(core) move some material to core that slipped through in a rebase

Summary:
This makes core independently buildable again, and adds a small
script to run as a sanity check.

Test Plan: checked that build_core.sh succeeds

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D2558

app/server/lib/IElectionStore.ts

@@ -0,0 +1,24 @@
+/**
+ * Get a revokable named exclusive lock with a TTL.  This is convenient for housekeeping
+ * tasks, which can be done by any server, but should preferably be only done by one
+ * at a time.
+ */
+export interface IElectionStore {
+  /**
+   * Try to get a lock called <name> for a specified duration.  If the named lock
+   * has already been taken, null is returned, otherwise a secret is returned.
+   * The secret can be used to remove the lock before the duration has expired.
+   */
+  getElection(name: string, durationInMs: number): Promise<string|null>;
+
+  /**
+   * Remove a named lock, presenting the secret returned by getElection() as
+   * a cross-check.
+   */
+  removeElection(name: string, electionKey: string): Promise<void>;
+
+  /**
+   * Close down access to the store.
+   */
+  close(): Promise<void>;
+}

app/server/lib/Permit.ts

@@ -0,0 +1,58 @@
+/**
+ * An exceptional grant of rights on a resource, for when work needs to be
+ * initiated by Grist systems rather than a user.  Cases where this may happen:
+ *
+ *   - Deletion of documents and workspaces in the trash
+ *
+ * Permits are stored in redis (or, in a single-process dev environment, in memory)
+ * as json, in keys that expire within minutes.  The keys should be effectively
+ * unguessable.
+ *
+ * To use a permit:
+ *
+ *   - Prepare a Permit object that includes the id of the document or
+ *     workspace to be operated on.
+ *
+ *   - It the operation you care about involves the database, check
+ *     that "allowSpecialPermit" is enabled for it in HomeDBManager
+ *     (currently only deletion of docs/workspaces has this enabled).
+ *
+ *   - Save the permit in the permit store, with setPermit, noting its
+ *     generated key.
+ *
+ *   - Call the API with a "Permit: <permit-key>" header.
+ *
+ *   - Optionally, remove the permit with removePermit().
+ */
+export interface Permit {
+  docId?: string;
+  workspaceId?: number;
+  org?: string|number;
+}
+
+/* A store of permits */
+export interface IPermitStore {
+
+  // Store a permit, and return the key it is stored in.
+  // Permits are transient, and will expire.
+  setPermit(permit: Permit): Promise<string>;
+
+  // Get any permit associated with the given key, or null if none.
+  getPermit(permitKey: string): Promise<Permit|null>;
+
+  // Remove any permit associated with the given key.
+  removePermit(permitKey: string): Promise<void>;
+
+  // Close down the permit store.
+  close(): Promise<void>;
+}
+
+// Create a well formatted permit key from a seed string.
+export function formatPermitKey(seed: string) {
+  return `permit-${seed}`;
+}
+
+// Check that permit key is well formatted.
+export function checkPermitKey(key: string): boolean {
+  return key.startsWith('permit-');
+}