ssh: improve handling of failed builds

When the wrapped backend fails, streamapi does not produce an artifact
tar on stdout. When this goes missing, we'll see a traceback. Hide that
traceback and adapt the exit code if necessary.

1 files changed, 17 insertions, 7 deletions

diff --git a/mdbp/ssh.py b/mdbp/ssh.py
index 8af6bde..50b7d25 100644
--- a/mdbp/ssh.py
+++ b/mdbp/ssh.py
@@ -3,6 +3,7 @@
 """mdbp backend wrapper via ssh"""
 
 import argparse
+import contextlib
 import io
 import json
 import pathlib
@@ -61,13 +62,22 @@ def main() -> None:
     assert proc.stdin is not None
     produce_request_tar(build, proc.stdin)
     proc.stdin.close()
-    with tarfile.open(fileobj=proc.stdout, mode="r|") as outtar:
-        for member in outtar:
-            if "/" in member.name or not member.isfile():
-                raise ValueError("expected flat tar as output")
-            outtar.extract(member, build["output"]["directory"],
-                           set_attrs=False)
-    sys.exit(proc.wait())
+    exitcode = 0
+    with contextlib.ExitStack() as stack:
+        try:
+            outtar = stack.enter_context(tarfile.open(fileobj=proc.stdout,
+                                                      mode="r|"))
+        except tarfile.ReadError as err:
+            if str(err) != "empty file":
+                raise
+            exitcode = 1
+        else:
+            for member in outtar:
+                if "/" in member.name or not member.isfile():
+                    raise ValueError("expected flat tar as output")
+                outtar.extract(member, build["output"]["directory"],
+                               set_attrs=False)
+    sys.exit(proc.wait() or exitcode)
 
 if __name__ == "__main__":
     main()