Improve error output from failing external programs (fix #20)

Print any stderr output from the program that failed, and make the command
that was run more legible in Nancy’s error message.

Author: rrthomas

nancy/__init__.py

@@ -181,13 +181,18 @@ def filter_bytes(
                         exe_path = Path(exe_path_str)
                     exe_args = external_args[1:]
                     debug(f"Running {exe_path} {b' '.join(exe_args)}")
-                    res = subprocess.run(
-                        [exe_path.resolve(strict=True)] + exe_args,
-                        capture_output=True,
-                        check=True,
-                        input=input,
-                    )
-                    return res.stdout
+                    try:
+                        res = subprocess.run(
+                            [exe_path.resolve(strict=True)] + exe_args,
+                            capture_output=True,
+                            check=True,
+                            input=input,
+                        )
+                        return res.stdout
+                    except subprocess.CalledProcessError as err:
+                        if err.stderr is not None:
+                            print(err.stderr.decode("iso-8859-1"), file=sys.stderr)
+                        die(f"Error code {err.returncode} running: {' '.join(map(str, err.cmd))}")
 
                 def command_to_str(
                     name: bytes,

tests/test-files/fail-with-error.in.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+printf "oh no!" >&2
+exit 1

tests/test-files/fail-with-error.nancy.txt

@@ -0,0 +1 @@
+$run(fail-with-error.in.sh)

tests/test_nancy.py

@@ -13,16 +13,11 @@
 from unittest import mock
 
 import pytest
+from nancy import main
 from pytest import CaptureFixture, LogCaptureFixture
-from testutils import (
-    check_links,
-    failing_cli_test,
-    failing_test,
-    passing_cli_test,
-    passing_test,
-)
 
-from nancy import main
+from testutils import (check_links, failing_cli_test, failing_test,
+                       passing_cli_test, passing_test)
 
 
 if sys.version_info[:2] >= (3, 11):  # pragma: no cover
@@ -72,13 +67,6 @@ def test_expand_of_run_output() -> None:
         passing_test(["expand-run-src"], "expand-run-expected")
 
 
-def test_failing_executable_test() -> None:
-    with chdir(tests_dir):
-        failing_test(
-            [os.getcwd()], "returned non-zero exit status 1", "false.nancy.txt"
-        )
-
-
 def test_non_existent_executable_test() -> None:
     with chdir(tests_dir):
         failing_test([os.getcwd()], "cannot find program 'foo'", "foo.nancy.txt")
@@ -350,6 +338,27 @@ def test_invalid_command_line_argument_causes_an_error(
     failing_cli_test(capsys, caplog, ["--foo", "a"], "unrecognized arguments: --foo")
 
 
+def test_failing_executable_test(
+    capsys: CaptureFixture[str],
+    caplog: LogCaptureFixture,
+) -> None:
+    with chdir(tests_dir):
+        failing_cli_test(capsys, caplog, ["false.nancy.txt"], "Error code 1")
+
+
+def test_failing_executable_error_message_test(
+    capsys: CaptureFixture[str],
+    caplog: LogCaptureFixture,
+) -> None:
+    with chdir(tests_dir):
+        failing_cli_test(
+            capsys,
+            caplog,
+            ["fail-with-error.nancy.txt"],
+            "oh no!",
+        )
+
+
 def test_running_on_a_nonexistent_path_causes_an_error_DEBUG_coverage(
     capsys: CaptureFixture[str],
     caplog: LogCaptureFixture,