thodg/libgit2

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b01f481..b8304b0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -145,3 +145,9 @@ v0.21 + 1
 * git_treebuilder_create now takes a repository so that it can query
   repository configuration.  Subsequently, git_treebuilder_write no
   longer takes a repository.
+
+* The git_submodule_update function was renamed to
+  git_submodule_update_strategy. git_submodule_update is now used to
+  provide functionalty similar to "git submodule update".
+ 
+ 
diff --git a/include/git2/submodule.h b/include/git2/submodule.h
index 8efc26e..31c68e3 100644
--- a/include/git2/submodule.h
+++ b/include/git2/submodule.h
@@ -10,6 +10,8 @@
 #include "common.h"
 #include "types.h"
 #include "oid.h"
+#include "remote.h"
+#include "checkout.h"
 
 /**
  * @file git2/submodule.h
@@ -106,6 +108,68 @@ typedef enum {
 	GIT_SUBMODULE_STATUS_WD_UNTRACKED)) != 0)
 
 /**
+ * Submodule update options structure
+ *
+ * Use the GIT_SUBMODULE_UPDATE_OPTIONS_INIT to get the default settings, like this:
+ *
+ *		git_submodule_update_options opts = GIT_SUBMODULE_UPDATE_OPTIONS_INIT;
+ */
+typedef struct git_submodule_update_options {
+	unsigned int version;
+
+	/**
+	 * These options are passed to the checkout step. To disable
+	 * checkout, set the `checkout_strategy` to
+	 * `GIT_CHECKOUT_NONE`. Generally you will want the use
+	 * GIT_CHECKOUT_SAFE to update files in the working
+	 * directory. Use the `clone_checkout_strategy` field
+	 * to set the checkout strategy that will be used in
+	 * the case where update needs to clone the repository.
+	 */
+	git_checkout_options checkout_opts;
+
+	/**
+	 * Callbacks to use for reporting fetch progress, and for acquiring
+	 * credentials in the event they are needed.
+	 */
+	git_remote_callbacks remote_callbacks;
+
+	/**
+	 * The checkout strategy to use when the sub repository needs to
+	 * be cloned. Use GIT_CHECKOUT_SAFE_CREATE to create all files
+	 * in the working directory for the newly cloned repository.
+	 */
+	unsigned int clone_checkout_strategy;
+
+	/**
+	 * The identity used when updating the reflog. NULL means to
+	 * use the default signature using the config.
+	 */
+	git_signature *signature;
+} git_submodule_update_options;
+
+#define GIT_SUBMODULE_UPDATE_OPTIONS_VERSION 1
+#define GIT_SUBMODULE_UPDATE_OPTIONS_INIT {GIT_CHECKOUT_OPTIONS_VERSION, {GIT_CHECKOUT_OPTIONS_VERSION, GIT_CHECKOUT_SAFE}, GIT_REMOTE_CALLBACKS_INIT, GIT_CHECKOUT_SAFE_CREATE}
+
+/**
+ * Update a submodule. This will clone a missing submodule and
+ * checkout the subrepository to the commit specified in the index of
+ * containing repository.
+ *
+ * @param submodule Submodule object
+ * @param init If the submodule is not initialized, setting this flag to true
+ *        will initialize the submodule before updating. Otherwise, this will
+ *        return an error if attempting to update an uninitialzed repository.
+ *        but setting this to true forces them to be updated.
+ * @param options configuration options for the update.  If NULL, the
+ *        function works as though GIT_SUBMODULE_UPDATE_OPTIONS_INIT was passed.
+ * @return 0 on success, any non-zero return value from a callback
+ *         function, or a negative value to indicate an error (use
+ *         `giterr_last` for a detailed error message).
+ */
+GIT_EXTERN(int) git_submodule_update(git_submodule *submodule, int init, git_submodule_update_options *options);
+
+/**
  * Lookup submodule information by name or path.
  *
  * Given either the submodule name or path (they are usually the same), this
@@ -403,7 +467,7 @@ GIT_EXTERN(git_submodule_ignore_t) git_submodule_set_ignore(
  * @return The current git_submodule_update_t value that will be used
  *         for this submodule.
  */
-GIT_EXTERN(git_submodule_update_t) git_submodule_update(
+GIT_EXTERN(git_submodule_update_t) git_submodule_update_strategy(
 	git_submodule *submodule);
 
