mesh-grants: hold a list that changes several loaded users' hashes at once with no rotation by the vault naming them, and raise user-list-held naming them (issue 774, ADR 0561 §7)

modules/mesh-grants/internal/grants/inputs.go

@@ -138,6 +138,22 @@ type HeldHash struct {
 	Generation int       `json:"generation,omitempty"`
 	At         time.Time `json:"at"`
 	Adopted    bool      `json:"adopted,omitempty"`
+	// Fingerprint is the vault's fingerprint of the value the hash is of: with the generation, what says the password
+	// changed, never the hash itself (a hash of the same password differs by its salt).
+	Fingerprint string `json:"fingerprint,omitempty"`
+	// Rotating is the vault's rotation of the account under way: a change of its hash it announces (ADR 0561 §4, §7).
+	Rotating *struct {
+		Login string `json:"login"`
+	} `json:"rotating,omitempty"`
+}
+
+// credentialOf is what says which password a held hash is of, never the hash: adopted, or the vault's generation and the
+// fingerprint of its value.
+func credentialOf(h HeldHash) string {
+	if h.Adopted {
+		return "adopted"
+	}
+	return fmt.Sprintf("generation %d %s", h.Generation, h.Fingerprint)
 }
 
 // readHashes reads the vault's bus-accounts; a refusal is a gap, and every user then has no hash held.