utils: add deterministic_umask context manager

Useful for when exporting file from cas.

Part of #870

2 files changed, 17 insertions, 1 deletions

diff --git a/buildstream/element.py b/buildstream/element.py
index b9643aee9..a77f7e6dc 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -1491,7 +1491,8 @@ class Element(Plugin):
                 for source in self.sources():
                     source._stage(import_dir)
 
-            vdirectory.import_files(import_dir)
+            with utils._deterministic_umask():
+                vdirectory.import_files(import_dir)
 
         # Ensure deterministic mtime of sources at build time
         vdirectory.set_deterministic_mtime()
diff --git a/buildstream/utils.py b/buildstream/utils.py
index 2960348e9..f4a329210 100644
--- a/buildstream/utils.py
+++ b/buildstream/utils.py
@@ -1307,3 +1307,18 @@ def _with_gc_disabled(func):
             # used by other objects during the course of running BuildStream.
             gc.collect()
     return _gc_disabled
+
+
+# _deterministic_umask()
+#
+# Context managed to apply a umask to a section that may be affected by a users
+# umask. Restores old mask afterwards.
+#
+@contextmanager
+def _deterministic_umask():
+    old_umask = os.umask(0o022)
+
+    try:
+        yield
+    finally:
+        os.umask(old_umask)