 /**
diff --git a/src/submodule.c b/src/submodule.c
index d2af144..d89dd04 100644
--- a/src/submodule.c
+++ b/src/submodule.c
@@ -475,7 +475,7 @@ int git_submodule_repo_init(
 
 	/* get the configured remote url of the submodule */
 	if ((error = git_buf_printf(&buf, "submodule.%s.url", sm->name)) < 0 ||
-		(error = git_repository_config(&cfg, sm->repo)) < 0 ||
+		(error = git_repository_config_snapshot(&cfg, sm->repo)) < 0 ||
 		(error = git_config_get_string(&configured_url, cfg, buf.ptr)) < 0 ||
 		(error = submodule_repo_init(&sub_repo, sm->repo, sm->path, configured_url, use_gitlink)) < 0)
 		goto done;
@@ -790,7 +790,7 @@ git_submodule_ignore_t git_submodule_set_ignore(
 	return old;
 }
 
-git_submodule_update_t git_submodule_update(git_submodule *submodule)
+git_submodule_update_t git_submodule_update_strategy(git_submodule *submodule)
 {
 	assert(submodule);
 	return (submodule->update < GIT_SUBMODULE_UPDATE_CHECKOUT) ?
@@ -835,6 +835,178 @@ git_submodule_recurse_t git_submodule_set_fetch_recurse_submodules(
 	return old;
 }
 
