ISEP/hooks/lib/push_target_dir.py

#!/usr/bin/env python3
"""hooks/lib/push_target_dir.py -- shared helper (inkstone/ISEP#40 S7-style
lib helper: source of truth for a small piece of logic, not a gate by
itself; nothing here decides allow/block on its own).

What it answers: given a shell command string that somewhere invokes
`git ... push ...`, which directory will that push actually execute in?

Why this exists (inkstone/ISEP#30 comment 3949, 脈絡見 InkStoneCo#57):
main-and-prod-push-guard.sh's stamp_ok() used to read `git rev-parse
--show-toplevel` from the *hook's own* cwd (= the session's real repo) as
"HERE", and compare it against the repo path the caller wrote into the
stamp ("WANT"). That works when the push target IS the repo the session is
standing in. It can never work otherwise: a `cd <other-repo> && git push
origin HEAD:main` changes the *push's* directory but not the hook's, so
HERE stays the real repo forever while WANT is (correctly) the other repo
-- the two can never match, no matter how carefully the caller follows the
gate's own instructions. This isn't a bad judgment call; that scenario
simply doesn't exist in the old model.

This module extracts the directory the push *actually* runs in, purely by
tokenizing -- it never executes anything. Two sources, first one found
wins (in the order a real shell would apply them):
  - a `cd <path>` chain preceding the push, scoped correctly across
    subshells: a `(` inherits the current directory from its parent at the
    moment it opens, but whatever a subshell `cd`s to does NOT leak back
    out to sibling commands after the matching `)` closes (this mirrors
    real bash: a subshell's cwd change is local to that subshell). This
    scoping is load-bearing, not cosmetic: without it, `(cd /repo-A &&
    true); git push origin main` would misattribute the later push (which
    really runs wherever the outer shell already was) to /repo-A, and a
    stale/legitimate stamp for /repo-A could then wrongly wave through a
    push into whatever the outer cwd actually is -- the exact "stamp
    opened for repo A also opens the door for repo B" shape 2026-08-11
    already burned us on once.
  - a `-C <path>` flag on the git invocation itself, which further wins
    over any `cd` chain (matches git's own precedence: `-C` sets the
    directory for that invocation regardless of the shell's cwd).

Multiple relative `cd`/`-C` hops are combined by plain string join here
(no `..`/`~`/`$()` resolution) -- resolving the combined expression to a
real, canonical, absolute path is left to the caller, which does it with a
read-only `cd "<expr>" && pwd` in a throwaway subshell. That two-step split
matters: this module only ever *parses*, so it stays side-effect-free even
when fed a hostile or malformed command; only the caller's final `cd`
touches the filesystem, and `cd` cannot execute anything, it can only fail
to find a directory.

If no `cd`/`-C` applies (the push runs wherever the hook itself is, i.e.
today's behaviour), or the command doesn't parse, prints nothing -- the
caller falls back to its existing cwd-based resolution. That fallback
direction is deliberately the *safe* one: on any parse ambiguity we hand
back "unknown" rather than guess, and an unresolved HERE can only make the
gate keep blocking (fail toward blocking), never open a door it wouldn't
have opened before.

Usage:
    printf '%s' "$CMD" | python3 push_target_dir.py
"""
import shlex
import sys

_SEPARATORS = {";", "&&", "||", "|", "&", "\n"}


def _join(base, path):
    """Combine a cwd-so-far (`base`, or None if unknown/hook-cwd) with a
    `cd`/`-C` argument written in the command. Absolute paths and `~`
    replace the base outright; anything else is appended textually --
    normalizing `..`/`.` is intentionally left to the caller's real `cd`."""
    if not path:
        return base
    if path == "-" or path.startswith("$"):
        # `cd -` (previous dir) and `$VAR`/`$(...)` expansions can't be
        # resolved by tokenizing alone -- treat as "unknown" rather than
        # guess wrong, which keeps the caller on its safe fallback path.
        return None
    if path.startswith("/") or path.startswith("~"):
        return path
    if base is None:
        return path
    return base.rstrip("/") + "/" + path


def _classify(tokens):
    if not tokens:
        return ("other", None)
    if tokens[0] == "cd" and len(tokens) > 1:
        return ("cd", tokens[1])
    if tokens[0] == "git" and "push" in tokens[1:]:
        return ("push", tokens)
    return ("other", None)


def _events(cmd):
    """Tokenize cmd into (kind, value) events in source order: 'enter'/
    'exit' for parens (subshell boundaries), 'cd'/'push'/'other' for
    statements split on the usual shell separators. Returns [] on any
    quoting error -- caller then falls back to cwd-based resolution."""
    try:
        lexer = shlex.shlex(cmd, posix=True, punctuation_chars=True)
        lexer.whitespace_split = True
        toks = list(lexer)
    except ValueError:
        return []

    events = []
    seg = []

    def flush():
        if seg:
            # _classify's "push" branch returns `tokens` by reference; copy
            # before clear() below, or the event's tuple would observe the
            # list emptied out from under it (aliasing, not a value copy).
            events.append(_classify(seg[:]))
            seg.clear()

    for tok in toks:
        if tok == "(":
            flush()
            events.append(("enter", None))
        elif tok == ")":
            flush()
            events.append(("exit", None))
        elif tok in _SEPARATORS:
            flush()
        else:
            seg.append(tok)
    flush()
    return events


def find_push_target(cmd):
    events = _events(cmd)
    if not events:
        return ""

    stack = [None]  # cwd-so-far per paren depth; None = "same as hook cwd"
    result = None
    saw_push = False

    for kind, val in events:
        if kind == "enter":
            stack.append(stack[-1])  # child subshell inherits current dir
        elif kind == "exit":
            if len(stack) > 1:
                stack.pop()          # subshell's own cd's don't leak out
        elif kind == "cd":
            stack[-1] = _join(stack[-1], val)
        elif kind == "push":
            saw_push = True
            c_path = None
            toks = val
            for j, t in enumerate(toks):
                if t == "-C" and j + 1 < len(toks):
                    c_path = toks[j + 1]
                    break
                if t.startswith("-C") and len(t) > 2:
                    c_path = t[2:]
                    break
            result = _join(stack[-1], c_path) if c_path else stack[-1]

    if not saw_push:
        return ""
    return result or ""


def main():
    cmd = sys.stdin.read()
    sys.stdout.write(find_push_target(cmd))


if __name__ == "__main__":
    main()