Don't crash when an executable is not found

pre_commit/parse_shebang.py

@@ -12,6 +12,10 @@ from pre_commit import five
 printable = frozenset(string.printable)
 
 
+class ExecutableNotFoundError(OSError):
+    pass
+
+
 def parse_bytesio(bytesio):
     """Parse the shebang from a file opened for reading binary."""
     if bytesio.read(2) != b'#!':
@@ -70,7 +74,9 @@ def normexe(orig_exe):
     if os.sep not in orig_exe:
         exe = find_executable(orig_exe)
         if exe is None:
-            raise OSError('Executable {0} not found'.format(orig_exe))
+            raise ExecutableNotFoundError(
+                'Executable `{0}` not found'.format(orig_exe),
+            )
         return exe
     else:
         return orig_exe

pre_commit/util.py

@@ -181,8 +181,11 @@ def cmd_output(*cmd, **kwargs):
         for key, value in kwargs.pop('env', {}).items()
     ) or None
 
+    try:
         cmd = parse_shebang.normalize_cmd(cmd)
-
+    except parse_shebang.ExecutableNotFoundError as e:
+        returncode, stdout, stderr = (-1, e.args[0].encode('UTF-8'), b'')
+    else:
         popen_kwargs.update(kwargs)
         proc = __popen(cmd, **popen_kwargs)
         stdout, stderr = proc.communicate()
@@ -197,7 +200,7 @@ def cmd_output(*cmd, **kwargs):
             returncode, cmd, retcode, output=(stdout, stderr),
         )
 
-    return proc.returncode, stdout, stderr
+    return returncode, stdout, stderr
 
 
 def rmtree(path):

testing/resources/not_found_exe/hooks.yaml

@@ -0,0 +1,5 @@
+-   id: not-found-exe
+    name: Not found exe
+    entry: i-dont-exist-lol
+    language: system
+    files: ''

tests/parse_shebang_test.py

@@ -108,7 +108,7 @@ def test_find_executable_path_ext(in_tmpdir):
 def test_normexe_does_not_exist():
     with pytest.raises(OSError) as excinfo:
         parse_shebang.normexe('i-dont-exist-lol')
-    assert excinfo.value.args == ('Executable i-dont-exist-lol not found',)
+    assert excinfo.value.args == ('Executable `i-dont-exist-lol` not found',)
 
 
 def test_normexe_already_full_path():

tests/repository_test.py

@@ -164,6 +164,16 @@ def test_system_hook_with_spaces(tempdir_factory, store):
     )
 
 
+@pytest.mark.integration
+def test_missing_executable(tempdir_factory, store):
+    _test_hook_repo(
+        tempdir_factory, store, 'not_found_exe',
+        'not-found-exe', ['/dev/null'],
+        b'Executable `i-dont-exist-lol` not found',
+        expected_return_code=1,
+    )
+
+
 @pytest.mark.integration
 def test_run_a_script_hook(tempdir_factory, store):
     _test_hook_repo(