+static int submodule_repo_create(
+	git_repository **out,
+	git_repository *parent_repo,
+	const char *path)
+{
+	int error = 0;
+	git_buf workdir = GIT_BUF_INIT, repodir = GIT_BUF_INIT;
+	git_repository_init_options initopt = GIT_REPOSITORY_INIT_OPTIONS_INIT;
+	git_repository *subrepo = NULL;
+
+	initopt.flags =
+		GIT_REPOSITORY_INIT_MKPATH |
+		GIT_REPOSITORY_INIT_NO_REINIT |
+		GIT_REPOSITORY_INIT_NO_DOTGIT_DIR |
+		GIT_REPOSITORY_INIT_RELATIVE_GITLINK;
+
+	/* Workdir: path to sub-repo working directory */
+	error = git_buf_joinpath(&workdir, git_repository_workdir(parent_repo), path);
+	if (error < 0)
+		goto cleanup;
+
+	initopt.workdir_path = workdir.ptr;
+
+	/**
+	 * Repodir: path to the sub-repo. sub-repo goes in:
+	 * <repo-dir>/modules/<name>/ with a gitlink in the 
+	 * sub-repo workdir directory to that repository.
+	 */
+	error = git_buf_join3(
+		&repodir, '/', git_repository_path(parent_repo), "modules", path);
+	if (error < 0)
+		goto cleanup;
+
+	error = git_repository_init_ext(&subrepo, repodir.ptr, &initopt);
+
+cleanup:
+	git_buf_free(&workdir);
+	git_buf_free(&repodir);
+
+	*out = subrepo;
+
+	return error;
+}
+
+/**
+ * Callback to override sub-repository creation when
+ * cloning a sub-repository.
+ */
+static int git_submodule_update_repo_init_cb(
+	git_repository **out,
+	const char *path,
+	int bare,
+	void *payload)
+{
+	GIT_UNUSED(bare);
+	git_submodule *sm = payload;
+
+	return submodule_repo_create(out, sm->repo, path);
+}
+
+int git_submodule_update(git_submodule *sm, int init, git_submodule_update_options *_update_options)
+{
+	int error;
+	unsigned int submodule_status;
+	git_config *config = NULL;
+	const char *submodule_url;
+	git_repository *sub_repo = NULL;
+	git_remote *remote = NULL;
+	git_object *target_commit = NULL;
+	git_buf buf = GIT_BUF_INIT;
+	git_submodule_update_options update_options = GIT_SUBMODULE_UPDATE_OPTIONS_INIT;
+	git_clone_options clone_options = GIT_CLONE_OPTIONS_INIT;
+
+	assert(sm);
+
+	if (_update_options)
+		memcpy(&update_options, _update_options, sizeof(git_submodule_update_options));
+
+	GITERR_CHECK_VERSION(&update_options, GIT_SUBMODULE_UPDATE_OPTIONS_VERSION, "git_submodule_update_options");
+
+	/* Copy over the remote callbacks */
+	clone_options.remote_callbacks = update_options.remote_callbacks;
+	clone_options.signature = update_options.signature;
+
+	/* Get the status of the submodule to determine if it is already initialized  */
+	if ((error = git_submodule_status(&submodule_status, sm)) < 0)
+		goto done;
+
+	/*
+	 * If submodule work dir is not already initialized, check to see
+	 * what we need to do (initialize, clone, return error...)
+	 */
+	if (submodule_status & GIT_SUBMODULE_STATUS_WD_UNINITIALIZED) {
+		/*
+		 * Work dir is not initialized, check to see if the submodule
+		 * info has been copied into .git/config
+		 */
+		if ((error = git_repository_config_snapshot(&config, sm->repo)) < 0 ||
+			(error = git_buf_printf(&buf, "submodule.%s.url", git_submodule_name(sm))) < 0)
+			goto done;
+
+		if ((error = git_config_get_string(&submodule_url, config, git_buf_cstr(&buf))) < 0) {
+			/*
+			 * If the error is not "not found" or if it is "not found" and we are not
+			 * initializing the submodule, then return error.
+			 */
+			if (error != GIT_ENOTFOUND)
+				goto done;
+
+			if (error == GIT_ENOTFOUND && !init) {
+				giterr_set(GITERR_SUBMODULE, "Submodule is not initialized.");
+				error = GIT_ERROR;
+				goto done;
+			}
+
+			/* The submodule has not been initialized yet - initialize it now.*/
+			if ((error = git_submodule_init(sm, 0)) < 0)
+				goto done;
+
+			git_config_free(config);
+			config = NULL;
+
+			if ((error = git_repository_config_snapshot(&config, sm->repo)) < 0 ||
+				(error = git_config_get_string(&submodule_url, config, git_buf_cstr(&buf))) < 0)
+				goto done;
+		}
+
+		/** submodule is initialized - now clone it **/
+		/* override repo creation */
+		clone_options.repository_cb = git_submodule_update_repo_init_cb;
+		clone_options.repository_cb_payload = sm;
+
+		/*
+		 * Do not perform checkout as part of clone, instead we 
+		 * will checkout the specific commit manually.
+		 */
+		clone_options.checkout_opts.checkout_strategy = GIT_CHECKOUT_NONE;
+		update_options.checkout_opts.checkout_strategy = update_options.clone_checkout_strategy;
+
+		if ((error = git_clone(&sub_repo, submodule_url, sm->path, &clone_options)) < 0 ||
+			(error = git_repository_set_head_detached(sub_repo, git_submodule_index_id(sm), update_options.signature, NULL)) < 0 ||
+			(error = git_checkout_head(sub_repo, &update_options.checkout_opts)) != 0)
+			goto done;
+	} else {
+		/**
+		 * Work dir is initialized - look up the commit in the parent repository's index,
+		 * update the workdir contents of the subrepository, and set the subrepository's
+		 * head to the new commit.
+		 */
+		if ((error = git_submodule_open(&sub_repo, sm)) < 0 ||
+			(error = git_object_lookup(&target_commit, sub_repo, git_submodule_index_id(sm), GIT_OBJ_COMMIT)) < 0 ||
+			(error = git_checkout_tree(sub_repo, target_commit, &update_options.checkout_opts)) != 0 ||
+			(error = git_repository_set_head_detached(sub_repo, git_submodule_index_id(sm), update_options.signature, NULL)) < 0)
+			goto done;
+
+		/* Invalidate the wd flags as the workdir has been updated. */
+		sm->flags = sm->flags &
+			~(GIT_SUBMODULE_STATUS_IN_WD |
+		  	GIT_SUBMODULE_STATUS__WD_OID_VALID |
+		  	GIT_SUBMODULE_STATUS__WD_SCANNED);
+	}
+
+done:
+	git_buf_free(&buf);
+	git_config_free(config);
+	git_object_free(target_commit);
+	git_remote_free(remote);
+	git_repository_free(sub_repo);
+
+	return error;
+}
+
 int git_submodule_init(git_submodule *sm, int overwrite)
 {
 	int error;
@@ -853,7 +1025,7 @@ int git_submodule_init(git_submodule *sm, int overwrite)
 
