static void attr_cache__free(git_attr_cache *cache)
 {
-	git_attr_cache *cache;
-
-	if (!repo)
+	if (!cache)
 		return;
 
-	cache = git_repository_attr_cache(repo);
-
 	if (cache->files != NULL) {
 		git_attr_file *file;
 
@@ -720,19 +705,93 @@ void git_attr_cache_flush(
 	git__free(cache->cfg_excl_file);
 	cache->cfg_excl_file = NULL;
 
-	cache->initialized = 0;
+	git_mutex_free(&cache->lock);
+
+	git__free(cache);
+}
+
+int git_attr_cache__init(git_repository *repo)
+{
+	int ret = 0;
+	git_attr_cache *cache = git_repository_attr_cache(repo);
+	git_config *cfg;
+
+	if (cache)
+		return 0;
+
+	if ((ret = git_repository_config__weakptr(&cfg, repo)) < 0)
+		return ret;
+
+	cache = git__calloc(1, sizeof(git_attr_cache));
+	GITERR_CHECK_ALLOC(cache);
+
+	/* set up lock */
+	if (git_mutex_init(&cache->lock) < 0) {
+		giterr_set(GITERR_OS, "Unable to initialize lock for attr cache");
+		git__free(cache);
+		return -1;
+	}
+
+	/* cache config settings for attributes and ignores */
+	ret = attr_cache__lookup_path(
+		&cache->cfg_attr_file, cfg, GIT_ATTR_CONFIG, GIT_ATTR_FILE_XDG);
+	if (ret < 0)
+		goto cancel;
+
+	ret = attr_cache__lookup_path(
+		&cache->cfg_excl_file, cfg, GIT_IGNORE_CONFIG, GIT_IGNORE_FILE_XDG);
+	if (ret < 0)
+		goto cancel;
+
+	/* allocate hashtable for attribute and ignore file contents,
+	 * hashtable for attribute macros, and string pool
+	 */
+	if ((ret = git_strmap_alloc(&cache->files)) < 0 ||
+		(ret = git_strmap_alloc(&cache->macros)) < 0 ||
+		(ret = git_pool_init(&cache->pool, 1, 0)) < 0)
+		goto cancel;
+
+	cache = git__compare_and_swap(&repo->attrcache, NULL, cache);
+	if (cache)
+		goto cancel; /* raced with another thread, free this but no error */
+
+	/* insert default macros */
+	return git_attr_add_macro(repo, "binary", "-diff -crlf -text");
+
+cancel:
+	attr_cache__free(cache);
+	return ret;
+}
+
+void git_attr_cache_flush(git_repository *repo)
+{
+	git_attr_cache *cache;
+
+	/* this could be done less expensively, but for now, we'll just free
+	 * the entire attrcache and let the next use reinitialize it...
+	 */
+	if (repo && (cache = git__swap(repo->attrcache, NULL)) != NULL)
+		attr_cache__free(cache);
 }
 
 int git_attr_cache__insert_macro(git_repository *repo, git_attr_rule *macro)
 {
-	git_strmap *macros = git_repository_attr_cache(repo)->macros;
+	git_attr_cache *cache = git_repository_attr_cache(repo);
+	git_strmap *macros = cache->macros;
 	int error;
 
 	/* TODO: generate warning log if (macro->assigns.length == 0) */
 	if (macro->assigns.length == 0)
 		return 0;
 
-	git_strmap_insert(macros, macro->match.pattern, macro, error);
+	if (git_mutex_lock(&cache->lock) < 0) {
+		giterr_set(GITERR_OS, "Unable to get attr cache lock");
+		error = -1;
+	} else {
+		git_strmap_insert(macros, macro->match.pattern, macro, error);
+		git_mutex_unlock(&cache->lock);
+	}
+
 	return (error < 0) ? -1 : 0;
 }
 

src/attr_file.c

@@ -23,6 +23,7 @@ int git_attr_file__new(
 
 	attrs = git__calloc(1, sizeof(git_attr_file));
 	GITERR_CHECK_ALLOC(attrs);
+	GIT_REFCOUNT_INC(attrs);
 
 	if (pool)
 		attrs->pool = pool;
@@ -152,11 +153,8 @@ void git_attr_file__clear_rules(git_attr_file *file)
 	git_vector_free(&file->rules);
 }
 
