fix(computers): a failed resolve reports apt's error, not the resolver's traceback

src/agent_env/env/envs/computer/utils/resolve_in_base.py

@@ -69,5 +69,18 @@ def _wheels(layer: dict, dest: Path) -> list[str]:
     return sorted(path.name for path in dest.glob("*.whl"))
 
 
+def cli(argv: list[str]) -> None:
+    """``main`` on ``argv``. A command that fails ends it with that command's exit code and no traceback, so the last
+    line of the output, which the composer reports as the reason, is the command's own error (apt's ``E:`` line); one
+    whose output was captured has it printed first."""
+    try:
+        main(*argv)
+    except subprocess.CalledProcessError as e:
+        captured = e.stderr or e.stdout
+        if captured:
+            sys.stderr.write(captured if isinstance(captured, str) else captured.decode(errors="replace"))
+        sys.exit(e.returncode)
+
+
 if __name__ == "__main__":
-    main(*sys.argv[1:])
+    cli(sys.argv[1:])

tst/unit/env/envs/test_computer_env.py

@@ -13,6 +13,7 @@
 import logging
 import re
 import shutil
+import subprocess
 import urllib.parse
 from datetime import timedelta
 from pathlib import Path
@@ -2046,6 +2047,34 @@ def _local(work, context):
     return builder.LocalBuilder("LocalSandboxProvider", work, base, "linux/arm64")
 
 
+def test_a_command_the_resolver_runs_that_fails_ends_it_with_the_commands_code_and_no_traceback(monkeypatch, capsys):
+    """The composer reports the output's last line as the reason: it must be the command's error, apt's ``E:`` line,
+    not a traceback naming the command."""
+    def fail(*_args):
+        raise subprocess.CalledProcessError(100, ["apt-get", "install"])
+
+    monkeypatch.setattr(resolve_in_base, "main", fail)
+
+    with pytest.raises(SystemExit) as exited:
+        resolve_in_base.cli(["request.json", "out"])
+
+    assert exited.value.code == 100
+    assert capsys.readouterr().err == ""
+
+
+def test_a_captured_command_that_fails_has_its_error_printed_last(monkeypatch, capsys):
+    def fail(*_args):
+        raise subprocess.CalledProcessError(2, ["dpkg-deb", "-f"], stderr="dpkg-deb: error: not a Debian archive\n")
+
+    monkeypatch.setattr(resolve_in_base, "main", fail)
+
+    with pytest.raises(SystemExit) as exited:
+        resolve_in_base.cli(["request.json", "out"])
+
+    assert exited.value.code == 2
+    assert capsys.readouterr().err.strip().splitlines()[-1] == "dpkg-deb: error: not a Debian archive"
+
+
 def test_the_resolver_installs_the_apt_packages_it_downloads_before_building_wheels(tmp_path, monkeypatch):
     ran = []
     monkeypatch.setattr(resolve_in_base, "KEYRINGS", tmp_path / "keyrings")