pmb.helpers.run_core: fix proxy env var logic

Fix that the list "cmd" was turned into a string if one of the proxy
vars was set in the environment. Add a test for this code path. Before
this patch:

$ FTP_PROXY=test pmbootstrap -v --details-to-stdout status
…
% cd /home/user/.local/var/pmbootstrap/cache_git/pmaports; git remote -v
run: FTP_PROXY=test ['git', 'remote', '-v']
ERROR: [Errno 2] No such file or directory: "FTP_PROXY=test ['git', 'remote', '-v']"

Fixes: 1a00c04f ("pmb.helpers.run_core: always configure proxy vars if set in environment")
Reviewed-by: Clayton Craft <clayton@craftyguy.net>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230713182337.6185-3-ollieparanoid@postmarketos.org%3E

pmb/helpers/run.py

@@ -1,6 +1,5 @@
 # Copyright 2023 Oliver Smith
 # SPDX-License-Identifier: GPL-3.0-or-later
-import shlex
 import pmb.helpers.run_core
 
 

pmb/helpers/run_core.py

@@ -341,11 +341,13 @@ def core(args, log_message, cmd, working_dir=None, output="log",
     sanity_checks(output, output_return, check)
 
     # Preserve proxy environment variables
+    env = {}
     for var in ["FTP_PROXY", "ftp_proxy", "HTTP_PROXY", "http_proxy",
                 "HTTPS_PROXY", "https_proxy", "HTTP_PROXY_AUTH"]:
         if var in os.environ:
-            # Prepend setting var to cmd string
-            cmd = f"{var}={os.environ[var]} {cmd}"
+            env[var] = os.environ[var]
+    if env:
+        cmd = ["sh", "-c", flat_cmd(cmd, env=env)]
 
     if args.sudo_timer and sudo:
         sudo_timer_start()

test/test_run_core.py

@@ -1,10 +1,11 @@
 # Copyright 2023 Oliver Smith
 # SPDX-License-Identifier: GPL-3.0-or-later
 """ Test pmb.helpers.run_core """
-import re
-import sys
-import subprocess
+import os
 import pytest
+import re
+import subprocess
+import sys
 import time
 
 import pmb_test  # noqa
@@ -127,7 +128,7 @@ def test_foreground_tui():
     assert func(["echo", "test"]) == 0
 
 
-def test_core(args):
+def test_core(args, monkeypatch):
     # Background
     func = pmb.helpers.run_core.core
     msg = "test"
@@ -157,6 +158,11 @@ def test_core(args):
         func(args, msg, ["sleep", "1"], output="log")
     assert re.search(r"^Command failed \(exit code -?\d*\): ", str(e.value))
 
+    # Preserve proxy environment variables
+    monkeypatch.setattr(os, "environ", {"FTP_PROXY": "testproxy"})
+    ret = func(args, msg, ["sh", "-c", 'echo "$FTP_PROXY"'], output_return=True)
+    assert ret == "testproxy\n"
+
 
 @pytest.mark.skip_ci
 def test_sudo_timer(args):