stat errors in subprocess environments with process substitution

[p-wall]

Description

When shadowenv is used in subprocess environments where stderr is redirected through process substitutions (e.g., in Claude Code's shell integration or similar tools), hookbook's DEBUG trap generates stat: cannot statx errors. These errors occur because the trap attempts to stat transient pipe/socket file descriptors that are created by process substitution.

Root Cause

The __hookbook_debug_handler in sh/hookbook.sh:82-87 checks if stderr is redirected to /dev/null by comparing the major device number of /dev/fd/2 with that of /dev/null. When stderr is redirected through process substitution (e.g., exec 2> >(command)), /dev/fd/2 points to a transient pipe or socket file descriptor (e.g., /proc/XXXX/fd/pipe:[XXXXX] or /proc/XXXX/fd/socket:[XXXXX]).

The stat command fails when trying to check these transient descriptors because:

1. They may be closed or unavailable by the time stat runs (race condition)
2. The path resolved by readlink -f may not be statable

This results in error messages like:

stat: cannot statx '/proc/XXXX/fd/pipe:[XXXXX]': No such file or directory

Reproduction

Run the following test script:

#!/usr/bin/env bash

# Run a subprocess that redirects stderr through process substitution
{
    # Redirect stderr through a process substitution
    # This makes /dev/fd/2 point to a pipe/socket
    exec 2> >(cat >&2)

    # Source hookbook
    source sh/hookbook.sh

    # Add hook
    test_func() { :; }
    hookbook_add_hook test_func

    # Run commands that trigger DEBUG trap
    # Each one will try to stat /dev/fd/2 which is now a pipe
    echo "Command 1" >&2
    :
    true
    echo "Command 2" >&2
    false || :
    echo "Command 3" >&2

} 2>&1

Expected: Commands execute without errors
Actual: stat: cannot statx errors are printed to stderr

Environment

• Affects Linux systems (uses stat -c "%t" and readlink -f)
• Occurs in subprocess environments with process substitution redirects
• Commonly encountered in tools like Claude Code that use socket-based IPC for shell integration