refs: allow overwrite of empty folders

We noticed that once a ref folder is created, there is no existing
command that can remove it. For example, once "foo/bar" is created,
even if the user deletes foo or all the refs under foo, the folder
will persist.

Now when the user attempts to create a ref "foo" either through commit
or refs --create, if a folder "foo" exists but is empty of refs, the
folder is removed and the new ref "foo" is created.

New unit tests in tests-ref.sh verify this functionality.

Closes: #354
Approved by: cgwalters

src/libostree/ostree-repo-refs.c

@@ -100,14 +100,51 @@ write_checksum_file_at (OstreeRepo   *self,
   {
     size_t l = strlen (sha256);
     char *bufnl = alloca (l + 2);
+    g_autoptr(GError) temp_error = NULL;
 
     memcpy (bufnl, sha256, l);
     bufnl[l] = '\n';
     bufnl[l+1] = '\0';
 
     if (!_ostree_repo_file_replace_contents (self, dfd, name, (guint8*)bufnl, l + 1,
-                                             cancellable, error))
-      goto out;
+                                             cancellable, &temp_error))
+      {
+        if (g_error_matches (temp_error, G_IO_ERROR, G_IO_ERROR_IS_DIRECTORY))
+          {
+            g_autoptr(GHashTable) refs = NULL;
+            GHashTableIter hashiter;
+            gpointer hashkey, hashvalue;
+
+            g_clear_error (&temp_error);
+
+            if (!ostree_repo_list_refs (self, name, &refs, cancellable, error))
+              goto out;
+
+            g_hash_table_iter_init (&hashiter, refs);
+
+            while ((g_hash_table_iter_next (&hashiter, &hashkey, &hashvalue)))
+              {
+                if (strcmp (name, (char *)hashkey) != 0)
+                  {
+                    g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                                 "Conflict: %s exists under %s when attempting write", (char*)hashkey, name);
+                    goto out;
+                  }
+              }
+
+            if (!glnx_shutil_rm_rf_at (dfd, name, cancellable, error))
+              goto out;
+
+            if (!_ostree_repo_file_replace_contents (self, dfd, name, (guint8*)bufnl, l + 1,
+                                                     cancellable, error))
+              goto out;
+          }
+        else
+          {
+            g_propagate_error (error, g_steal_pointer (&temp_error));
+            goto out;
+          }
+      }
   }
 
   ret = TRUE;