Two mechanisms, matching what the make engine actually did:
Lazy defaults. A stack setting is a function myos_default_<VAR>, called only
when the variable has no value, and called again at every reference. That is
exactly a recursive ?=: an explicit value wins, and the default follows a
DOMAIN that a .env changes later. The prefix is what makes it safe; the first
version used a bare function named after the variable, and the test suite
caught it running /usr/bin/host for a stack group called host.
Templates. myos env-update fills a .env from the .env.dist files, expanding
${VAR} against the current values and running $(command), forward references
included.
Also fixed: the project .env now wins over /etc/conf.d/myos, which is what the
documentation claimed and the code did not.
share/make/shim.mk lets a project keep make as a front end: every myos command
becomes a target that shells out to bin/myos, and the project keeps its own
targets and its stack .mk files. It sits outside make/ because the legacy
engine globs every .mk in there.
A stack can now ship <name>.env and <name>.sh next to its compose files. The
hook is sourced with the tag helpers available, which is what the computing
.mk files of the catalogue were using make for: 29 of its 44 .mk files only
exist to build variables like the fabio tags.
Converting stack/host/fabio.mk by hand gives byte-identical output for the
route tag, and drops a trailing comma the make version left in the listener
list.
Also: APP_HOST and APP_URI are computed (the tag helpers build on them),
--color controls the escape codes rather than always emitting them, and
make test-portability runs the CLI under busybox ash and dash.
Yann Autissier