(core) When getting error details for on-demand formulas, provide an explanation

Summary:
Since formula errors are typically obtained from the Python data engine, they
were not returning any info for errors in on-demand tables (not loaded into the
data engine). This change implements a detailed message to explain such errors,
mainly to point out that on-demand table is the reason.

Test Plan: Added a check to the OnDemand test that formula error details are shown.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4317

app/client/widgets/FormulaEditor.ts

@@ -480,6 +480,7 @@ function _isInIdentifier(line: string, column: number) {
 
 /**
  * Open a formula editor. Returns a Disposable that owns the editor.
+ * This is used for the editor in the side panel.
  */
 export function openFormulaEditor(options: {
   gristDoc: GristDoc,

app/server/lib/ActiveDoc.ts

@@ -138,7 +138,7 @@ import {
   OptDocSession
 } from './DocSession';
 import {createAttachmentsIndex, DocStorage, REMOVE_UNUSED_ATTACHMENTS_DELAY} from './DocStorage';
-import {expandQuery} from './ExpandedQuery';
+import {expandQuery, getFormulaErrorForExpandQuery} from './ExpandedQuery';
 import {GranularAccess, GranularAccessForBundle} from './GranularAccess';
 import {OnDemandActions} from './OnDemandActions';
 import {getLogMetaFromDocSession, getPubSubPrefix, getTelemetryMetaFromDocSession} from './serverUtils';
@@ -1169,6 +1169,11 @@ export class ActiveDoc extends EventEmitter {
     this._log.info(docSession, "getFormulaError(%s, %s, %s, %s)",
       docSession, tableId, colId, rowId);
     await this.waitForInitialization();
+    const onDemand = this._onDemandActions.isOnDemand(tableId);
+    if (onDemand) {
+      // It's safe to use this.docData after waitForInitialization().
+      return getFormulaErrorForExpandQuery(this.docData!, tableId, colId);
+    }
     return this._pyCall('get_formula_error', tableId, colId, rowId);
   }
 

app/server/lib/ExpandedQuery.ts

@@ -1,5 +1,6 @@
 import { ServerQuery } from 'app/common/ActiveDocAPI';
 import { ApiError } from 'app/common/ApiError';
+import { CellValue } from 'app/common/DocActions';
 import { DocData } from 'app/common/DocData';
 import { parseFormula } from 'app/common/Formula';
 import { removePrefix } from 'app/common/gutil';
@@ -133,6 +134,24 @@ export function expandQuery(iquery: ServerQuery, docData: DocData, onDemandFormu
   return query;
 }
 
+export function getFormulaErrorForExpandQuery(docData: DocData, tableId: string, colId: string): CellValue {
+  // On-demand tables may produce several kinds of error messages, e.g. "Formula not supported" or
+  // "Cannot find column". We construct the full query to get the basic message for the requested
+  // column, then tack on the detail, which is fine to be the same for all of them.
+  const iquery: ServerQuery = {tableId, filters: {}};
+  const expanded = expandQuery(iquery, docData, true);
+  const constantValue = expanded.constants?.[colId];
+  if (constantValue?.length === 2) {
+    return [GristObjCode.Exception, constantValue[1],
+`Not supported in on-demand tables.
+
+This table is marked as an on-demand table. Such tables don't support most formulas. \
+For proper formula support, unmark it as on-demand.
+`];
+  }
+  return null;
+}
+
 /**
  * Build a query that relates two homogeneous tables sharing a common set of columns,
  * returning rows that exist in both tables (if they have differences), and rows from