-void git_attr_file__free(git_attr_file *file)
+static void attr_file_free(git_attr_file *file)
 {
-	if (!file)
-		return;
-
 	git_attr_file__clear_rules(file);
 
 	if (file->pool_is_allocated) {
@@ -168,6 +166,13 @@ void git_attr_file__free(git_attr_file *file)
 	git__free(file);
 }
 
+void git_attr_file__free(git_attr_file *file)
+{
+	if (!file)
+		return;
+	GIT_REFCOUNT_DEC(file, attr_file_free);
+}
+
 uint32_t git_attr_file__name_hash(const char *name)
 {
 	uint32_t h = 5381;

src/attr_file.h

@@ -64,6 +64,7 @@ typedef struct {
 } git_attr_assignment;
 
 typedef struct {
+	git_refcount rc;
 	char *key;				/* cache "source#path" this was loaded from */
 	git_vector rules;		/* vector of <rule*> or <fnmatch*> */
 	git_pool *pool;

src/attrcache.h

@@ -11,12 +11,12 @@
 #include "strmap.h"
 
 typedef struct {
-	int initialized;
-	git_pool pool;
-	git_strmap *files;	 /* hash path to git_attr_file of rules */
-	git_strmap *macros;	 /* hash name to vector<git_attr_assignment> */
 	char *cfg_attr_file; /* cached value of core.attributesfile */
 	char *cfg_excl_file; /* cached value of core.excludesfile */
+	git_strmap *files;	 /* hash path to git_attr_file of rules */
+	git_strmap *macros;	 /* hash name to vector<git_attr_assignment> */
+	git_mutex lock;
+	git_pool  pool;
 } git_attr_cache;
 
 extern int git_attr_cache__init(git_repository *repo);

src/config_file.c

@@ -180,11 +180,15 @@ static int config_open(git_config_backend *cfg, git_config_level_t level)
 
 	b->level = level;
 
-	b->values = git_strmap_alloc();
-	GITERR_CHECK_ALLOC(b->values);
+	if ((res = git_strmap_alloc(&b->values)) < 0)
+		return res;
 
 	git_array_init(b->readers);
 	reader = git_array_alloc(b->readers);
+	if (!reader) {
+		git_strmap_free(b->values);
+		return -1;
+	}
 	memset(reader, 0, sizeof(struct reader));
 
 	reader->file_path = git__strdup(b->file_path);
@@ -205,6 +209,7 @@ static int config_open(git_config_backend *cfg, git_config_level_t level)
 
 	reader = git_array_get(b->readers, 0);
 	git_buf_free(&reader->buffer);
+
 	return res;
 }
 
@@ -218,8 +223,10 @@ static int config_refresh(git_config_backend *cfg)
 
 	for (i = 0; i < git_array_size(b->readers); i++) {
 		reader = git_array_get(b->readers, i);
+
 		res = git_futils_readbuffer_updated(
-			&reader->buffer, reader->file_path, &reader->file_mtime, &reader->file_size, &updated);
+			&reader->buffer, reader->file_path,
+			&reader->file_mtime, &reader->file_size, &updated);
 
 		if (res < 0)
 			return (res == GIT_ENOTFOUND) ? 0 : res;
@@ -233,10 +240,9 @@ static int config_refresh(git_config_backend *cfg)
 
 	/* need to reload - store old values and prep for reload */
 	old_values = b->values;
-	b->values = git_strmap_alloc();
-	GITERR_CHECK_ALLOC(b->values);
-
-	if ((res = config_parse(b, reader, b->level, 0)) < 0) {
+	if ((res = git_strmap_alloc(&b->values)) < 0) {
+		b->values = old_values;
+	} else if ((res = config_parse(b, reader, b->level, 0)) < 0) {
 		free_vars(b->values);
 		b->values = old_values;
 	} else {

src/diff_driver.c

@@ -66,7 +66,7 @@ git_diff_driver_registry *git_diff_driver_registry_new()
 	if (!reg)
 		return NULL;
 
