--full-auto and --sandbox workspace-write flags cause indefinite hang with orphaned child processes

What version of Codex is running?

codex-cli 0.66.0

What subscription do you have?

PRO

Which model were you using?

gpt-5.1-codex-max

What platform is your computer?

Linux 6.8.0-87-generic x86_64

What issue are you seeing?

When using --full-auto or --sandbox workspace-write flags with codex exec, commands hang indefinitely even for simple operations that complete in milliseconds without these flags.

Test results:

| Command | Result | Time |
|---------|--------|------|
| codex exec "echo test" | ✅ Works | ~2s |
| codex exec --json "echo test" | ✅ Works | ~2s |
| codex exec --full-auto "echo test" | ❌ HANGS | >2min |
| codex exec --sandbox workspace-write "echo test" | ❌ HANGS | >2min |

Symptoms:

• Command output shows successful completion in JSONL log
• Codex never returns control to caller
• Child processes remain orphaned in S (sleeping) state
• exit_code: null, status: "in_progress" persists indefinitely

Note: This is separate from issue #7846 (process substitution). This bug occurs with the --full-auto/--sandbox flags themselves, even with simple commands like echo.

What steps can reproduce the bug?

Minimal reproduction:

```bash
# Step 1: This WORKS (no sandbox flags)
timeout 30s codex exec --dangerously-bypass-approvals-and-sandbox \
"Run: echo 'hello world'"
# Result: Completes in ~2-5 seconds

# Step 2: This HANGS (with --full-auto)
timeout 30s codex exec --dangerously-bypass-approvals-and-sandbox \
--full-auto "Run: echo 'hello world'"
# Result: Times out after 30s

# Step 3: This also HANGS (with --sandbox workspace-write)
timeout 30s codex exec --dangerously-bypass-approvals-and-sandbox \
--sandbox workspace-write "Run: echo 'hello world'"
# Result: Times out after 30s

# Step 4: Check for orphaned processes after timeout
pgrep -af 'codex|echo' | grep -v pgrep
```

### Full reproduction script:

```bash
#!/usr/bin/env bash
set -euo pipefail
echo "=== Test 1: Without --full-auto ==="
time timeout 20s codex exec --dangerously-bypass-approvals-and-sandbox \
"Run: echo test" 2>&1 | tail -3

echo "=== Test 2: With --full-auto (will hang) ==="
time timeout 20s codex exec --dangerously-bypass-approvals-and-sandbox \
--full-auto "Run: echo test" 2>&1 | tail -3
echo "Exit code: $?"
```

What is the expected behavior?

• Commands with --full-auto should complete in similar time as without the flag (~2-5s for simple commands)
• All child processes should be terminated when Codex exits
• No orphaned processes should remain after command completion
• exit_code should be set to actual return value (0 for success)

Additional information

Root cause analysis:

The issue appears to be in process group management when sandbox mode is active:

1. Missing setpgid(0, 0) - Child processes not placed in own process group
2. Signal propagation - Signals sent only to direct child PID, not entire group
3. Grandchild orphaning - Sandbox intermediate processes create orphans
4. Pipe deadlock - Codex waits for EOF on pipes kept open by orphans

### Related issues:

• #7846 - Process substitution hang (different trigger, same root cause)
• #4726 - Missing child.wait() (closed)
• #3836 - Async process management

### Workaround:
Remove --full-auto flag. Use --json alone if JSON output needed.