External storage: split checkBackend and configure (follow-up #545) (#567)

This fixes an issue with external storage in saas environment.

See https://github.com/gristlabs/grist-core/pull/546/files#r1259340397

Co-authored-by: Florent FAYOLLE <florent.fayolle@beta.gouv.fr>

app/server/lib/ICreate.ts

@@ -32,6 +32,8 @@ export interface ICreate {
   sessionSecret(): string;
   // Check configuration of the app early enough to show on startup.
   configure?(): Promise<void>;
+  // Optionally perform sanity checks on the configured storage, throwing a fatal error if it is not functional
+  checkBackend?(): Promise<void>;
   // Return a string containing 1 or more HTML tags to insert into the head element of every
   // static page.
   getExtraHeadHtml?(): string;
@@ -119,6 +121,13 @@ export function makeSimpleCreator(opts: {
       return secret;
     },
     async configure() {
+      for (const s of storage || []) {
+        if (s.check()) {
+          break;
+        }
+      }
+    },
+    async checkBackend() {
       for (const s of storage || []) {
         if (s.check()) {
           await s.checkBackend?.();