 	/* write "submodule.NAME.url" */
 
-	if ((git_submodule_resolve_url(&effective_submodule_url, sm->repo, sm->url)) < 0 ||
+	if ((error = git_submodule_resolve_url(&effective_submodule_url, sm->repo, sm->url)) < 0 ||
 		(error = git_buf_printf(&key, "submodule.%s.url", sm->name)) < 0 ||
 		(error = git_config__update_entry(
 			cfg, key.ptr, effective_submodule_url.ptr, overwrite != 0, false)) < 0)
@@ -1867,16 +2039,31 @@ static int lookup_head_remote_key(git_buf *remote_name, git_repository *repo)
 	if ((error = git_repository_head(&head, repo)) < 0)
 		return error;
 
+	/**
+	 * If head does not refer to a branch, then return
+	 * GIT_ENOTFOUND to indicate that we could not find
+	 * a remote key for the local tracking branch HEAD points to.
+	 **/
+	if (!git_reference_is_branch(head)) {
+		giterr_set(GITERR_INVALID,
+			"HEAD does not refer to a branch.");
+		error = GIT_ENOTFOUND;
+		goto done;
+	}
+
 	/* lookup remote tracking branch of HEAD */
-	if (!(error = git_branch_upstream_name(
-			&upstream_name, repo, git_reference_name(head))))
-	{
-		/* lookup remote of remote tracking branch */
-		error = git_branch_remote_name(remote_name, repo, upstream_name.ptr);
+	if ((error = git_branch_upstream_name(
+		&upstream_name,
+		repo,
+		git_reference_name(head))) < 0)
+		goto done;
 
-		git_buf_free(&upstream_name);
-	}
+	/* lookup remote of remote tracking branch */
+	if ((error = git_branch_remote_name(remote_name, repo, upstream_name.ptr)) < 0)
+		goto done;
 
+done:
+	git_buf_free(&upstream_name);
 	git_reference_free(head);
 
 	return error;
diff --git a/tests/submodule/init.c b/tests/submodule/init.c
index c03bf46..d07bc9a 100644
--- a/tests/submodule/init.c
+++ b/tests/submodule/init.c
@@ -71,3 +71,41 @@ void test_submodule_init__relative_url(void)
 	git_buf_free(&absolute_url);
 	git_config_free(cfg);
 }
+
+void test_submodule_init__relative_url_detached_head(void)
+{
+	git_submodule *sm;
+	git_config *cfg;
+	git_buf absolute_url = GIT_BUF_INIT;
+	const char *config_url;
+	git_reference *head_ref = NULL;
+	git_object *head_commit = NULL;
+
+	g_repo = setup_fixture_submodule_simple();
+
+	/* Put the parent repository into a detached head state. */
+	cl_git_pass(git_repository_head(&head_ref, g_repo));
+	cl_git_pass(git_reference_peel(&head_commit, head_ref, GIT_OBJ_COMMIT));
+
+	cl_git_pass(git_repository_set_head_detached(g_repo, git_commit_id((git_commit *)head_commit), NULL, NULL));
+
+	cl_assert(git_path_dirname_r(&absolute_url, git_repository_workdir(g_repo)) > 0);
+	cl_git_pass(git_buf_joinpath(&absolute_url, absolute_url.ptr, "testrepo.git"));
+
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "testrepo"));
+
+	/* verify that the .gitmodules is set with an absolute path*/
+	cl_assert_equal_s("../testrepo.git", git_submodule_url(sm));
+
+	/* init and verify that absolute path is written to .git/config */
+	cl_git_pass(git_submodule_init(sm, false));
+
+	cl_git_pass(git_repository_config(&cfg, g_repo));
+
+	git_config_get_string(&config_url, cfg, "submodule.testrepo.url");
+	cl_assert_equal_s(absolute_url.ptr, config_url);
+
+	git_buf_free(&absolute_url);
+	git_config_free(cfg);
+
+}
diff --git a/tests/submodule/lookup.c b/tests/submodule/lookup.c
index 34de592..fa452fb 100644
--- a/tests/submodule/lookup.c
+++ b/tests/submodule/lookup.c
@@ -49,7 +49,7 @@ void test_submodule_lookup__accessors(void)
 	cl_assert(git_oid_streq(git_submodule_wd_id(sm), oid) == 0);
 
