fix(assemble): an apostrophe glued to a ${ or the closing '' (#244)

[defangdevs]

Motivation

bin/assemble-module.py's nix_escape() escaped in two passes — ''→''', then ${→''${. Each rule is right alone and wrong in combination: the escape for ${ starts with '', and Nix lexes a run of apostrophes greedily, three at a time. So an ordinary shell line

echo "key '${LOCAL_WEBHOOK_SPAWN_KEY:-event}' is unusable" >&2

reached the generated module as ' + ''${…, which Nix reads as an escaped '' plus a live antiquotation. nix run .#assemble succeeded; every later nix build failed with error: undefined variable 'event' pointing into modules/agent-box.nix, with nothing wrong in the source file. With a ${FOO} (no :-default) there is no undefined variable and the payload is silently mangled instead — that half ships. Found in #243, which worked around it by rewording the line. Fixes #244.

Real Nix showed the same flaw at the other end of a payload: text ending in an odd number of apostrophes glues onto the host's closing '' and the string never terminates.

What changed

• bin/assemble-module.py — one pass over '+\$\{|'+|\$\{, so a quote run and the ${ behind it are escaped together. Pairs become '''; an odd trailing apostrophe becomes ''\', which leaves the following ''${ (or the host's closing '') unambiguous. The result is now splice-anywhere safe.
• tests/test-assemble-module.py (new) — decodes escaped text the way Nix's IND_STRING lexer rules do (raising on a live ${, an unterminated string, or trailing text), and round-trips every string up to 5 characters over ' $ { } \ a — 9331 of them — through nix_escape and back. Plus explicit expected escapes, the exact fix(sessions): let a session name be as long as a dispatch can mint #243 line, and two resolve()-level tests for @@include: vs @@include-verbatim:. One test re-runs the corpus through the old two-pass escaping and asserts it fails, so a decoder that accepted everything would not pass silently.
• flake.nix — new eval-level check assemble-module-escaping (both systems), plus a CI step. module-generated-up-to-date cannot catch an escaping bug: it regenerates the file with the same assembler, so the check and the bug agree on the wrong bytes.
• modules/src/webhook-spawn.sh — the fix(sessions): let a session name be as long as a dispatch can mint #243 workaround (key printed unquoted, with a comment pointing at assemble-module.py: a single quote directly before ${ escapes wrong — silent corruption or a broken generated module #244) is replaced by the natural wording. This is also the regression test with real Nix in it: the committed module now carries ''\'''${LOCAL_WEBHOOK_SPAWN_KEY:-event}', so module-single-file, vm-closure and golden-snapshot all parse it. The golden fixture shows the rendered script back with the apostrophes in place.

The model is validated against real Nix

The committed test uses a Python model of the lexer, because the check runs in a build sandbox with no nix. The model was checked against the real thing on this box — all 9331 escaped strings evaluate back to their source through nix-instantiate --eval --json:

$ python3 tests/test-assemble-module.py --nix
all 9331 escaped strings round-trip through real Nix

--nix is kept in the file so anyone can redo that.

User-visible and security effects

None on a deployed box beyond one diagnostic's wording. agent-box-webhook-spawn now says

agent-box-webhook-spawn: key '<key>' does not fit a session name (max 150 characters)

The escaping change is a no-op for every payload in the tree today (no modules/src/* file contains the pattern — one could not, the tree would not build), so modules/agent-box.nix changes only by that reworded line.

Checks run

On this aarch64-linux box (all exit 0):

• assemble-module-escaping — 7 tests OK
• python3 tests/test-assemble-module.py --nix — 9331/9331 through real Nix
• module-generated-up-to-date, module-single-file, multi-user, download-route, webhook-route
• golden-snapshot — failed with exactly the one expected diagnostic diff, regenerated with nix run .#update-golden, green after

VM tests and vm-closure are x86_64-linux-only; CI covers them.