fix: clear programs cache if >90% of heap is used (#643)

## PR Checklist

- [x] Addresses an existing open issue: fixes #642
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/eslint-plugin-expect-type/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/eslint-plugin-expect-type/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

I played around with different strategies a bit, but ended up doing
basically the same thing as
microsoft/DefinitelyTyped-tools#1024. Therefore:

Co-authored-by: Andrew Branch <andrew@wheream.io>

src/utils/programs.test.ts

@@ -0,0 +1,47 @@
+import ts from "typescript";
+import { describe, expect, it, vi } from "vitest";
+
+import { getProgramForVersion } from "./programs.js";
+
+const mockGetHeapStatistics = vi.fn<() => Record<string, number>>();
+
+vi.mock("node:v8", () => ({
+	get default() {
+		return { getHeapStatistics: mockGetHeapStatistics };
+	},
+}));
+
+const originalProgram = ts.createProgram({
+	options: {},
+	rootNames: [],
+});
+
+function actGetProgramForVersion() {
+	return getProgramForVersion("tsconfig.json", ts, "current", originalProgram);
+}
+
+describe("getProgramForVersion", () => {
+	it("reuses old programs when <90% of the heap is used", () => {
+		mockGetHeapStatistics.mockReturnValue({
+			heap_size_limit: 100,
+			used_heap_size: 90,
+		});
+
+		const programA = actGetProgramForVersion();
+		const programB = actGetProgramForVersion();
+
+		expect(programA).toBe(programB);
+	});
+
+	it("does not reuse old programs when >90% of the heap is used", () => {
+		mockGetHeapStatistics.mockReturnValue({
+			heap_size_limit: 100,
+			used_heap_size: 91,
+		});
+
+		const programA = actGetProgramForVersion();
+		const programB = actGetProgramForVersion();
+
+		expect(programA).not.toBe(programB);
+	});
+});

src/utils/programs.ts

@@ -2,6 +2,7 @@ import type * as ts from "typescript";
 
 import fs from "node:fs";
 import path from "node:path";
+import v8 from "node:v8";
 
 export type TSModule = typeof ts;
 
@@ -46,19 +47,25 @@ function createProgram(configFile: string, ts: TSModule): ts.Program {
 export function getProgramForVersion(
 	configFile: string,
 	ts: TSModule,
-	versionName: string,
-	lintProgram: ts.Program,
+	version: string,
+	originalProgram: ts.Program,
 ): ts.Program {
-	let versionToProgram = programCache.get(lintProgram);
+	let versionToProgram = programCache.get(originalProgram);
 	if (versionToProgram === undefined) {
 		versionToProgram = new Map<string, ts.Program>();
-		programCache.set(lintProgram, versionToProgram);
+		programCache.set(originalProgram, versionToProgram);
 	}
 
-	let newProgram = versionToProgram.get(versionName);
+	let newProgram = versionToProgram.get(version);
 	if (newProgram === undefined) {
 		newProgram = createProgram(configFile, ts);
-		versionToProgram.set(versionName, newProgram);
+		versionToProgram.set(version, newProgram);
+	}
+
+	const heapStats = v8.getHeapStatistics();
+	const heapUsage = heapStats.used_heap_size / heapStats.heap_size_limit;
+	if (heapUsage > 0.9) {
+		versionToProgram.clear();
 	}
 
 	return newProgram;