storage.Directory.export_to_tar: mtime=_magic...aevri/mtime1

Change the default value of mtime when doing export_to_tar() from `0` to
`_utils._magic_timestamp`. This avoids problems in other software, which
assume that an mtime of `0` means the file does not exist.

There is more than one example where files with an mtime of zero are
treated as non-existant by other software. e.g. [ninja][1] and [Template
Toolkit][2].

The OSTree project also [express an intention][3] to move from an mtime
of 0 to an mtime of 1:

> For this reason, OSTree acts as though all timestamps are set to
> time_t 0, so that comparisons will be considered up-to-date. Note that
> for a few releases, OSTree used 1 to fix warnings such as GNU Tar
> emitting "implausibly old time stamp" with 0; however, until we have a
> mechanism to transition cleanly to 1, for compatibilty OSTree is
> reverted to use zero again.

From the comments on export_to_tar(), the motivation for having an mtime
of 0 was to have reproducible results, rather than it specifically being
the value 0.

Additionally, the reproducible builds project has a [page on
archives][4]; it mentions the benefits of setting all the files to have
the same mtime, or clamping the mtime. It makes no mention of a
motivation for the mtime to be specifically 0.

Fixes #914

[1]: https://github.com/ninja-build/ninja/issues/1120
[2]: https://github.com/abw/Template2/blob/8d7d37200af436f1ad43628278d3caad257c8e27/lib/Template/Provider.pm#L635
[3]: https://ostree.readthedocs.io/en/latest/manual/repo/
[4]: https://reproducible-builds.org/docs/archives/

1 files changed, 1 insertions, 1 deletions

diff --git a/buildstream/storage/_filebaseddirectory.py b/buildstream/storage/_filebaseddirectory.py
index 735179cb6..b919413f0 100644
--- a/buildstream/storage/_filebaseddirectory.py
+++ b/buildstream/storage/_filebaseddirectory.py
@@ -157,7 +157,7 @@ class FileBasedDirectory(Directory):
     # First, it sorts the results of os.listdir() to ensure the ordering of
     # the files in the archive is the same.  Second, it sets a fixed
     # timestamp for each entry. See also https://bugs.python.org/issue24465.
-    def export_to_tar(self, tf, dir_arcname, mtime=0):
+    def export_to_tar(self, tf, dir_arcname, mtime=_magic_timestamp):
         # We need directories here, including non-empty ones,
         # so list_relative_paths is not used.
         for filename in sorted(os.listdir(self.external_directory)):