 	cl_assert(git_submodule_ignore(sm) == GIT_SUBMODULE_IGNORE_NONE);
-	cl_assert(git_submodule_update(sm) == GIT_SUBMODULE_UPDATE_CHECKOUT);
+	cl_assert(git_submodule_update_strategy(sm) == GIT_SUBMODULE_UPDATE_CHECKOUT);
 
 	git_submodule_free(sm);
 
diff --git a/tests/submodule/modify.c b/tests/submodule/modify.c
index 582d416..9bb48ba 100644
--- a/tests/submodule/modify.c
+++ b/tests/submodule/modify.c
@@ -160,7 +160,7 @@ void test_submodule_modify__edit_and_save(void)
 	cl_assert_equal_i(
 		GIT_SUBMODULE_IGNORE_UNTRACKED, git_submodule_ignore(sm1));
 	cl_assert_equal_i(
-		GIT_SUBMODULE_UPDATE_REBASE, git_submodule_update(sm1));
+		GIT_SUBMODULE_UPDATE_REBASE, git_submodule_update_strategy(sm1));
 	cl_assert_equal_i(
 		GIT_SUBMODULE_RECURSE_YES, git_submodule_fetch_recurse_submodules(sm1));
 
@@ -179,7 +179,7 @@ void test_submodule_modify__edit_and_save(void)
 	/* check that revert was successful */
 	cl_assert_equal_s(old_url, git_submodule_url(sm1));
 	cl_assert_equal_i((int)old_ignore, (int)git_submodule_ignore(sm1));
-	cl_assert_equal_i((int)old_update, (int)git_submodule_update(sm1));
+	cl_assert_equal_i((int)old_update, (int)git_submodule_update_strategy(sm1));
 	cl_assert_equal_i(
 		old_fetchrecurse, git_submodule_fetch_recurse_submodules(sm1));
 
@@ -202,7 +202,7 @@ void test_submodule_modify__edit_and_save(void)
 	cl_assert_equal_i(
 		(int)GIT_SUBMODULE_IGNORE_UNTRACKED, (int)git_submodule_ignore(sm1));
 	cl_assert_equal_i(
-		(int)GIT_SUBMODULE_UPDATE_REBASE, (int)git_submodule_update(sm1));
+		(int)GIT_SUBMODULE_UPDATE_REBASE, (int)git_submodule_update_strategy(sm1));
 	cl_assert_equal_i(GIT_SUBMODULE_RECURSE_YES, git_submodule_fetch_recurse_submodules(sm1));
 
 	/* call reload and check that the new values are loaded */
@@ -212,7 +212,7 @@ void test_submodule_modify__edit_and_save(void)
 	cl_assert_equal_i(
 		(int)GIT_SUBMODULE_IGNORE_UNTRACKED, (int)git_submodule_ignore(sm1));
 	cl_assert_equal_i(
-		(int)GIT_SUBMODULE_UPDATE_REBASE, (int)git_submodule_update(sm1));
+		(int)GIT_SUBMODULE_UPDATE_REBASE, (int)git_submodule_update_strategy(sm1));
 	cl_assert_equal_i(GIT_SUBMODULE_RECURSE_YES, git_submodule_fetch_recurse_submodules(sm1));
 
