A user, bytes, and an archive — because most of what people install is

not a service

A shell, a terminal, a chat client, a desktop are a package plus
configuration in somebody's home. A mesh with no notion of a user can own
/etc and nothing anybody looks at, which is most of the reason to manage
a machine at all.

Three shapes, and the vocabulary test asserts the count precisely because
widening it widens what a compromised control plane can express:

  user     a login, its shell and its groups
  archive  a set of files, fetched by digest and unpacked
  (file)   gains `bytes` for what is not text, and `owner`

`user` also makes "zsh is my login shell" declared state. chsh is a
command, the link may not carry one, and a shell settable only by hand is
a shell the mesh cannot manage.

Groups are additive and never pruned — usermod without --append REPLACES
them, which would silently remove every group that makes a login able to
use the machine. A machine's own groups are not the mesh's to know about.

The archive is the one place this host reaches out on its own; everywhere
else it holds one outbound connection and fetches nothing. So it carries
the discipline the bootstrap already uses for images: pinned by digest,
and the digest checked before a single file is written.

Two decisions in the unpacker worth naming:

- an entry naming a path outside the archive is REFUSED, not sanitised.
  Rewriting it to land inside would put a file somewhere nobody asked for
  and report success. Found by the test: the first version quietly
  relocated it.
- symlinks and device nodes are refused rather than skipped, or an
  archive that needed one arrives silently incomplete.

A partial host does archives and refuses users: an archive needs a
filesystem and a way to fetch; a user needs a user database it is allowed
to write.

internal/system/system.go

@@ -60,6 +60,61 @@ type System interface {
 	// ServiceBoot is "enabled" or "disabled" — whether the unit starts at boot.
 	ServiceBoot(ctx context.Context, run Runner, unit string) (string, error)
 	SetServiceBoot(ctx context.Context, run Runner, unit, boot string) error
+
+	// CreateUser makes a login. Home and shell may be empty, meaning the system's own defaults —
+	// a declaration that says nothing about them must not impose an opinion.
+	CreateUser(ctx context.Context, run Runner, name, home, shell string) error
+	// SetUserShell changes an existing login's shell, which is what makes "zsh is my shell"
+	// declared state rather than a command the link may not carry.
+	SetUserShell(ctx context.Context, run Runner, name, shell string) error
+	// AddUserToGroup is additive and never removes. A machine's own groups are not the mesh's to
+	// know about, and a declaration that pruned them would take away what somebody set by hand.
+	AddUserToGroup(ctx context.Context, run Runner, name, group string) error
+}
+
+// Login is what the machine's user database says about a login.
+type Login struct {
+	Home  string
+	Shell string
+}
+
+// LookUpUser reads a login from the user database.
+//
+// Shared rather than per-system: `getent passwd` gives the same seven colon-separated fields
+// everywhere this host runs, and a second implementation would be a second thing to get wrong in
+// the same way.
+//
+// **Absent is an answer, an error is not.** A user database that cannot be read must not be
+// reported as "no such user" — that is absence read as fact, the exact confusion this package
+// takes trouble over elsewhere. `getent` exits 2 for "not found" and other codes for failures, so
+// the two are distinguished rather than collapsed.
+func LookUpUser(ctx context.Context, run Runner, name string) (Login, bool, error) {
+	out, err := run(ctx, "getent", "passwd", name)
+	if err != nil {
+		// getent's own convention: 2 means the key was not found, which is the only failure that
+		// means "no such user".
+		if strings.Contains(err.Error(), "exit status 2") {
+			return Login{}, false, nil
+		}
+		return Login{}, false, fmt.Errorf(
+			"the user database did not answer about %q, so nothing can be said about it: %w",
+			name, err)
+	}
+	fields := strings.Split(strings.TrimSpace(out), ":")
+	if len(fields) < 7 {
+		return Login{}, false, fmt.Errorf("the user database gave %q for %q, which is not a passwd entry",
+			strings.TrimSpace(out), name)
+	}
+	return Login{Home: fields[5], Shell: fields[6]}, true, nil
+}
+
+// GroupsOf is every group a login is in.
+func GroupsOf(ctx context.Context, run Runner, name string) ([]string, error) {
+	out, err := run(ctx, "id", "-nG", name)
+	if err != nil {
+		return nil, err
+	}
+	return strings.Fields(out), nil
 }
 
 // Supports reports whether this host can apply a shape.
@@ -110,6 +165,7 @@ func everyShape() []declaration.Type {
 	return []declaration.Type{
 		declaration.TypeDirectory, declaration.TypeFile, declaration.TypeService,
 		declaration.TypePackage, declaration.TypeContainer, declaration.TypeAction,
+		declaration.TypeUser, declaration.TypeArchive,
 	}
 }
 
@@ -120,6 +176,10 @@ func everyShape() []declaration.Type {
 func portableShapes() []declaration.Type {
 	return []declaration.Type{
 		declaration.TypeDirectory, declaration.TypeFile, declaration.TypeAction,
+		// An archive is a file that arrives in a bundle rather than in the declaration. It needs
+		// only a filesystem and a way to fetch, so a partial host can do it; a user needs a user
+		// database it is allowed to write, which it does not have.
+		declaration.TypeArchive,
 	}
 }