fix(orchestrate): demux Docker logs in a loop, not by recursion

package.json

@@ -1,6 +1,6 @@
 {
   "name": "perfkit",
-  "version": "0.1.9",
+  "version": "0.1.10",
   "description": "Benchmark harness for Fonoster projects: DigitalOcean runs, one report.json schema, one branded report page",
   "license": "MIT",
   "author": "Pedro Sanders <psanders@fonoster.com>",

src/orchestrate/createDockerEngine.ts

@@ -60,15 +60,17 @@ export function createDockerEngine(socketPath: string): EngineRequest {
 export function demuxLogs(buffer: Buffer): { stdout: string; stderr: string } {
   const out: Buffer[] = [];
   const err: Buffer[] = [];
-  const read = (offset: number): void => {
-    if (offset + 8 > buffer.length) return;
+  // A loop, not recursion: a long command output arrives as tens of
+  // thousands of frames, past the call stack's depth.
+  let offset = 0;
+  while (offset + 8 <= buffer.length) {
     const stream = buffer[offset];
     const size = buffer.readUInt32BE(offset + 4);
-    const frame = buffer.subarray(offset + 8, offset + 8 + size);
-    (stream === 2 ? err : out).push(frame);
-    read(offset + 8 + size);
-  };
-  read(0);
+    (stream === 2 ? err : out).push(
+      buffer.subarray(offset + 8, offset + 8 + size)
+    );
+    offset += 8 + size;
+  }
   return {
     stdout: Buffer.concat(out).toString("utf8"),
     stderr: Buffer.concat(err).toString("utf8")

test/orchestrate/helpers.unit.test.ts

@@ -99,6 +99,24 @@ describe("orchestration helpers", () => {
     ).to.deep.equal({ stdout: "out put", stderr: "err" });
   });
 
+  it("should demux a long log of small frames without overflowing the stack", () => {
+    // A 6-hour sampler file read over SSH arrives as tens of thousands of
+    // small frames; a recursive reader overflowed the stack on it.
+    const frame = (stream: number, text: string) => {
+      const body = Buffer.from(text);
+      const header = Buffer.alloc(8);
+      header[0] = stream;
+      header.writeUInt32BE(body.length, 4);
+      return Buffer.concat([header, body]);
+    };
+    const lines = Array.from({ length: 200_000 }, (_, i) => `${i}\n`);
+    const { stdout, stderr } = demuxLogs(
+      Buffer.concat(lines.map((line) => frame(1, line)))
+    );
+    expect(stdout).to.equal(lines.join(""));
+    expect(stderr).to.equal("");
+  });
+
   it("should read docker info and refuse what it can't read", () => {
     expect(
       parseDockerInfo("8;17179869184;Ubuntu 24.04.1 LTS;27.3.1\n")