thodg/libgit2

diff --git a/include/git2/repository.h b/include/git2/repository.h
index 0b56a08..ff81b75 100644
--- a/include/git2/repository.h
+++ b/include/git2/repository.h
@@ -194,9 +194,12 @@ GIT_EXTERN(const char *) git_repository_workdir(git_repository *repo);
  *
  * @param repo A repository object
  * @param workdir The path to a working directory
+ * @param update_gitlink Create/update gitlink in workdir and set config
+ *        "core.worktree" (if workdir is not the parent of the .git directory)
  * @return 0, or an error code
  */
-GIT_EXTERN(int) git_repository_set_workdir(git_repository *repo, const char *workdir);
+GIT_EXTERN(int) git_repository_set_workdir(
+	git_repository *repo, const char *workdir, int update_gitlink);
 
 /**
  * Check if a repository is bare
diff --git a/src/repository.c b/src/repository.c
index bee7b3e..a293171 100644
--- a/src/repository.c
+++ b/src/repository.c
@@ -741,15 +741,24 @@ static int repo_init_config(const char *git_dir, bool is_bare, bool is_reinit)
 #define GIT_DESC_CONTENT "Unnamed repository; edit this file 'description' to name the repository.\n"
 
 static int repo_write_template(
-	const char *git_dir, const char *file, mode_t mode, const char *content)
+	const char *git_dir,
+	bool allow_overwrite,
+	const char *file,
+	mode_t mode,
+	const char *content)
 {
 	git_buf path = GIT_BUF_INIT;
-	int fd, error = 0;
+	int fd, error = 0, flags;
 
 	if (git_buf_joinpath(&path, git_dir, file) < 0)
 		return -1;
 
-	fd = p_open(git_buf_cstr(&path), O_WRONLY | O_CREAT | O_EXCL, mode);
+	if (allow_overwrite)
+		flags = O_WRONLY | O_CREAT | O_TRUNC;
+	else
+		flags = O_WRONLY | O_CREAT | O_EXCL;
+
+	fd = p_open(git_buf_cstr(&path), flags, mode);
 
 	if (fd >= 0) {
 		error = p_write(fd, content, strlen(content));
@@ -811,7 +820,7 @@ static int repo_init_structure(const char *git_dir, int is_bare)
 	/* Make template files as needed */
 	for (i = 0; tmpl[i].file != NULL; ++i) {
 		if (repo_write_template(
-				git_dir, tmpl[i].file, tmpl[i].mode, tmpl[i].content) < 0)
+				git_dir, false, tmpl[i].file, tmpl[i].mode, tmpl[i].content) < 0)
 			return -1;
 	}
 
@@ -943,8 +952,47 @@ const char *git_repository_workdir(git_repository *repo)
 	return repo->workdir;
 }
 
-int git_repository_set_workdir(git_repository *repo, const char *workdir)
+static int write_gitlink(
+	const char *in_dir, const char *to_repo)
+{
+	int error;
+	git_buf buf = GIT_BUF_INIT;
+	struct stat st;
+
+	if (git_path_dirname_r(&buf, to_repo) < 0 ||
+		git_path_to_dir(&buf) < 0)
+		return -1;
+
+	/* don't write gitlink to natural workdir */
+	if (git__suffixcmp(to_repo, "/" DOT_GIT "/") == 0 &&
+		strcmp(in_dir, buf.ptr) == 0)
+		return GIT_PASSTHROUGH;
+
+	if (git_buf_joinpath(&buf, in_dir, DOT_GIT) < 0)
+		return -1;
+
+	if (!p_stat(buf.ptr, &st) && !S_ISREG(st.st_mode)) {
+		giterr_set(GITERR_REPOSITORY,
+			"Cannot overwrite gitlink file into path '%s'", in_dir);
+		return GIT_EEXISTS;
+	}
+
+	git_buf_clear(&buf);
+
+	if (git_buf_printf(&buf, "%s %s", GIT_FILE_CONTENT_PREFIX, to_repo) < 0)
+		return -1;
+
+	error = repo_write_template(in_dir, true, DOT_GIT, 0644, buf.ptr);
+
+	git_buf_free(&buf);
+
+	return error;
+}
+
+int git_repository_set_workdir(
+	git_repository *repo, const char *workdir, int update_gitlink)
 {
+	int error = 0;
 	git_buf path = GIT_BUF_INIT;
 
 	assert(repo && workdir);
@@ -952,11 +1000,37 @@ int git_repository_set_workdir(git_repository *repo, const char *workdir)
 	if (git_path_prettify_dir(&path, workdir, NULL) < 0)
 		return -1;
 
-	git__free(repo->workdir);
+	if (repo->workdir && strcmp(repo->workdir, path.ptr) == 0)
+		return 0;
 
