_stream.py: Preserve stdout FD across checkout-to-stdout

In order to support things which might need to use the stdout FD
after checkout has completed writing a tarball to it, preserve it
by means of `os.dup()` and `os.dup2()`

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

1 files changed, 10 insertions, 3 deletions

diff --git a/buildstream/_stream.py b/buildstream/_stream.py
index e7a71978b..6e2e8b25b 100644
--- a/buildstream/_stream.py
+++ b/buildstream/_stream.py
@@ -423,9 +423,16 @@ class Stream():
                 else:
                     if location == '-':
                         with target.timed_activity("Creating tarball"):
-                            with os.fdopen(sys.stdout.fileno(), 'wb') as fo:
-                                with tarfile.open(fileobj=fo, mode="w|") as tf:
-                                    sandbox_vroot.export_to_tar(tf, '.')
+                            # Save the stdout FD to restore later
+                            saved_fd = os.dup(sys.stdout.fileno())
+                            try:
+                                with os.fdopen(sys.stdout.fileno(), 'wb') as fo:
+                                    with tarfile.open(fileobj=fo, mode="w|") as tf:
+                                        sandbox_vroot.export_to_tar(tf, '.')
+                            finally:
+                                # No matter what, restore stdout for further use
+                                os.dup2(saved_fd, sys.stdout.fileno())
+                                os.close(saved_fd)
                     else:
                         with target.timed_activity("Creating tarball '{}'"
                                                    .format(location)):