[PATCH 3/4] tmp-objdir: preserve errno when directory creation fails

[Justin Tobler <jltobler@gmail.com>]

When `tmp_objdir_create()` fails to create the temporary object
directory, it cleans up after itself before returning NULL. Depending on
where the failure occurred, this cleanup may clobber `errno`, leaving
callers without a way to reliably tell why creating the directory
failed.

In a subsequent commit, ODB transactions create their temporary object
directory eagerly and report a more specific error regarding creation
failure. Preserve `errno` across the cleanup so that callers can inspect
it accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
---
 tmp-objdir.c | 8 ++++++++
 tmp-objdir.h | 3 ++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/tmp-objdir.c b/tmp-objdir.c
index 5307717354..4b91d86787 100644
--- a/tmp-objdir.c
+++ b/tmp-objdir.c
@@ -148,10 +148,13 @@ static int setup_tmp_objdir(const char *root)
 {
 	char *path;
 	int ret = 0;
+	int saved_errno;
 
 	path = xstrfmt("%s/pack", root);
 	ret = mkdir(path, 0777);
+	saved_errno = errno;
 	free(path);
+	errno = saved_errno;
 
 	return ret;
 }
@@ -163,6 +166,7 @@ struct tmp_objdir *tmp_objdir_create(struct repository *r,
 	struct odb_source_files *files = odb_source_files_downcast(r->objects->source);
 	static int installed_handlers;
 	struct tmp_objdir *t;
+	int saved_errno;
 
 	if (the_tmp_objdir)
 		BUG("only one tmp_objdir can be used at a time");
@@ -185,8 +189,10 @@ struct tmp_objdir *tmp_objdir_create(struct repository *r,
 		chdir_notify_register(tmp_objdir_reparent, t);
 
 	if (!mkdtemp(t->path.buf)) {
+		saved_errno = errno;
 		/* free, not destroy, as we never touched the filesystem */
 		tmp_objdir_free(t);
+		errno = saved_errno;
 		return NULL;
 	}
 
@@ -197,7 +203,9 @@ struct tmp_objdir *tmp_objdir_create(struct repository *r,
 	}
 
 	if (setup_tmp_objdir(t->path.buf)) {
+		saved_errno = errno;
 		tmp_objdir_destroy(t);
+		errno = saved_errno;
 		return NULL;
 	}
 
diff --git a/tmp-objdir.h b/tmp-objdir.h
index a301b01d84..4ef6750a01 100644
--- a/tmp-objdir.h
+++ b/tmp-objdir.h
@@ -26,7 +26,8 @@ struct tmp_objdir;
 
 /*
  * Create a new temporary object directory with the specified prefix and
- * install the directory as the primary write target; returns NULL on failure.
+ * install the directory as the primary write target; returns NULL and sets
+ * errno on failure.
  */
 struct tmp_objdir *tmp_objdir_create(struct repository *r, const char *prefix,
 				     int will_destroy);
-- 
2.56.0.173.gf57c9a7388