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A failed file syscall lost both its class and its message

Found 2026-08-16 while triaging Track T's regression-cascade-343a52551808: the uforth corpus is diffed against CPython byte for byte, and a missing INCLUDE printed the bare path.

Measured

open("/nonexistent/f.txt")
# CPython  FileNotFoundError: [Errno 2] No such file or directory: '/nonexistent/f.txt'
# NilPy    FileNotFoundError: /nonexistent/f.txt

open("/tmp", "w")
# CPython  IsADirectoryError: [Errno 21] Is a directory: '/tmp'
# NilPy    FileNotFoundError: /tmp                      <- wrong CLASS

try:
    open("/nonexistent/f.txt")
except IOError as e:      # the most common spelling of this guard
    ...
# CPython  caught
# NilPy    NOT caught

Cause, both halves

  1. Class. Every PyPal* failure path raised FileNotFoundError.Create(path) unconditionally. __pxxrawsyscall returns the raw kernel value, so the errno was right there in r and simply not read.
  2. Alias vs subclass. IOError = class(OSError) makes IOError a SIBLING of FileNotFoundError, not an ancestor, so except IOError: misses it. CPython makes IOError and EnvironmentError aliases — IOError is OSError is True. pylib's own comment said "aliases of OSError" while the line below it said class(OSError); the comment was right and the code was not.

Fix

pyos_raise_ioerror(err, path, path2) in pylib maps errno → subclass and builds CPython's message (including the -> 'dst' tail for two-path calls), and the four raise sites call it. IOError = OSError; / EnvironmentError = OSError;.

Why it matters

The message half is what a program PRINTS — nothing crashes and no value is wrong, so only a differential corpus catches it. The class half is worse: a program that distinguishes "not there" from "not allowed" takes the wrong branch silently, and one that guards with except IOError: — which CPython accepts and runs — does not catch at all. That is the upward-compatibility direction that is not negotiable.

Gate

test/test_nilpy_oserror_class_and_message.npy, six shapes diffed against CPython byte for byte; the uforth filetest word set identical to CPython; gate.sh quick green.