Every credential provider says whether it still holds a consumer

holds() for postgres, mssql, mongodb, minio, lavinmq, mosquitto, mailu
and gitea, so the harness makes again a login the backend lost (hq issue
120). Each checks the mesh's password as the consumer presents it, or
compares it read-only, and returns false only when the backend says the
credential is absent or wrong; an unreachable backend throws.

modules/lavinmq/client.ts

@@ -94,6 +94,39 @@ export class LavinmqClient {
     await this.api("PUT", `/permissions/${v}/${u}`, { configure: ".*", write: ".*", read: ".*" });
   }
 
+  /**
+   * Whether a consumer's user exists with exactly this password and full permissions on its own
+   * vhost. Read-only: the stored hash is salted SHA-256, the scheme `rabbitHash` writes, so the
+   * password is checked by hashing it with the stored salt rather than by logging in. `false` when
+   * the user or its permission is gone or the password differs; an unreachable API rejects
+   * (novox/hq issue 120).
+   */
+  async holdsConsumer(login: string, password: string): Promise<boolean> {
+    const v = encodeURIComponent(login);
+    const u = encodeURIComponent(login);
+    const user = await this.getOrNull<{ password_hash?: string; hashing_algorithm?: string }>(`/users/${u}`);
+    if (!user?.password_hash) return false;
+    if (user.hashing_algorithm && !/sha256/i.test(user.hashing_algorithm)) {
+      throw new Error(`lavinmq user ${login} is hashed with ${user.hashing_algorithm}, which this check cannot verify`);
+    }
+    const stored = Buffer.from(user.password_hash, "base64");
+    if (stored.length < 5 || rabbitHash(password, stored.subarray(0, 4)) !== user.password_hash) return false;
+    const perm = await this.getOrNull<{ configure?: string; write?: string; read?: string }>(`/permissions/${v}/${u}`);
+    return perm?.configure === ".*" && perm?.write === ".*" && perm?.read === ".*";
+  }
+
+  /** A GET that answers null for a 404 and rejects on anything else that is not 2xx. */
+  private async getOrNull<T>(path: string): Promise<T | null> {
+    const resp = await fetch(`${this.conn.base}/api${path}`, {
+      headers: {
+        Authorization: "Basic " + Buffer.from(`${this.conn.adminUser}:${this.conn.adminPassword}`).toString("base64"),
+      },
+    });
+    if (resp.status === 404) return null;
+    if (!resp.ok) throw new Error(`lavinmq management API GET ${path} -> ${resp.status}: ${await resp.text()}`);
+    return (await resp.json()) as T;
+  }
+
   /** Remove a consumer's vhost and user, idempotently. A DELETE of what is already gone is tolerated. */
   async removeConsumer(login: string): Promise<void> {
     const v = encodeURIComponent(login);