-	if ((reg->drivers = git_strmap_alloc()) == NULL) {
+	if (git_strmap_alloc(&reg->drivers) < 0) {
 		git_diff_driver_registry_free(reg);
 		return NULL;
 	}

src/repository.h

@@ -108,7 +108,7 @@ struct git_repository {
 	git_submodule_cache *_submodules;
 
 	git_cache objects;
-	git_attr_cache attrcache;
+	git_attr_cache *attrcache;
 	git_diff_driver_registry *diff_drivers;
 
 	char *path_repository;
@@ -123,7 +123,7 @@ struct git_repository {
 
 GIT_INLINE(git_attr_cache *) git_repository_attr_cache(git_repository *repo)
 {
-	return &repo->attrcache;
+	return repo->attrcache;
 }
 
 int git_repository_head_tree(git_tree **tree, git_repository *repo);

src/sortedcache.c

@@ -20,7 +20,7 @@ int git_sortedcache_new(
 
 	if (git_pool_init(&sc->pool, 1, 0) < 0 ||
 		git_vector_init(&sc->items, 4, item_cmp) < 0 ||
-		(sc->map = git_strmap_alloc()) == NULL)
+		git_strmap_alloc(&sc->map) < 0)
 		goto fail;
 
 	if (git_rwlock_init(&sc->lock)) {
@@ -39,8 +39,7 @@ int git_sortedcache_new(
 	return 0;
 
 fail:
-	if (sc->map)
-		git_strmap_free(sc->map);
+	git_strmap_free(sc->map);
 	git_vector_free(&sc->items);
 	git_pool_clear(&sc->pool);
 	git__free(sc);

src/strmap.h

@@ -22,7 +22,9 @@ typedef khiter_t git_strmap_iter;
 #define GIT__USE_STRMAP \
 	__KHASH_IMPL(str, static kh_inline, const char *, void *, 1, kh_str_hash_func, kh_str_hash_equal)
 
-#define git_strmap_alloc()  kh_init(str)
+#define git_strmap_alloc(hp) \
+	((*(hp) = kh_init(str)) == NULL) ? giterr_set_oom(), -1 : 0
+
 #define git_strmap_free(h)  kh_destroy(str, h), h = NULL
 #define git_strmap_clear(h) kh_clear(str, h)
 

src/submodule.c

@@ -1638,8 +1638,7 @@ static int submodule_cache_alloc(
 		return -1;
 	}
 
-	cache->submodules = git_strmap_alloc();
-	if (!cache->submodules) {
+	if (git_strmap_alloc(&cache->submodules) < 0) {
 		submodule_cache_free(cache);
 		return -1;
 	}

tests/clar_libgit2.h

@@ -11,7 +11,7 @@
  *
  * Use this wrapper around all `git_` library calls that return error codes!
  */
-#define cl_git_pass(expr) cl_git_pass_(expr, __FILE__, __LINE__)
+#define cl_git_pass(expr) cl_git_pass_((expr), __FILE__, __LINE__)
 
 #define cl_git_pass_(expr, file, line) do { \
 	int _lg2_error; \

tests/core/strmap.c

@@ -3,12 +3,22 @@
 
 GIT__USE_STRMAP;
 
+git_strmap *g_table;
+
+void test_core_strmap__initialize(void)
+{
+	cl_git_pass(git_strmap_alloc(&g_table));
+	cl_assert(g_table != NULL);
+}
+
+void test_core_strmap__cleanup(void)
+{
+	git_strmap_free(g_table);
+}
+
 void test_core_strmap__0(void)
 {
-	git_strmap *table = git_strmap_alloc();
-	cl_assert(table != NULL);
-	cl_assert(git_strmap_num_entries(table) == 0);
-	git_strmap_free(table);
+	cl_assert(git_strmap_num_entries(g_table) == 0);
 }
 
 static void insert_strings(git_strmap *table, int count)
@@ -37,21 +47,17 @@ void test_core_strmap__1(void)
 {
 	int i;
 	char *str;
-	git_strmap *table = git_strmap_alloc();
-	cl_assert(table != NULL);
 
-	insert_strings(table, 20);
+	insert_strings(g_table, 20);
 
