Judge the installer's user list against mesh-host with the groups grant (hq issue 525)

The controller now asks mesh-delivery.groups, and mesh-host a76baa4b carries
that grant in the installer's first user list (mesh-host #85). The captured
copy is moved to that commit so the genesis test judges the list that ships.

testdata/beside/mesh-host/internal/apply/placement_guard.go.captured

@@ -31,6 +31,13 @@ package apply
 //  5. **A mount source that is a refused directory or a refused access is refused with it**: the container
 //     would otherwise bind the very path the engine would not place, and the runtime creates a missing one
 //     as root.
+//  6. **A file is judged as a directory is** (novox/hq issue 495): never one of the machine's own roots, nor
+//     in a tree nothing is placed in (rule 2), and never below a directory or an access refused here. A file
+//     or an archive below a refused path is refused with it, whichever module declares it and in whatever
+//     order. Rules 3 and 4 are a directory's owner and do not reach a file: a file below /etc is the mesh's
+//     ordinary configuration, and one below a home is judged by home_links.go and agent_home_guard.go.
+//     A refused directory also gates the rest of its module, as a step that did not complete does
+//     (apply.go's `gated`): what a module declares after its directory was written for that directory.
 //
 // Each is a failed resource with its reason in the node's report; nothing is touched. Paths are judged as
 // declared and again with every link in them resolved, so a link at /srv/x pointing at /etc places nothing.
@@ -400,6 +407,7 @@ const (
 	placingDirectory placing = iota
 	placingAccess
 	placingMount
+	placingFile
 )
 
 // refusePath says why a path is not placed or mounted; nil when it may be. module is the resource's module,
@@ -466,7 +474,7 @@ func refuseOne(path string, kind placing, module, owner string) *PlacementRefuse
 	for _, part := range strings.Split(path, "/") {
 		ssh = ssh || part == ".ssh"
 	}
-	if ssh && kind != placingDirectory {
+	if ssh && (kind == placingAccess || kind == placingMount) {
 		return &PlacementRefusedError{Path: path, Why: "it is an account's .ssh, which holds its keys and who may " +
 			"log in as it, and is never an access or a mount"}
 	}
@@ -502,8 +510,12 @@ func moduleOfID(id string) string {
 	return module
 }
 
-// refusedPlaces judges every directory and access of a declaration before anything is applied, and answers
-// each refusal by path: what is refused is refused again as a container's mount source.
+// refusedPlaces judges every directory, access, file and archive of a declaration before anything is applied,
+// and answers each refusal by path: what is refused is refused again as a container's mount source.
+//
+// Directories and accesses first, so a file or an archive below one of them is refused with it whatever order
+// they are declared in (novox/hq issue 495): the engine refused Docker's data root as a directory and then wrote
+// a file into it, because nothing judged the file's path.
 func refusedPlaces(resources []declaration.Resource) map[string]error {
 	out := map[string]error{}
 	for _, r := range resources {
@@ -520,9 +532,60 @@ func refusedPlaces(resources []declaration.Resource) map[string]error {
 			out[filepath.Clean(r.Target())] = err
 		}
 	}
+	trees := make(map[string]error, len(out))
+	for path, err := range out {
+		trees[path] = err
+	}
+	for _, r := range resources {
+		var err error
+		switch res := r.(type) {
+		case *declaration.File:
+			if err = belowRefused(res.Path, trees); err == nil {
+				err = refusePath(res.Path, placingFile, moduleOfID(res.ID), "")
+			}
+		case *declaration.Archive:
+			// Only below a refused path: where an archive may unpack is otherwise the archive's own judgement
+			// (archive.go), and the engine's own module unpacks its versions into its own tree.
+			err = belowRefused(res.Path, trees)
+		default:
+			continue
+		}
+		if err != nil {
+			out[filepath.Clean(r.Target())] = err
+		}
+	}
 	return out
 }
 
+// belowRefused says why a path below a refused directory or access is refused with it; nil when it is below
+// none. Judged as declared and with its links resolved, as refusePath judges.
+func belowRefused(path string, refused map[string]error) error {
+	clean := filepath.Clean(path)
+	if !filepath.IsAbs(clean) {
+		return nil
+	}
+	for _, p := range []string{clean, resolved(clean)} {
+		// The deepest refused tree it is below, so the same declaration always gives the same reason.
+		var tree string
+		for t := range refused {
+			if below(p, t) && len(t) > len(tree) {
+				tree = t
+			}
+		}
+		if tree == "" {
+			continue
+		}
+		why := refused[tree]
+		reason := why.Error()
+		var refusal *PlacementRefusedError
+		if errors.As(why, &refusal) {
+			reason = refusal.Why
+		}
+		return &PlacementRefusedError{Path: clean, Why: "it is below " + tree + ", which is refused: " + reason}
+	}
+	return nil
+}
+
 // refuseMounts says why a container's mounts are not made; nil when they may be.
 func refuseMounts(c *declaration.Container, refused map[string]error) error {
 	for _, v := range c.Volumes {