 	/* open a second copy of the repo and compare submodule */
@@ -223,7 +223,7 @@ void test_submodule_modify__edit_and_save(void)
 	cl_assert_equal_i(
 		GIT_SUBMODULE_IGNORE_UNTRACKED, git_submodule_ignore(sm2));
 	cl_assert_equal_i(
-		GIT_SUBMODULE_UPDATE_REBASE, git_submodule_update(sm2));
+		GIT_SUBMODULE_UPDATE_REBASE, git_submodule_update_strategy(sm2));
 	cl_assert_equal_i(
 		GIT_SUBMODULE_RECURSE_NO, git_submodule_fetch_recurse_submodules(sm2));
 
diff --git a/tests/submodule/update.c b/tests/submodule/update.c
new file mode 100644
index 0000000..ebf864d
--- /dev/null
+++ b/tests/submodule/update.c
@@ -0,0 +1,380 @@
+#include "clar_libgit2.h"
+#include "posix.h"
+#include "path.h"
+#include "submodule_helpers.h"
+#include "fileops.h"
+
+static git_repository *g_repo = NULL;
+
+void test_submodule_update__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+void test_submodule_update__unitialized_submodule_no_init(void)
+{
+	git_submodule *sm;
+	git_submodule_update_options update_options = GIT_SUBMODULE_UPDATE_OPTIONS_INIT;
+
+	g_repo = setup_fixture_submodule_simple();
+
+	/* get the submodule */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "testrepo"));
+
+	/* updating an unitialized repository throws */
+	cl_git_fail_with(
+		GIT_ERROR,
+		git_submodule_update(sm, 0, &update_options));
+
+	git_submodule_free(sm);
+}
+
+struct update_submodule_cb_payload {
+	int update_tips_called;
+	int checkout_progress_called;
+	int checkout_notify_called;
+};
+
+static void checkout_progress_cb(
+	const char *path,
+	size_t completed_steps,
+	size_t total_steps,
+	void *payload)
+{
+	struct update_submodule_cb_payload *update_payload = payload;
+
+	GIT_UNUSED(path);
+	GIT_UNUSED(completed_steps);
+	GIT_UNUSED(total_steps);
+
+	update_payload->checkout_progress_called = 1;
+}
+
+static int checkout_notify_cb(
+	git_checkout_notify_t why,
+	const char *path,
+	const git_diff_file *baseline,
+	const git_diff_file *target,
+	const git_diff_file *workdir,
+	void *payload)
+{
+	struct update_submodule_cb_payload *update_payload = payload;
+
+	GIT_UNUSED(why);
+	GIT_UNUSED(path);
+	GIT_UNUSED(baseline);
+	GIT_UNUSED(target);
+	GIT_UNUSED(workdir);
+
+	update_payload->checkout_notify_called = 1;
+
+	return 0;
+}
+
+static int update_tips(const char *refname, const git_oid *a, const git_oid *b, void *data)
+{
+	struct update_submodule_cb_payload *update_payload = data;
+
+	GIT_UNUSED(refname);
+	GIT_UNUSED(a);
+	GIT_UNUSED(b);
+
+	update_payload->update_tips_called = 1;
+
+	return 1;
+}
+
+void test_submodule_update__update_submodule(void)
+{
+	git_submodule *sm;
+	git_submodule_update_options update_options = GIT_SUBMODULE_UPDATE_OPTIONS_INIT;
+	unsigned int submodule_status = 0;
+	struct update_submodule_cb_payload update_payload = { 0 };
+
+	g_repo = setup_fixture_submodule_simple();
+
+	update_options.checkout_opts.progress_cb = checkout_progress_cb;
+	update_options.checkout_opts.progress_payload = &update_payload;
+
+	update_options.remote_callbacks.update_tips = update_tips;
+	update_options.remote_callbacks.payload = &update_payload;
+
+	/* get the submodule */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "testrepo"));
+
+	/* verify the initial state of the submodule */
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_WD_UNINITIALIZED);
+
+	/* initialize and update the submodule */
+	cl_git_pass(git_submodule_init(sm, 0));
+	cl_git_pass(git_submodule_update(sm, 0, &update_options));
+
+	/* verify state */
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_IN_WD);
+
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+
+	/* verify that the expected callbacks have been called. */
+	cl_assert_equal_i(1, update_payload.checkout_progress_called);
+	cl_assert_equal_i(1, update_payload.update_tips_called);
+
+	git_submodule_free(sm);
+}
+
+void test_submodule_update__update_and_init_submodule(void)
+{
+	git_submodule *sm;
