Debugging computerd against real FUSE

This skill captures the recipe for isolating bugs in computerd that only fire when the kernel-FUSE backend is the one wired up. Most unit tests run against SQLiteTestStorage and an in-process VFS or the userspace FUSE_MOUNT=shim polling driver — neither exercises the kernel-FUSE callback path. Several production-only bugs (write buffer never spilled to the VFS, spawn(cwd=/mount) deadlocks) have shipped to next and not been caught by the regular suites because the harness is hard to drive end-to-end from a dev box.

The workflow here bypasses workerd, wrangler, and the vitest-pool-workers harness. It boots the computerd SEA binary in a real docker container with /dev/fuse and SYS_ADMIN, and drives its capnweb endpoints from a plain Node script.

When to reach for this

• A user report of "wrote a file via FUSE, RPC reads see 0 bytes."
• A user report of "exec hangs forever" or "computerd /health stops responding after the first shell call."
• Adding a new computerd op (FUSE callback, sync RPC, runner feature) and you want a smoke test that doesn't depend on Docker-in-DO or Cloudflare Containers.
• The harness vitest suite (packages/computer/src/test-harness/) hangs at ws.shell.exec against the docker container and you need to attribute the wedge to computerd vs workerd vs the network shim.
• Confirming a fix actually settles in the deployed binary: the SEA build is what wrangler ships, and reproducing locally with the same binary is the only way to be sure.

What you need

• Docker, with /dev/fuse and --privileged available. In Pi's sandbox this works through DinD; on a Linux dev box it works natively; on macOS you need colima start --vm-type=vz (the default qemu vm doesn't surface /dev/fuse).
• A fresh computerd binary at artifacts/computerd/computerd-linux-x64. Build it with npm run build:bin --workspace @cloudflare/computerd, or just npm run build:all from the repo root if you need the docker image too.

Boot a real-FUSE computerd container

There's already a recipe — packages/computer/test-harness/run-computerd.sh. It picks a host port, runs the binary with --privileged --device /dev/fuse --cap-add SYS_ADMIN --cap-add MKNOD, installs fuse3 + libfuse2t64 from apt, waits for /health, prints the URL on stdout and the container id on stderr.

# Start
COMPUTERD_HARNESS_PORT=18080 bash packages/computer/test-harness/run-computerd.sh
# prints: http://127.0.0.1:18080      on stdout
# prints: COMPUTERD_HARNESS_CID=<id>        on stderr

# Verify
curl -sf http://127.0.0.1:18080/health
docker logs <CID> | tail
# should see: computerd listening on 0.0.0.0:8080 mount=/workspace backend=linux

# Tear down
docker kill <CID>

In a tmux session:

tmux new-session -d -s computerd \
  "COMPUTERD_HARNESS_PORT=18080 bash packages/computer/test-harness/run-computerd.sh; sleep 600"
for i in 1 2 3 4 5; do
  sleep 5
  if curl -sf http://127.0.0.1:18080/health 2>/dev/null; then echo ready; break; fi
done

backend=fuse is the success signal. If you see backend=shim the FUSE mount didn't take and you're testing the wrong path. backend=none means FUSE_MOUNT=none was set; FUSE_MOUNT=fuse with a missing /dev/fuse would have failed startup outright.

Drive computerd from a Node script

computerd serves a composite WorkspaceRPC over /ws (capnweb WebSocket) and /api (capnweb HTTP batch). The @cloudflare/computer-rpc/client package wraps the WS form and the /driver subpath exposes pushOnce/pullOnce against a Node-side Database.

Set up a probe project once:

mkdir -p /tmp/computerd-probe && cd /tmp/computerd-probe
cat > package.json <<'EOF'
{
  "type": "module",
  "private": true,
  "dependencies": {
    "@cloudflare/dofs": "file:/workspace/packages/dofs",
    "@cloudflare/computer-rpc": "file:/workspace/packages/rpc",
    "ws": "^8.18.0"
  }
}
EOF
npm install --ignore-scripts --no-audit --no-fund

Now you can write probes that hold both sides of the wire. The canonical shape, with the WebSocket impl pinned to the ws package (Node's built-in WebSocket doesn't negotiate the permessage-deflate extension that computerd advertises):

import { Database, WorkspaceFilesystem, initializeSchema } from "@cloudflare/dofs";
import { SQLiteTestStorage } from "@cloudflare/dofs/testing";
import { createWorkspaceClient } from "@cloudflare/computer-rpc/client";
import { pullOnce, pushOnce } from "@cloudflare/computer-rpc/driver";
import { WebSocket } from "ws";

const url = process.env.COMPUTERD_URL;                // e.g. http://127.0.0.1:18080
const wsUrl = `${url.replace(/^http(s?):\/\//, "ws$1://")}/ws`;

const storage = new SQLiteTestStorage();
const db = new Database(storage);
initializeSchema(db, () => Date.now());
const fs = new WorkspaceFilesystem(db, { now: () => Date.now() });

