Fix MutabilityWithReadOnly linter rule to avoid the out of memory issue

[AkhilaIlla]

Daniel Jurek: Akhila Illa -- I traced through spectral's execution of the rules la... | Azure SDK > API Spec Review | Microsoft Teams

[danieljurek]

Logging shows that this code path is hit every time and it seems that memory is getting filled with empty arrays: https://github.com/Azure/azure-openapi-validator/blob/main/packages/rulesets/src/spectral/functions/Extensions/mutability-with-read-only.ts#L7-L13

  if (
    prop.readOnly === undefined ||
    prop["x-ms-mutability"] === undefined ||
    prop["x-ms-mutability"].length === 0
  ) {
    return [];
  }

I suspect that rule expressions which cover the "empty array returning scenarios" would prevent out of memory errors.

[danieljurek]

Here's a write up of what we did: https://github.com/Azure/azure-openapi-validator/wiki/Instrumenting-rule-executions-in-spectral

[mikeharder]

I believe the fix, is to change the given clause of this rule (L266):

azure-openapi-validator/packages/rulesets/src/spectral/az-common.ts

Lines 259 to 268 in f299af5

	MutabilityWithReadOnly: {
	description:
	'Verifies whether a model property which has a readOnly property set has the appropriate `x-ms-mutability` options. If `readonly: true`, `x-ms-mutability` must be `["read"]`. If `readonly: false`, `x-ms-mutability` can be any of the `x-ms-mutability` options.',
	message: "{{error}}",
	severity: "error",
	resolved: true,
	formats: [oas2],
	given: ["$[paths,'x-ms-paths']..?(@property === 'readOnly')^"],
	then: {
	function: mutabilityWithReadOnly,

To this, which should be functionally equivalent (given what happens in function mutabilityWithReadOnly), but much more performant, since it checks for x-ms-mutability earlier.

given: ["$[paths,'x-ms-paths']..*[?(@ != null && @.readOnly !== undefined && @['x-ms-mutability'] !== undefined)]"],