-	cl_assert(git_strmap_exists(table, "aaaaaaaaa"));
-	cl_assert(git_strmap_exists(table, "ggggggggg"));
-	cl_assert(!git_strmap_exists(table, "aaaaaaaab"));
-	cl_assert(!git_strmap_exists(table, "abcdefghi"));
+	cl_assert(git_strmap_exists(g_table, "aaaaaaaaa"));
+	cl_assert(git_strmap_exists(g_table, "ggggggggg"));
+	cl_assert(!git_strmap_exists(g_table, "aaaaaaaab"));
+	cl_assert(!git_strmap_exists(g_table, "abcdefghi"));
 
 	i = 0;
-	git_strmap_foreach_value(table, str, { i++; free(str); });
+	git_strmap_foreach_value(g_table, str, { i++; free(str); });
 	cl_assert(i == 20);
-
-	git_strmap_free(table);
 }
 
 void test_core_strmap__2(void)
@@ -59,44 +65,36 @@ void test_core_strmap__2(void)
 	khiter_t pos;
 	int i;
 	char *str;
-	git_strmap *table = git_strmap_alloc();
-	cl_assert(table != NULL);
 
-	insert_strings(table, 20);
+	insert_strings(g_table, 20);
 
-	cl_assert(git_strmap_exists(table, "aaaaaaaaa"));
-	cl_assert(git_strmap_exists(table, "ggggggggg"));
-	cl_assert(!git_strmap_exists(table, "aaaaaaaab"));
-	cl_assert(!git_strmap_exists(table, "abcdefghi"));
+	cl_assert(git_strmap_exists(g_table, "aaaaaaaaa"));
+	cl_assert(git_strmap_exists(g_table, "ggggggggg"));
+	cl_assert(!git_strmap_exists(g_table, "aaaaaaaab"));
+	cl_assert(!git_strmap_exists(g_table, "abcdefghi"));
 
-	cl_assert(git_strmap_exists(table, "bbbbbbbbb"));
-	pos = git_strmap_lookup_index(table, "bbbbbbbbb");
-	cl_assert(git_strmap_valid_index(table, pos));
-	cl_assert_equal_s(git_strmap_value_at(table, pos), "bbbbbbbbb");
-	free(git_strmap_value_at(table, pos));
-	git_strmap_delete_at(table, pos);
+	cl_assert(git_strmap_exists(g_table, "bbbbbbbbb"));
+	pos = git_strmap_lookup_index(g_table, "bbbbbbbbb");
+	cl_assert(git_strmap_valid_index(g_table, pos));
+	cl_assert_equal_s(git_strmap_value_at(g_table, pos), "bbbbbbbbb");
+	free(git_strmap_value_at(g_table, pos));
+	git_strmap_delete_at(g_table, pos);
 
-	cl_assert(!git_strmap_exists(table, "bbbbbbbbb"));
+	cl_assert(!git_strmap_exists(g_table, "bbbbbbbbb"));
 
 	i = 0;
-	git_strmap_foreach_value(table, str, { i++; free(str); });
+	git_strmap_foreach_value(g_table, str, { i++; free(str); });
 	cl_assert(i == 19);
-
-	git_strmap_free(table);
 }
 
 void test_core_strmap__3(void)
 {
 	int i;
 	char *str;
-	git_strmap *table = git_strmap_alloc();
-	cl_assert(table != NULL);
 
-	insert_strings(table, 10000);
+	insert_strings(g_table, 10000);
 
 	i = 0;
-	git_strmap_foreach_value(table, str, { i++; free(str); });
+	git_strmap_foreach_value(g_table, str, { i++; free(str); });
 	cl_assert(i == 10000);
-
-	git_strmap_free(table);
 }