+	git_submodule_update_options update_options = GIT_SUBMODULE_UPDATE_OPTIONS_INIT;
+	unsigned int submodule_status = 0;
+
+	g_repo = setup_fixture_submodule_simple();
+
+	/* get the submodule */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "testrepo"));
+
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_WD_UNINITIALIZED);
+
+	/* update (with option to initialize sub repo) */
+	cl_git_pass(git_submodule_update(sm, 1, &update_options));
+
+	/* verify expected state */
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+
+	git_submodule_free(sm);
+}
+
+void test_submodule_update__update_already_checked_out_submodule(void)
+{
+	git_submodule *sm = NULL;
+	git_checkout_options checkout_options = GIT_CHECKOUT_OPTIONS_INIT;
+	git_submodule_update_options update_options = GIT_SUBMODULE_UPDATE_OPTIONS_INIT;
+	unsigned int submodule_status = 0;
+	git_reference *branch_reference = NULL;
+	git_object *branch_commit = NULL;
+	struct update_submodule_cb_payload update_payload = { 0 };
+
+	g_repo = setup_fixture_submodule_simple();
+
+	update_options.checkout_opts.progress_cb = checkout_progress_cb;
+	update_options.checkout_opts.progress_payload = &update_payload;
+
+	/* Initialize and update the sub repository */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "testrepo"));
+
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_WD_UNINITIALIZED);
+
+	cl_git_pass(git_submodule_update(sm, 1, &update_options));
+
+	/* verify expected state */
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+
+	/* checkout the alternate_1 branch */
+	checkout_options.checkout_strategy = GIT_CHECKOUT_SAFE;
+
+	cl_git_pass(git_reference_lookup(&branch_reference, g_repo, "refs/heads/alternate_1"));
+	cl_git_pass(git_reference_peel(&branch_commit, branch_reference, GIT_OBJ_COMMIT));
+	cl_git_pass(git_checkout_tree(g_repo, branch_commit, &checkout_options));
+	cl_git_pass(git_repository_set_head(g_repo, git_reference_name(branch_reference), NULL, NULL));
+
+	/*
+	 * Verify state after checkout of parent repository. The submodule ID in the
+	 * HEAD commit and index should be updated, but not the workdir.
+	 */
+
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_IN_WD |
+		GIT_SUBMODULE_STATUS_WD_MODIFIED);
+
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+
+	/*
+	 * Update the submodule and verify the state.
+	 * Now, the HEAD, index, and Workdir commits should all be updated to
+	 * the new commit.
+	 */
+	cl_git_pass(git_submodule_update(sm, 0, &update_options));
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+
+	/* verify that the expected callbacks have been called. */
+	cl_assert_equal_i(1, update_payload.checkout_progress_called);
+	
+	git_submodule_free(sm);
+	git_object_free(branch_commit);
+	git_reference_free(branch_reference);
+}
+
+void test_submodule_update__update_blocks_on_dirty_wd(void)
+{
+	git_submodule *sm = NULL;
+	git_checkout_options checkout_options = GIT_CHECKOUT_OPTIONS_INIT;
+	git_submodule_update_options update_options = GIT_SUBMODULE_UPDATE_OPTIONS_INIT;
+	unsigned int submodule_status = 0;
+	git_reference *branch_reference = NULL;
+	git_object *branch_commit = NULL;
+	struct update_submodule_cb_payload update_payload = { 0 };
+
+	g_repo = setup_fixture_submodule_simple();
+
+	update_options.checkout_opts.notify_flags = GIT_CHECKOUT_NOTIFY_CONFLICT;
+	update_options.checkout_opts.notify_cb = checkout_notify_cb;
+	update_options.checkout_opts.notify_payload = &update_payload;
+
+	/* Initialize and update the sub repository */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "testrepo"));
+
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_WD_UNINITIALIZED);
+
+	cl_git_pass(git_submodule_update(sm, 1, &update_options));
+
+	/* verify expected state */
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+
+	/* checkout the alternate_1 branch */
+	checkout_options.checkout_strategy = GIT_CHECKOUT_SAFE;
+