-	repo->workdir = git_buf_detach(&path);
-	repo->is_bare = 0;
-	return 0;
+	if (update_gitlink) {
+		git_config *config;
+
+		if (git_repository_config__weakptr(&config, repo) < 0)
+			return -1;
+
+		error = write_gitlink(path.ptr, git_repository_path(repo));
+
+		/* passthrough error means gitlink is unnecessary */
+		if (error == GIT_PASSTHROUGH)
+			error = git_config_delete(config, "core.worktree");
+		else if (!error)
+			error = git_config_set_string(config, "core.worktree", path.ptr);
+
+		if (!error)
+			error = git_config_set_bool(config, "core.bare", false);
+	}
+
+	if (!error) {
+		char *old_workdir = repo->workdir;
+
+		repo->workdir = git_buf_detach(&path);
+		repo->is_bare = 0;
+
+		git__free(old_workdir);
+	}
+
+	return error;
 }
 
 int git_repository_is_bare(git_repository *repo)
diff --git a/tests-clar/repo/setters.c b/tests-clar/repo/setters.c
index 6242d85..cd6e389 100644
--- a/tests-clar/repo/setters.c
+++ b/tests-clar/repo/setters.c
@@ -2,6 +2,8 @@
 #include "buffer.h"
 #include "posix.h"
 #include "util.h"
+#include "path.h"
+#include "fileops.h"
 
 static git_repository *repo;
 
@@ -16,7 +18,7 @@ void test_repo_setters__cleanup(void)
 {
 	git_repository_free(repo);
 	cl_fixture_cleanup("testrepo.git");
-	cl_must_pass(p_rmdir("new_workdir"));
+	cl_fixture_cleanup("new_workdir");
 }
 
 void test_repo_setters__setting_a_workdir_turns_a_bare_repository_into_a_standard_one(void)
@@ -24,7 +26,7 @@ void test_repo_setters__setting_a_workdir_turns_a_bare_repository_into_a_standar
 	cl_assert(git_repository_is_bare(repo) == 1);
 
 	cl_assert(git_repository_workdir(repo) == NULL);
-	cl_git_pass(git_repository_set_workdir(repo, "./new_workdir"));
+	cl_git_pass(git_repository_set_workdir(repo, "./new_workdir", false));
 
 	cl_assert(git_repository_workdir(repo) != NULL);
 	cl_assert(git_repository_is_bare(repo) == 0);
@@ -32,9 +34,30 @@ void test_repo_setters__setting_a_workdir_turns_a_bare_repository_into_a_standar
 
 void test_repo_setters__setting_a_workdir_prettifies_its_path(void)
 {
-	cl_git_pass(git_repository_set_workdir(repo, "./new_workdir"));
+	cl_git_pass(git_repository_set_workdir(repo, "./new_workdir", false));
 
-	cl_assert(git__suffixcmp(git_repository_workdir(repo), "/") == 0);
+	cl_assert(git__suffixcmp(git_repository_workdir(repo), "new_workdir/") == 0);
+}
+
+void test_repo_setters__setting_a_workdir_creates_a_gitlink(void)
+{
+	git_config *cfg;
+	const char *val;
+	git_buf content = GIT_BUF_INIT;
+
+	cl_git_pass(git_repository_set_workdir(repo, "./new_workdir", true));
+
+	cl_assert(git_path_isfile("./new_workdir/.git"));
+
+	cl_git_pass(git_futils_readbuffer(&content, "./new_workdir/.git"));
+	cl_assert(git__prefixcmp(git_buf_cstr(&content), "gitdir: ") == 0);
+	cl_assert(git__suffixcmp(git_buf_cstr(&content), "testrepo.git/") == 0);
+	git_buf_free(&content);
+
+	cl_git_pass(git_repository_config(&cfg, repo));
+	cl_git_pass(git_config_get_string(&val, cfg, "core.worktree"));
+	cl_assert(git__suffixcmp(val, "new_workdir/") == 0);
+	git_config_free(cfg);
 }
 
 void test_repo_setters__setting_a_new_index_on_a_repo_which_has_already_loaded_one_properly_honors_the_refcount(void)
diff --git a/tests-clar/status/worktree.c b/tests-clar/status/worktree.c
index 3670b72..fed81e5 100644
--- a/tests-clar/status/worktree.c
+++ b/tests-clar/status/worktree.c
@@ -459,7 +459,7 @@ void test_status_worktree__status_file_without_index_or_workdir(void)
 	cl_git_pass(p_mkdir("wd", 0777));
 
 	cl_git_pass(git_repository_open(&repo, cl_fixture("testrepo.git")));
-	cl_git_pass(git_repository_set_workdir(repo, "wd"));
+	cl_git_pass(git_repository_set_workdir(repo, "wd", false));
 
 	cl_git_pass(git_index_open(&index, "empty-index"));
 	cl_assert_equal_i(0, git_index_entrycount(index));
@@ -500,7 +500,7 @@ void test_status_worktree__status_file_with_clean_index_and_empty_workdir(void)
 	cl_git_pass(p_mkdir("wd", 0777));
 
 	cl_git_pass(git_repository_open(&repo, cl_fixture("testrepo.git")));
-	cl_git_pass(git_repository_set_workdir(repo, "wd"));
+	cl_git_pass(git_repository_set_workdir(repo, "wd", false));
 
 	cl_git_pass(git_index_open(&index, "my-index"));
 	fill_index_wth_head_entries(repo, index);