ecschema-metadata: Race condition when creating FormatterSpec

[grigasp]

Describe the bug

Calling FormatterSpec.create several times in parallel on same units causes the following error:

UnitConverter.ts:87 Uncaught (in promise) _BentleyError: Source and target units do not have matching base units
    at UnitConverter.processUnits (UnitConverter.ts:87:13)
    at async SchemaUnitProvider.getConversion (SchemaUnitProvider.ts:180:24)
    at async _FormatterSpec.getUnitConversions (FormatterSpec.ts:81:28)
    at async _FormatterSpec.create (FormatterSpec.ts:110:47)

To Reproduce

Calling this function should reproduce the error on await Promise.all line:

async function createFormatterSpecMetersToMillimeters(imodel: IModelConnection) {
  const schemaContext = new SchemaContext();
  schemaContext.addLocater(new ECSchemaRpcLocater(imodel.getRpcProps()));
  const unitsProvider = new SchemaUnitProvider(schemaContext);

  const persistenceUnit = await unitsProvider.findUnitByName("Units.M");

  // matches AecUnits:LENGTH_SHORT
  const formatProps = {
    schemaItemType: "Format",
    label: "realu",
    type: "Decimal",
    precision: 2,
    formatTraits: ["KeepSingleZero", "KeepDecimalPoint", "ShowUnitLabel"],
    name: "Formats.DefaultRealU(2)[Units.MM]",
    parent: "Formats.DefaultRealU",
    composite: { units: [{ name: "Units.MM" }] },
  };
  const format = await Format.createFromJSON("", unitsProvider, formatProps);

  const [formatterSpec1, formatterSpec2] = await Promise.all([
    FormatterSpec.create("", format, unitsProvider, persistenceUnit),
    FormatterSpec.create("", format, unitsProvider, persistenceUnit),
  ]);

  // eslint-disable-next-line no-console
  console.log(`5 meters = ${formatterSpec1.applyFormatting(5)} = ${formatterSpec2.applyFormatting(5)}`);
}

Expected behavior

Both FormatterSpec objects are created successfully and can be used to format values.

Desktop (please complete the applicable information):

• OS: Windows 11
• Browser: Chrome, Firefox
• iTwin.js Version: 4.10.1

Additional context

Originally reported in iTwin/presentation#848