Fix exec in prewarm forked process sometimes inheriting env vars from grandparent process

kitty/child.c

@@ -181,8 +181,26 @@ spawn(PyObject *self UNUSED, PyObject *args) {
     return PyLong_FromLong(pid);
 }
 
+#ifdef __APPLE__
+#include <crt_externs.h>
+#else
+extern char **environ;
+#endif
+
+static PyObject*
+clearenv_py(PyObject *self UNUSED, PyObject *args UNUSED) {
+#ifdef __APPLE__
+    char **e = *_NSGetEnviron();
+    if (e) *e = NULL;
+#else
+    if (environ) *environ = NULL;
+#endif
+    Py_RETURN_NONE;
+}
+
 static PyMethodDef module_methods[] = {
     METHODB(spawn, METH_VARARGS),
+    {"clearenv", clearenv_py, METH_NOARGS, ""},
     {NULL, NULL, 0, NULL}        /* Sentinel */
 };
 

kitty/fast_data_types.pyi

@@ -1485,3 +1485,4 @@ class SingleKey:
 
 def set_use_os_log(yes: bool) -> None: ...
 def get_docs_ref_map() -> bytes: ...
+def clearenv() -> None: ...

kitty/prewarm.py

@@ -24,9 +24,9 @@ from typing import (
 from kitty.constants import kitty_exe, running_in_kitty
 from kitty.entry_points import main as main_entry_point
 from kitty.fast_data_types import (
-    CLD_EXITED, CLD_KILLED, CLD_STOPPED, get_options, install_signal_handlers,
-    read_signals, remove_signal_handlers, safe_pipe, set_options,
-    set_use_os_log
+    CLD_EXITED, CLD_KILLED, CLD_STOPPED, clearenv, get_options,
+    install_signal_handlers, read_signals, remove_signal_handlers, safe_pipe,
+    set_options, set_use_os_log
 )
 from kitty.options.types import Options
 from kitty.shm import SharedMemory
@@ -293,6 +293,13 @@ def child_main(cmd: Dict[str, Any], ready_fd: int = -1, prewarm_type: str = 'dir
     env = cmd.get('env')
     if env is not None:
         os.environ.clear()
+        # os.environ.clear() does not delete all existing env vars from the
+        # libc environ pointer in some circumstances, I havent figured out
+        # which exactly. Presumably there is something that alters the
+        # libc environ pointer?? The environ pointer is used by os.exec and
+        # therefore by subprocess and friends, so we need to ensure it is
+        # cleared.
+        clearenv()
         os.environ.update(env)
     argv = cmd.get('argv')
     if argv:

kitty_tests/prewarm.py

@@ -28,13 +28,15 @@ class Prewarm(BaseTest):
 
         cwd = tempfile.gettempdir()
         env = {'TEST_ENV_PASS': 'xyz'}
-        cols = 117
+        cols = 317
         stdin_data = 'from_stdin'
         pty = self.create_pty(cols=cols)
         ttyname = os.ttyname(pty.slave_fd)
         opts = get_options()
         opts.config_overrides = 'font_family prewarm',
+        os.environ['SHOULD_NOT_BE_PRESENT'] = '1'
         p = fork_prewarm_process(opts, use_exec=True)
+        del os.environ['SHOULD_NOT_BE_PRESENT']
         if p is None:
             return
         p.take_from_worker_fd(create_file=True)
@@ -44,7 +46,7 @@ import os, json; from kitty.utils import *; from kitty.fast_data_types import ge
         'ttyname': os.ttyname(sys.stdout.fileno()),
         'cols': read_screen_size().cols,
         'cwd': os.getcwd(),
-        'env': os.environ.get('TEST_ENV_PASS'),
+        'env': os.environ.copy(),
         'pid': os.getpid(),
         'font_family': get_options().font_family,
         'stdin': sys.stdin.read(),
@@ -59,7 +61,8 @@ import os, json; from kitty.utils import *; from kitty.fast_data_types import ge
         self.assertTrue(data['cterm'])
         self.ae(data['ttyname'], ttyname)
         self.ae(os.path.realpath(data['cwd']), os.path.realpath(cwd))
-        self.ae(data['env'], env['TEST_ENV_PASS'])
+        self.ae(data['env']['TEST_ENV_PASS'], env['TEST_ENV_PASS'])
+        self.assertNotIn('SHOULD_NOT_BE_PRESENT', data['env'])
         self.ae(data['font_family'], 'prewarm')
         self.ae(int(p.from_worker.readline()), data['pid'])