A module may need a secret of its own, and the provisioner watches

Two things, both found by trying to write a real postgres module and
discovering it could not be said.

A database has a superuser password, a broker an administrator, a
registry an account. None of them is *for* anybody — they are not the
credential a consumer is given, and the mechanism that hands those out
has a consumer in the middle of it. So a module may declare what it needs
and where to put it, and the mesh generates one per node, seals it, and
reads it no more than it reads any other.

Per node, deliberately: a module running on three machines has three
passwords. One in the manifest instead would put the same secret on every
machine that ever runs it, in a file anybody can read, for ever. Made
once and kept, or a running database would be handed a password it was
not started with; remade when the machine's sealing key changes, like
everything else sealed here.

A need declared and not made is refused rather than skipped, because a
module whose own credential is silently absent starts, fails to
authenticate, and the reason is three layers from the machine reporting
it.

And the provisioner can watch. That is what lets it be a module rather
than a binary somebody places: run once, it needs invoking after every
declaration by a timer or a unit wired to a file; watching, it is an
ordinary long-running service the host already supervises. It polls
rather than watching the filesystem, because the host writes atomically —
the file is replaced, so a watch on the path stops seeing anything after
the first replacement, and a watcher that silently stops working is worse
than a poll. Credentials are compared by digest and never held: this runs
for as long as the machine is up.

internal/catalogue/manifest.go

@@ -199,6 +199,19 @@ type Manifest struct {
 	// makes `restart-on` precise.
 	Secrets map[string]string `json:"secrets,omitempty"`
 
+	// Needs is a secret this module needs for itself, and where to put it.
+	//
+	// Not tied to a consumer. A database has a superuser password, a broker has an administrator,
+	// a registry has an account — each is a secret the module needs in order to be itself, and
+	// none of them is *for* anybody. Keyed by a name of the module's choosing, valued by the file
+	// it lands in.
+	//
+	// **Generated per node and sealed to it**, like everything else the mesh hands out, so a
+	// module running on three machines has three passwords and the mesh can read none of them. A
+	// manifest carrying one instead would put the same secret on every machine that ever runs the
+	// module, in a file anybody can read, for ever.
+	Needs map[string]string `json:"needs,omitempty"`
+
 	// Grants is a directory this module wants the credentials of its consumers written into, per
 	// provision it offers — one file per consumer, named for it, holding the value alone.
 	//
@@ -235,6 +248,9 @@ const (
 	ArtifactArchive = "archive"
 )
 
+// NeedID is the resource identity of the file a module's own secret lands in.
+func NeedID(name string) string { return "needs-" + name }
+
 // SecretID is the resource identity of the file a module is given a credential in.
 func SecretID(requirement string) string { return "secret-" + requirement }
 
@@ -374,6 +390,15 @@ func ParseManifest(raw []byte) (Manifest, error) {
 				"%s binds %q and does not require it", m.Module, to))
 		}
 	}
+	for name, where := range m.Needs {
+		if !strings.HasPrefix(where, "/") {
+			problems = append(problems, fmt.Sprintf(
+				"%s needs %q at %q, which is not an absolute path", m.Module, name, where))
+		}
+		if name == "" {
+			problems = append(problems, m.Module+" needs a secret with no name")
+		}
+	}
 	for to, where := range m.Secrets {
 		if !strings.HasPrefix(where, "/") {
 			problems = append(problems, fmt.Sprintf(