lib: Add "open dfd iter handling noent" helper, port tree-wide

Follow up to a previous patch that addressed a double-close; I
realized we already had a helper for doing "open dfd iter, do nothing
if we get ENOENT".  Raise it to libotuil, and port all consumers.

Closes: #863
Approved by: jlebon

src/libotutil/ot-fs-utils.c

@@ -172,6 +172,31 @@ ot_openat_ignore_enoent (int dfd,
   return ret;
 }
 
+/* Like glnx_dirfd_iterator_init_at(), but if %ENOENT, then set
+ * @out_exists to %FALSE, and return successfully.
+ */
+gboolean
+ot_dfd_iter_init_allow_noent (int dfd,
+                              const char *path,
+                              GLnxDirFdIterator *dfd_iter,
+                              gboolean *out_exists,
+                              GError **error)
+{
+  glnx_fd_close int fd = glnx_opendirat_with_errno (dfd, path, TRUE);
+  if (fd < 0)
+    {
+      if (errno != ENOENT)
+        return glnx_throw_errno_prefix (error, "opendirat");
+      *out_exists = FALSE;
+      return TRUE;
+    }
+  if (!glnx_dirfd_iterator_init_take_fd (fd, dfd_iter, error))
+    return FALSE;
+  fd = -1;
+  *out_exists = TRUE;
+  return TRUE;
+}
+
 GBytes *
 ot_file_mapat_bytes (int dfd,
                      const char *path,

src/libotutil/ot-fs-utils.h

@@ -21,6 +21,7 @@
 #pragma once
 
 #include "ot-unix-utils.h"
+#include "libglnx.h"
 
 G_BEGIN_DECLS
 
@@ -53,6 +54,12 @@ gboolean ot_openat_ignore_enoent (int dfd,
                                   int *out_fd,
                                   GError **error);
 
+gboolean ot_dfd_iter_init_allow_noent (int dfd,
+                                       const char *path,
+                                       GLnxDirFdIterator *dfd_iter,
+                                       gboolean *out_exists,
+                                       GError **error);
+
 GBytes *ot_file_mapat_bytes (int dfd,
                              const char *path,
                              GError **error);