MGS3002: VCS unavailable

A vcs call could not produce the value it was asked for. Either no VCS is resolved for the workspace, or one is resolved and the command it ran failed.

[MGS3002] read git metadata: exit status 128

Why this raises instead of returning ""

These accessors used to return an empty string, and vcs.commit() returned a record with every field blank. Both are gone.

An empty string is a value a branch name or a commit subject can legitimately hold, so testing for it could never distinguish "there is no answer" from "the answer is empty". It also made the check optional: a magusfile that forgot if (h == "") interpolated an empty commit into a version string or an image tag, and nothing surfaced until somebody read the artifact.

Resolution

Building outside a checkout is a real case - a release tarball, a container build context, a vendored copy - so catch it where that is expected:

fun commit() > str {
    try {
        return vcs\shortHash();
    } catch (e) {
        return "unknown";
    }
}

To ask whether a VCS exists at all without catching, use vcs\name(). It returns "" when nothing is resolved and never raises - the detection half of the pair, the same split as os\env and os\lookupEnv.

Note that name() reports the resolved DRIVER. With git installed it answers "git" even outside a repository, because the driver resolved and the repository is what is missing.

Catching it by code

The error reaches catch as a map, so a magusfile can branch on the code rather than matching text:

try {
    final h = vcs\shortHash();
} catch (e) {
    if (e["code"] == "MGS3002") {
        magus\log.info("no VCS here; stamping the build as unknown");
    }
}

What still returns a value

vcs\isDirty() reports false when no VCS is resolved at all - a known state that genuinely answers "is anything dirty?". It RAISES only when a VCS exists and its status probe fails, because then the question was never answered. A drift gate must never be told "clean" by a check that did not run.