fix(compiler): cap DuckDB threads so a cgroup-capped instance doesn't OOM at boot

packages/compiler/src/writers.ts

@@ -32,7 +32,14 @@ const DUCKDB_TEMP_DIRECTORY = join(tmpdir(), "openislands-duckdb");
  * /app owned by root in the Docker image: "IO Error: Failed to create directory .tmp").
  */
 export function createInMemoryDuckDB(): Promise<InstanceType<typeof DuckDBInstance>> {
-  return DuckDBInstance.create(":memory:", { temp_directory: DUCKDB_TEMP_DIRECTORY });
+  return DuckDBInstance.create(":memory:", {
+    temp_directory: DUCKDB_TEMP_DIRECTORY,
+    // ponytail: DuckDB caps memory_limit to the cgroup (~80%) but scales threads to HOST cores,
+    // ignoring the CPU cgroup — 12 threads on an 819 MiB cap OOM at boot ("failed to pin block")
+    // at ~68 MiB/thread. 4 keeps >125 MiB/thread (DuckDB's floor) and clamps to core count on
+    // smaller boxes. Raise only for a big-data deployment that also has the RAM for it.
+    threads: "4",
+  });
 }
 
 /** Where a dataset's rows physically live: its content source, plus the table name when that source is a SQLite database. */

packages/compiler/test/writers.test.ts

@@ -3,7 +3,7 @@ import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { describe, expect, it } from "vitest";
 import { LocalContentStore } from "@openislands/storage";
-import { resolveWriter } from "../src/writers.js";
+import { createInMemoryDuckDB, resolveWriter } from "../src/writers.js";
 
 function tempDir(): string {
   const dir = mkdtempSync(join(tmpdir(), "oi-writers-"));
@@ -44,3 +44,23 @@ describe("appendLines EOL preservation", () => {
     expect(content).not.toContain("\r");
   });
 });
+
+describe("createInMemoryDuckDB resource bounds", () => {
+  // Guards the container OOM fix: an unbounded thread count over a cgroup-capped memory_limit
+  // fails to pin buffers at boot, and a default temp_directory spills to a non-writable cwd.
+  it("caps threads and sets a writable temp_directory", async () => {
+    const instance = await createInMemoryDuckDB();
+    const conn = await instance.connect();
+    try {
+      const reader = await conn.runAndReadAll(
+        "SELECT current_setting('threads') AS threads, current_setting('temp_directory') AS temp_directory",
+      );
+      const [row] = reader.getRowObjects();
+      expect(Number(row.threads)).toBe(4);
+      expect(String(row.temp_directory)).toContain("openislands-duckdb");
+    } finally {
+      conn.closeSync();
+      instance.closeSync();
+    }
+  });
+});