// Stage some local state. IMPORTANT: paths must be inside the
// container's MOUNT_POINT (default /workspace) or computerd's FUSE
// driver won't surface them on the host — computerd serves only the
// `/workspace/...` subtree of vfs_nodes through the kernel mount.
await fs.mkdir("/workspace", { recursive: true });
await fs.writeFile("/workspace/probe.txt", "hello from probe");

const client = createWorkspaceClient({ url: wsUrl, WebSocketImpl: WebSocket });
try {
  const pushed = await pushOnce(db, client.sync);
  console.log("pushed", pushed);
  const result = await pullOnce(db, client.sync);
  console.log("pulled", result);
} finally {
  await client.close();
}

Run with COMPUTERD_URL=http://127.0.0.1:18080 node probe.mjs.

To verify the push actually landed on the FUSE side:

docker exec <CID> sh -c 'ls -la /workspace && cat /workspace/probe.txt'

If ls /workspace is empty but the push reported pushed=N, the RPC apply path landed bytes in vfs_nodes but FUSE isn't surfacing them. Check that:

• Your absolute paths start with MOUNT_POINT (default /workspace). Writing to /probe.txt succeeds at the RPC layer but FUSE only serves the MOUNT_POINT subtree.
• computerd actually logs backend=linux (not backend=shim).
• You aren't reading the wrong container — docker ps after a zombie docker rm -f can show stale entries.

Drive container-side writes

The cleanest way to simulate a container-side FUSE write — the exact path computerd was designed to handle — is docker exec into the running container:

docker exec <CID> sh -c 'echo container-write > /workspace/r2/hello.txt'

This is a real FUSE write: kernel → fuse-native → computerd's writeBuf op → computerd's in-memory buffer for that file → on release/flush/fsync the buffer spills into the backing VFS (commit 68407fc). On the host side you can then pullOnce and see the new entry land in applyChanges.

Avoid client.shell.exec() while debugging FUSE bugs in this environment. It calls spawn with cwd=MOUNT_POINT by default, which can deadlock under real FUSE (see "Known deadlock patterns" below). If you need exec-shaped behaviour from outside the container, docker exec is closer to what wrangler's container runtime would do anyway — computerd isn't the parent of the spawned process.

Known deadlock patterns

FUSE buffer never spilled to the VFS

Symptom: container cat /workspace/foo returns the bytes you wrote, but a host-side pullOnce sees the file at size 0.

Mechanism: the FUSE driver in packages/computerd/src/fuse/driver.ts buffers writes in an in-memory files Map keyed by path. Reads through FUSE pull from that buffer, but the backing VFS only sees the empty inode the create op registered. If release / flush / fsync don't spill the buffer, RPC consumers see an empty file.

Fix shipped: commit 68407fc — flushEntry(path) runs on each of those ops.

Test for it: packages/computerd/src/fuse/driver.test.ts and the companion test at 8111d68. Mirror that shape if you find a new buffer-vs-VFS desync.

computerd's event loop blocked in spawn

Symptom: client.shell.exec against a real-FUSE computerd hangs forever. /health stops responding. docker exec <CID> ls /workspace also hangs once the deadlock fires.

Mechanism: Runner.exec calls spawn("/bin/sh", { cwd: "/workspace" }). libuv's uv_spawn forks and waits on a status pipe for the child to exec. The child, between fork and exec, does chdir("/workspace"). The kernel issues a FUSE LOOKUP against the computerd mount. The callback needs to run on computerd's event loop, which is blocked in the pipe read. Deadlock.

Diagnostic:

// against a fresh computerd
const t = Date.now();
await client.shell.exec({ command: "echo hi", cwd: "/tmp", timeoutMs: 5000 });
// returns in <100ms — control case
await client.shell.exec({ command: "echo hi", cwd: "/workspace", timeoutMs: 5000 });
// hangs forever — under real FUSE

Same computerd, same client, same network — only cwd differs. If that flips success→hang, you've reproduced the deadlock.

Fix: do the chdir inside the shell, not inside spawn's fork-and-exec dance. Prefix the command with cd ... && exec ... so the chdir runs after the shell is up and computerd's event loop is responsive again. Pre-flight existence via dofs's stat(db, path) (reads SQLite directly, no FUSE callbacks) to preserve the existing ENOENT-cwd error contract.

