Summary

• Add ProcessMonitor abstraction for waitpid-based process liveness detection
• Capture stderr to file BEFORE subprocess spawn (catches pre-main dyld errors)
• Pass BoxFilesystemLayout through spawn chain (DRY - no redundant recreation)
• Remove stderr from ExitInfo::Signal (CrashReport reads directly from file)
• Simplify CrashCapture::install() to only take exit_file parameter

Details

ProcessMonitor Abstraction

Extracts duplicated waitpid polling logic into a minimal struct:

pub enum ProcessExit {
    Code(i32),   // Process exited, we captured the code
    Unknown,     // Process dead, but we're not parent (ECHILD)
}

pub struct ProcessMonitor { pid: u32 }

impl ProcessMonitor {
    pub fn try_wait(&self) -> Option<ProcessExit>
    pub async fn wait_for_exit(&self) -> ProcessExit
}

This encapsulates the Unix parent/child constraint for waitpid():

• Owned processes (spawned): can call waitpid() to get exit code
• Attached processes (reconnected): get ECHILD, fall back to kill(pid, 0)

Stderr Capture Before Spawn

Previously, stderr was redirected inside the shim's main(), losing pre-main errors (dyld library loading failures). Now the parent creates the stderr file BEFORE spawn:

// spawn.rs
let stderr_file = std::fs::File::create(&layout.stderr_file_path())?;
cmd.stderr(Stdio::from(stderr_file));  // Captures ALL stderr

Layout Pass-Through

Eliminates redundant FilesystemLayout recreation in spawn_subprocess() by passing BoxFilesystemLayout through the call chain.

Test plan

• cargo build --package boxlite
• cargo test --package boxlite process (12 tests pass)
• cargo test --package boxlite guest_connect (8 tests pass)
• cargo test --package boxlite crash_report (7 tests pass)