+	cl_git_pass(git_reference_lookup(&branch_reference, g_repo, "refs/heads/alternate_1"));
+	cl_git_pass(git_reference_peel(&branch_commit, branch_reference, GIT_OBJ_COMMIT));
+	cl_git_pass(git_checkout_tree(g_repo, branch_commit, &checkout_options));
+	cl_git_pass(git_repository_set_head(g_repo, git_reference_name(branch_reference), NULL, NULL));
+
+	/*
+	 * Verify state after checkout of parent repository. The submodule ID in the
+	 * HEAD commit and index should be updated, but not the workdir.
+	 */
+
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_IN_WD |
+		GIT_SUBMODULE_STATUS_WD_MODIFIED);
+
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+
+	/*
+	 * Create a conflicting edit in the subrepository to verify that
+	 * the submodule update action is blocked.
+	 */
+	cl_git_write2file("submodule_simple/testrepo/branch_file.txt", "a conflicting edit", 0,
+		O_WRONLY | O_CREAT | O_TRUNC, 0755);
+
+	cl_git_fail(git_submodule_update(sm, 0, &update_options));
+
+	/* verify that the expected callbacks have been called. */
+	cl_assert_equal_i(1, update_payload.checkout_notify_called);
+
+	/* verify that the submodule state has not changed. */
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+
+	git_submodule_free(sm);
+	git_object_free(branch_commit);
+	git_reference_free(branch_reference);
+}
+
+void test_submodule_update__can_force_update(void)
+{
+	git_submodule *sm = NULL;
+	git_checkout_options checkout_options = GIT_CHECKOUT_OPTIONS_INIT;
+	git_submodule_update_options update_options = GIT_SUBMODULE_UPDATE_OPTIONS_INIT;
+	unsigned int submodule_status = 0;
+	git_reference *branch_reference = NULL;
+	git_object *branch_commit = NULL;
+
+	g_repo = setup_fixture_submodule_simple();
+
+	/* Initialize and update the sub repository */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "testrepo"));
+
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_WD_UNINITIALIZED);
+
+	cl_git_pass(git_submodule_update(sm, 1, &update_options));
+
+	/* verify expected state */
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+
+	/* checkout the alternate_1 branch */
+	checkout_options.checkout_strategy = GIT_CHECKOUT_SAFE;
+
+	cl_git_pass(git_reference_lookup(&branch_reference, g_repo, "refs/heads/alternate_1"));
+	cl_git_pass(git_reference_peel(&branch_commit, branch_reference, GIT_OBJ_COMMIT));
+	cl_git_pass(git_checkout_tree(g_repo, branch_commit, &checkout_options));
+	cl_git_pass(git_repository_set_head(g_repo, git_reference_name(branch_reference), NULL, NULL));
+
+	/*
+	 * Verify state after checkout of parent repository. The submodule ID in the
+	 * HEAD commit and index should be updated, but not the workdir.
+	 */
+	cl_git_pass(git_submodule_status(&submodule_status, sm));
+	cl_assert_equal_i(submodule_status, GIT_SUBMODULE_STATUS_IN_HEAD |
+		GIT_SUBMODULE_STATUS_IN_INDEX |
+		GIT_SUBMODULE_STATUS_IN_CONFIG |
+		GIT_SUBMODULE_STATUS_IN_WD |
+		GIT_SUBMODULE_STATUS_WD_MODIFIED);
+
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "be3563ae3f795b2b4353bcce3a527ad0a4f7f644") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+
+	/*
+	 * Create a conflicting edit in the subrepository to verify that
+	 * the submodule update action is blocked.
+	 */
+	cl_git_write2file("submodule_simple/testrepo/branch_file.txt", "a conflicting edit", 0,
+		O_WRONLY | O_CREAT | O_TRUNC, 0777);
+
+	/* forcefully checkout and verify the submodule state was updated. */
+	update_options.checkout_opts.checkout_strategy = GIT_CHECKOUT_FORCE;
+	cl_git_pass(git_submodule_update(sm, 0, &update_options));
+	cl_assert(git_oid_streq(git_submodule_head_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+	cl_assert(git_oid_streq(git_submodule_wd_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+	cl_assert(git_oid_streq(git_submodule_index_id(sm), "a65fedf39aefe402d3bb6e24df4d4f5fe4547750") == 0);
+
+	git_submodule_free(sm);
+	git_object_free(branch_commit);
+	git_reference_free(branch_reference);
+}