Avoid backtracking in npm warning filter

src/main.ts

@@ -1253,10 +1253,28 @@ function insetLines(text: string): string {
     return text.trimEnd().replace(/(^|\n)/g, "$1> ");
 }
 
-function reduceSpew(message: string): string {
+export function reduceSpew(message: string): string {
     // These are uninteresting in general and actually problematic when there are
     // thousands of instances of ENOSPC (which also appears as an error anyway)
-    return message.replace(/npm WARN.*\n/g, "");
+    const chunks: string[] = [];
+    let position = 0;
+    while (true) {
+        const warningStart = message.indexOf("npm WARN", position);
+        if (warningStart < 0) {
+            chunks.push(message.slice(position));
+            break;
+        }
+
+        const lineEnd = message.indexOf("\n", warningStart);
+        if (lineEnd < 0) {
+            chunks.push(message.slice(position));
+            break;
+        }
+
+        chunks.push(message.slice(position, warningStart));
+        position = lineEnd + 1;
+    }
+    return chunks.join("");
 }
 
 function makeMarkdownLink(url: string) {

test/main.test.ts

@@ -1,4 +1,4 @@
-import { getTscRepoResult, detectTypeScriptImplementation, detectTypeScriptNpmImplementation, downloadTsRepoAsync, mainAsync } from '../src/main.js'
+import { getTscRepoResult, detectTypeScriptImplementation, detectTypeScriptNpmImplementation, downloadTsRepoAsync, mainAsync, reduceSpew } from '../src/main.js'
 import * as path from "node:path"
 import { createCopyingOverlayFS } from '../src/utils/overlayFS.js'
 import type { SpawnResult } from '../src/utils/execUtils.js';
@@ -116,6 +116,13 @@ describe("main", () => {
         })).toBe("corsa");
     });
 
+    it("removes npm warnings in linear time", () => {
+        expect(reduceSpew("before npm WARN ignored\nnpm WARN also ignored\nafter")).toBe("before after");
+
+        const unterminatedWarnings = "npm WARN".repeat(10_000);
+        expect(reduceSpew(unterminatedWarnings)).toBe(unterminatedWarnings);
+    });
+
     it.skip("build-only correctly caches", async () => {
         const { status, summary } = await getTscRepoResult(
             {