tests/threads/diff.c

+#include "clar_libgit2.h"
+#include "thread-utils.h"
+
+static git_repository *g_repo;
+static git_tree *a, *b;
+static git_atomic counts[4];
+
+void test_threads_diff__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+static void run_in_parallel(
+	int repeats, int threads, void *(*func)(void *),
+	void (*before_test)(void), void (*after_test)(void))
+{
+	int r, t, *id = git__calloc(threads, sizeof(int));
+#ifdef GIT_THREADS
+	git_thread *th = git__calloc(threads, sizeof(git_thread));
+#else
+	void *th = NULL;
+#endif
+
+	cl_assert(id != NULL && th != NULL);
+
+	for (r = 0; r < repeats; ++r) {
+		g_repo = cl_git_sandbox_reopen(); /* reopen sandbox to flush caches */
+
+		if (before_test) before_test();
+
+		for (t = 0; t < threads; ++t) {
+			id[t] = t;
+#ifdef GIT_THREADS
+			cl_git_pass(git_thread_create(&th[t], NULL, func, &id[t]));
+#else
+			cl_assert(func(&id[t]) == &id[t]);
+#endif
+		}
+
+#ifdef GIT_THREADS
+		for (t = 0; t < threads; ++t)
+			cl_git_pass(git_thread_join(th[t], NULL));
+		memset(th, 0, threads * sizeof(git_thread));
+#endif
+
+		if (after_test) after_test();
+	}
+
+	git__free(id);
+	git__free(th);
+}
+
+static void setup_trees(void)
+{
+	cl_git_pass(git_revparse_single(
+		(git_object **)&a, g_repo, "0017bd4ab1^{tree}"));
+	cl_git_pass(git_revparse_single(
+		(git_object **)&b, g_repo, "26a125ee1b^{tree}"));
+
+	memset(counts, 0, sizeof(counts));
+}
+
+#define THREADS 20
+
+static void free_trees(void)
+{
+	git_tree_free(a); a = NULL;
+	git_tree_free(b); b = NULL;
+
+	cl_assert_equal_i(288, git_atomic_get(&counts[0]));
+	cl_assert_equal_i(112, git_atomic_get(&counts[1]));
+	cl_assert_equal_i( 80, git_atomic_get(&counts[2]));
+	cl_assert_equal_i( 96, git_atomic_get(&counts[3]));
+}
+
+static void *run_index_diffs(void *arg)
+{
+	int thread = *(int *)arg;
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	git_diff *diff = NULL;
+	size_t i;
+	int exp[4] = { 0, 0, 0, 0 };
+
+//	fprintf(stderr, "%d >>>\n", thread);
+
+	switch (thread & 0x03) {
+	case 0: /* diff index to workdir */;
+		cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, NULL, &opts));
+		break;
+	case 1: /* diff tree 'a' to index */;
+		cl_git_pass(git_diff_tree_to_index(&diff, g_repo, a, NULL, &opts));
+		break;
+	case 2: /* diff tree 'b' to index */;
+		cl_git_pass(git_diff_tree_to_index(&diff, g_repo, b, NULL, &opts));
+		break;
+	case 3: /* diff index to workdir (explicit index) */;
+		{
+			git_index *idx;
+			cl_git_pass(git_repository_index(&idx, g_repo));
+			cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, idx, &opts));
+			git_index_free(idx);
+			break;
+		}
+	}
+
+//	fprintf(stderr, "%d <<<\n", thread);
+
+	/* keep some diff stats to make sure results are as expected */
+
+	i = git_diff_num_deltas(diff);
+	git_atomic_add(&counts[0], (int32_t)i);
+	exp[0] = (int)i;
+
+	while (i > 0) {
+		switch (git_diff_get_delta(diff, --i)->status) {
+		case GIT_DELTA_MODIFIED: exp[1]++; git_atomic_inc(&counts[1]); break;
+		case GIT_DELTA_ADDED:    exp[2]++; git_atomic_inc(&counts[2]); break;
+		case GIT_DELTA_DELETED:  exp[3]++; git_atomic_inc(&counts[3]); break;
+		default: break;
+		}
+	}
+
+//	fprintf(stderr, "%2d: [%d] total %d (M %d A %d D %d)\n",
+//			thread, (int)(thread & 0x03), exp[0], exp[1], exp[2], exp[3]);
+
+	switch (thread & 0x03) {
+	case 0: case 3:
+		cl_assert_equal_i(8, exp[0]); cl_assert_equal_i(4, exp[1]);
+		cl_assert_equal_i(0, exp[2]); cl_assert_equal_i(4, exp[3]);
+		break;
+	case 1:
+		cl_assert_equal_i(12, exp[0]); cl_assert_equal_i(3, exp[1]);
+		cl_assert_equal_i(7, exp[2]); cl_assert_equal_i(2, exp[3]);
+		break;
+	case 2:
+		cl_assert_equal_i(8, exp[0]); cl_assert_equal_i(3, exp[1]);
+		cl_assert_equal_i(3, exp[2]); cl_assert_equal_i(2, exp[3]);
+		break;
+	}
+
+	git_diff_free(diff);
+
+	return arg;
+}
+
+void test_threads_diff__concurrent_diffs(void)
+{
+	g_repo = cl_git_sandbox_init("status");
+
+	run_in_parallel(
+		20, 32, run_index_diffs, setup_trees, free_trees);
+}