(core) Fix WelcomePage to use an explicit action URL, and parse submitted body in time to log it with errors.

Test Plan: Tested manually that "Preflight" error goes away in Safari.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2644

app/server/lib/FlexServer.ts

@@ -972,6 +972,10 @@ export class FlexServer implements GristServer {
         let body: string|undefined;
         let permitKey: string|undefined;
         try {
+          const user = getUser(req);
+          const row = {...req.body, UserID: userId, Name: user.name, Email: user.loginEmail};
+          body = JSON.stringify(mapValues(row, value => [value]));
+
           // Take an extra step to translate the special urlId to a docId. This is helpful to
           // allow the same urlId to be used in production and in test. We need the docId for the
           // specialPermit below, which we need to be able to write to this doc.
@@ -985,10 +989,6 @@ export class FlexServer implements GristServer {
             throw new Error(`Can't resolve ${urlId}: ${docAuth.error}`);
           }
 
-          const user = getUser(req);
-          const row = {...req.body, UserID: userId, Name: user.name, Email: user.loginEmail};
-          body = JSON.stringify(mapValues(row, value => [value]));
-
           permitKey = await this._docWorkerMap.setPermit({docId});
           const res = await fetch(await this.getHomeUrlByDocId(docId, `/api/docs/${docId}/tables/Responses/data`), {
             method: 'POST',