Other event-loop blockers to suspect

Any computerd code path that does sync work against MOUNT_POINT/the FUSE mount can wedge: fs.statSync(cwd), fs.readdirSync(MOUNT_POINT), fs.realpathSync(...) — all synchronous, all block the event loop, all issue FUSE callbacks to computerd itself. Rule of thumb when adding computerd code: anything that goes through Node's fs against the mount point must be async (so libuv's threadpool services the call) OR must read the dofs Database / VFS directly (pure SQL, no FUSE involvement).

Recipe library

Reset between probes

A wedged computerd container can leave fuse-native zombie processes that docker kill refuses to reap. The cleanest fix:

# Try kill first
docker kill <CID> 2>&1
# If it complains about a PID being a zombie, just abandon that
# port and pick a new one
COMPUTERD_HARNESS_PORT=18081 bash packages/computer/test-harness/run-computerd.sh

The zombie containers don't hold the host port once docker has nominally released them, but they linger in docker ps until the next daemon restart. Harmless for further work.

Stage a read-only mount fixture

Mirrors what the workspace indexer does. Useful when reproducing M3.5-class issues (read-only mount enforcement) without going through the full Workspace constructor:

import { Database, ROOT_INODE, WorkspaceFilesystem, initializeSchema, invalidateReadOnlyMountCache } from "@cloudflare/dofs";
import { SQLiteTestStorage } from "@cloudflare/dofs/testing";

const storage = new SQLiteTestStorage();
const db = new Database(storage);
initializeSchema(db, () => Date.now());
const fs = new WorkspaceFilesystem(db);

const root = "/workspace/r2";
// Indexer step 1: row at read-write so writeFile can land
db.run("INSERT INTO _vfs_mounts (root, kind, mode, indexed) VALUES (?, ?, 'read-write', 0)", root, "fixture");
invalidateReadOnlyMountCache(db);

// Indexer step 2: materialize
await fs.mkdir(root, { recursive: true });
await fs.writeFile(`${root}/hello.txt`, "hello world");

// Indexer step 3: stamp mount_root on every inode under the root.
// Path-to-inode resolver:
function resolveInode(db, absPath) {
  const parts = absPath.split("/").filter(Boolean);
  let inode = ROOT_INODE;
  for (const part of parts) {
    const row = db.one("SELECT child_inode FROM vfs_dirents WHERE parent_inode = ? AND name = ?", inode, part);
    if (!row) return undefined;
    inode = row.child_inode;
  }
  return inode;
}
const rootInode = resolveInode(db, root);
const queue = [rootInode];
const subtree = [rootInode];
while (queue.length) {
  const parent = queue.shift();
  for (const c of db.all("SELECT child_inode FROM vfs_dirents WHERE parent_inode = ?", parent)) {
    subtree.push(c.child_inode);
    queue.push(c.child_inode);
  }
}
for (const inode of subtree) {
  db.run("UPDATE vfs_nodes SET mount_root = ? WHERE inode = ?", root, inode);
}

// Indexer step 4: flip to read-only and invalidate the cache
db.run("UPDATE _vfs_mounts SET mode = 'read-only', indexed = 1 WHERE root = ?", root);
invalidateReadOnlyMountCache(db);

After staging this and pushOnce'ing to computerd, the container's FUSE view will surface /workspace/r2/hello.txt. Container-side writes to that subtree will propagate back via the next pullOnce, where applyChanges skips them with reason: "read-only".

Reference: a full M3.5-style probe

A working end-to-end probe that stages the mount, pushes, container-writes via docker exec, pulls, and asserts the expected applied/skipped split lives at script/computerd-mount-probe.mjs if it's still in the tree. Use it as the template for new probes.

What this skill is NOT

• Not a substitute for unit tests. Unit tests are still where fast feedback comes from; this is for the bugs only the kernel can reproduce.
• Not for benchmarking. Use script/run-fs-bench.sh for that.
• Not for testing the sync wire alone — packages/rpc/tests/wire.test.ts already covers WS round-trips with no FUSE in the loop.

When to escalate

If you've isolated a wedge to computerd's event loop and the fix isn't obvious from the patterns above, the next step is node --inspect-brk against the computerd binary. The SEA bundle preserves source maps for the bundled JS, so breakpoints in packages/computerd/src/exec/runner.ts resolve. Run computerd outside docker (it'll fall back to backend=shim or backend=none depending on your host) so the inspector port is reachable, or docker run --publish 9229:9229 ... and pass --inspect-brk=0.0.0.0:9229 through computerd's launch env.

If even that doesn't surface it, the wedge is probably below the JS layer — in fuse-native or libuv. At that point reach for strace -fp <computerd-pid> inside the container; the FUSE callbacks all surface as read(/dev/fuse, ...) and the per-call timing will show you exactly which op deadlocks.