From 902c313b4dfbfd19bf5825c0f44ef845f7d0f67b Mon Sep 17 00:00:00 2001
From: Brett Smith <brettcsmith@brettcsmith.org>
Date: Tue, 5 Jan 2021 16:46:42 -0500
Subject: [PATCH] cliutil: New function can_run.

---
 conservancy_beancount/cliutil.py | 18 ++++++++++++++++++
 tests/test_cliutil.py            | 12 ++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/conservancy_beancount/cliutil.py b/conservancy_beancount/cliutil.py
index 89e5601..2e04695 100644
--- a/conservancy_beancount/cliutil.py
+++ b/conservancy_beancount/cliutil.py
@@ -20,6 +20,7 @@ import os
 import pkg_resources
 import re
 import signal
+import subprocess
 import sys
 import traceback
 import types
@@ -37,6 +38,7 @@ from typing import (
     Any,
     BinaryIO,
     Callable,
+    Container,
     IO,
     Iterable,
     NamedTuple,
@@ -279,6 +281,22 @@ def add_version_argument(parser: argparse.ArgumentParser) -> argparse.Action:
         help="Show program version and license information",
     )
 
+def can_run(
+        cmd: Sequence[str],
+        stdout: Optional[int]=subprocess.DEVNULL,
+        stderr: Optional[int]=None,
+        ok_returncodes: Container[int]=frozenset([0]),
+) -> bool:
+    try:
+        with subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=stdout, stderr=stderr) as proc:
+            # Typing says this can be None, but I don't think that's true
+            # given that we passed stdin=PIPE.
+            proc.stdin.close()  # type:ignore[union-attr]
+    except (OSError, subprocess.SubprocessError):
+        return False
+    else:
+        return proc.returncode in ok_returncodes
+
 def date_arg(arg: str) -> datetime.date:
     return datetime.datetime.strptime(arg, '%Y-%m-%d').date()
 
diff --git a/tests/test_cliutil.py b/tests/test_cliutil.py
index f5ae44c..20ea61e 100644
--- a/tests/test_cliutil.py
+++ b/tests/test_cliutil.py
@@ -20,6 +20,8 @@ import pytest
 
 from pathlib import Path
 
+from . import testutil
+
 from conservancy_beancount import cliutil
 
 FILE_NAMES = ['-foobar', '-foo.bin']
@@ -227,3 +229,13 @@ def test_version_argument(argparser, capsys, arg):
 ])
 def test_diff_year(date, diff, expected):
     assert cliutil.diff_year(date, diff) == expected
+
+@pytest.mark.parametrize('cmd,expected', [
+    (['true'], True),
+    (['true', '--version'], True),
+    (['false'], False),
+    (['false', '--version'], False),
+    ([str(testutil.TESTS_DIR)], False),
+])
+def test_can_run(cmd, expected):
+    assert cliutil.can_run(cmd) == expected