fix: watch parent-directory glob inputs in build programs #764

rerun_if_changed_glob("../inputs/**/*.in") silently fingerprints an empty set, even when matching files exist beside the package. Adding or removing those files therefore does not rerun build.mcpp on either the build-program cache path or the project fast path.

This affects workspace members using shared vcpkg overlay directories: mcpp-plugins 0.18.1's mcpp::deps::watch_tree derives a path relative to manifest_dir() and emits exactly this form when the overlay lives outside the member.

Reproduction

Create this layout:

workspace/
  inputs/a.in
  app/mcpp.toml
  app/build.mcpp
  app/src/main.cpp

Use a normal executable package for app, and a build program which declares:

mcpp::rerun_if_changed_glob("../inputs/**/*.in");

Have the program enumerate manifest_dir()/../inputs and generate a function returning the number of .in files, as in tests/e2e/194_build_mcpp_glob_input.sh but with a sibling input directory.

1. Build and run app: the generated function returns 1.
2. Add inputs/b.in, without editing the manifest, build program or existing source files.
3. Build and run app again.

Expected: the build program reruns and the executable returns 2.

Actual: the build program is not invalidated and the generated function still returns 1. In the real workspace, build.mcpp.cache records glob cbf29ce484222325 ../vcpkg-scripts/ports/** (the empty-set fingerprint) while that directory contains multiple ports.

Cause and proposed fix

modules/buildmcpp/src/directives.cppm::glob_fingerprint always starts its recursive iterator at the package root. path_matches_glob cannot match an external file which was never enumerated. The source scanner already derives a glob's literal directory prefix to choose its walk root; the rerun fingerprint should share that derivation rather than independently assuming every input lives under the package.

The fix should preserve sorted path-set fingerprints, content-independent glob invalidation, output/.git exclusions and the no-directory-symlink rule. Regression coverage should exercise sibling and ancestor directories, previously missing input directories, and actual builds taking the fast path.

Environment

• mcpp 2026.10.3.1, latest main at 6e3cb974d199546112ef13f65597a912a06cb899
• Windows 11, LLVM 22.1.8 / MSVC ABI
• mcpp-plugins 0.18.1

The iterator's root restriction is platform independent.