fix(setup): retain bounded startup diagnostics without losing failures

src/process/ExecFileBoundedOutput.ts

@@ -1,4 +1,5 @@
 import { spawn } from "node:child_process";
+import { BoundedOutputExcerpt } from "./BoundedOutputExcerpt.js";
 
 export interface BoundedCommandOutput {
   readonly exitCode: string | number | null;
@@ -21,31 +22,10 @@ export const execFileBoundedOutput = (
       env: options.env,
       stdio: ["ignore", "pipe", "pipe"],
     });
-    const headLimit = Math.ceil(options.limit / 2);
-    const tailLimit = options.limit - headLimit;
-    const stdout = { head: Buffer.alloc(0), tail: Buffer.alloc(0), total: 0 };
-    const stderr = { head: Buffer.alloc(0), tail: Buffer.alloc(0), total: 0 };
-    let outputTruncated = false;
-    const capture =
-      (stream: typeof stdout) =>
-      (chunk: Buffer): void => {
-        stream.total += chunk.length;
-        const headCount = Math.min(
-          chunk.length,
-          Math.max(0, headLimit - stream.head.length),
-        );
-        if (headCount > 0)
-          stream.head = Buffer.concat([
-            stream.head,
-            chunk.subarray(0, headCount),
-          ]);
-        const rest = chunk.subarray(headCount);
-        if (rest.length > 0 && tailLimit > 0)
-          stream.tail = Buffer.concat([stream.tail, rest]).subarray(-tailLimit);
-        if (stream.total > options.limit) outputTruncated = true;
-      };
-    child.stdout.on("data", capture(stdout));
-    child.stderr.on("data", capture(stderr));
+    const stdout = new BoundedOutputExcerpt(options.limit);
+    const stderr = new BoundedOutputExcerpt(options.limit);
+    child.stdout.on("data", (chunk: Buffer) => stdout.append(chunk));
+    child.stderr.on("data", (chunk: Buffer) => stderr.append(chunk));
     let spawnError: NodeJS.ErrnoException | undefined;
     child.on("error", (error: NodeJS.ErrnoException) => {
       spawnError = error;
@@ -53,21 +33,9 @@ export const execFileBoundedOutput = (
     child.on("close", (code, signal) => {
       resolve({
         exitCode: spawnError?.code ?? code ?? signal,
-        stdout: retainedOutput(stdout, options.limit),
-        stderr: retainedOutput(stderr, options.limit),
-        outputTruncated,
+        stdout: stdout.text,
+        stderr: stderr.text,
+        outputTruncated: stdout.truncated || stderr.truncated,
       });
     });
   });
-
-const retainedOutput = (
-  stream: {
-    readonly head: Buffer;
-    readonly tail: Buffer;
-    readonly total: number;
-  },
-  limit: number,
-): string =>
-  stream.total > limit
-    ? `${stream.head.toString("utf8")} … [output omitted] … ${stream.tail.toString("utf8")}`
-    : Buffer.concat([stream.head, stream.tail]).toString("utf8");