Makefile

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+SHELL  := /bin/bash
+CC     := cc
+CFLAGS := -Wall -O2
+TARGET := fa2json
+SRCS   := fs-watcher.c json-writer.c
+
+.PHONY: all test dev clean
+
+all: $(TARGET)
+
+$(TARGET): $(SRCS)
+	$(CC) $(CFLAGS) -o $@ $^
+
+test: $(TARGET)
+	node test/test.mjs
+
+dev: $(TARGET)
+	node test/dev.mjs $(ARGS)
+
+clean:
+	rm -f $(TARGET)

test/dev.mjs

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+#!/usr/bin/env node
+// Developer mode: set up loop device, stream fa2json output through jq,
+// tear down on exit. Optionally launch a terminal at the mount point.
+//
+// Usage:
+//   sudo node test/dev.mjs
+//   sudo node test/dev.mjs --terminal konsole
+//   sudo node test/dev.mjs --terminal "konsole -e bash"
+
+import { spawn } from 'node:child_process';
+import { createInterface } from 'node:readline';
+import { setup, spawnFa2json } from './lib/setup.mjs';
+
+const terminalArg = (() => {
+	const i = process.argv.indexOf('--terminal');
+	return i !== -1 ? process.argv.slice(i + 1).join(' ') : null;
+})();
+
+const { mnt, teardown } = await setup();
+console.error(`Mount point: ${mnt}`);
+
+// Pipe fa2json stdout through jq for pretty coloured output
+const fa2json = spawnFa2json(mnt);
+const jq = spawn('jq', ['-C', '--unbuffered', '.'], { stdio: ['pipe', 'inherit', 'inherit'] });
+fa2json.stdout.pipe(jq.stdin);
+
+fa2json.on('exit', async () => {
+	jq.stdin.end();
+	await teardown();
+	process.exit(0);
+});
+
+// Launch optional terminal at mount point
+if (terminalArg) {
+	const [cmd, ...args] = terminalArg.split(' ');
+	spawn(cmd, [...args, '--workdir', mnt], { detached: true, stdio: 'ignore' }).unref();
+	console.error(`Launched: ${terminalArg} --workdir ${mnt}`);
+}
+
+// Clean teardown on Ctrl+C
+process.on('SIGINT', async () => {
+	fa2json.kill('SIGTERM');
+});

test/lib/setup.mjs

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+import { execSync, spawn } from 'node:child_process';
+import { mkdtempSync, mkdirSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+const FA2JSON = new URL('../../fa2json', import.meta.url).pathname;
+
+export async function setup() {
+	// Create image file and format
+	const img = execSync('mktemp /tmp/fa2json-test-XXXXXX.img').toString().trim();
+	execSync(`truncate -s 10M ${img}`);
+	execSync(`mkfs.ext4 -q ${img}`);
+
+	// Create mount point and mount
+	const mnt = execSync('mktemp -d /tmp/fa2json-mnt-XXXXXX').toString().trim();
+	execSync(`sudo mount ${img} ${mnt}`);
+
+	// Hand ownership to current user, then sync before fa2json starts
+	execSync(`sudo chown ${process.getuid()} ${mnt}`);
+	execSync('sync');
+
+	async function teardown() {
+		try { execSync(`sudo umount ${mnt}`); } catch {}
+		try { execSync(`rm -f ${img}`); } catch {}
+		try { execSync(`rmdir ${mnt}`); } catch {}
+	}
+
+	return { img, mnt, teardown };
+}
+
+export function spawnFa2json(mnt) {
+	const proc = spawn('sudo', [FA2JSON, mnt], { stdio: ['ignore', 'pipe', 'inherit'] });
+	return proc;
+}

test/test.mjs

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+#!/usr/bin/env node
+// Automated test runner. Exit 0 = pass, exit 1 = fail.
+// Requires root (sudo) for mount and fa2json.
+
+import { createInterface } from 'node:readline';
+import { promises as fs } from 'node:fs';
+import { join } from 'node:path';
+import { setup, spawnFa2json } from './lib/setup.mjs';
+
+// Fanotify mask flags
+const FAN_ATTRIB      = 0x4;
+const FAN_CLOSE_WRITE = 0x8;
+const FAN_CREATE      = 0x100;
+const FAN_DELETE      = 0x200;
+const FAN_RENAME      = 0x10000000;
+const FAN_ONDIR       = 0x40000000;
+
+const { mnt, teardown } = await setup();
+const fa2json = spawnFa2json(mnt);
+
+// Event buffer and marker machinery
+const events = [];
+let markerResolve = null;
+let markerName = null;
+let markerCounter = 0;
+
+const rl = createInterface({ input: fa2json.stdout });
+rl.on('line', line => {
+	const event = JSON.parse(line);
+	// Strip mount prefix from all path fields
+	for (const key of ['name', 'old', 'new']) {
+		if (event[key]) event[key] = event[key].slice(mnt.length);
+	}
+	events.push(event);
+	if (markerResolve && event.name === `/.marker_${markerName}` && (event.mask & FAN_CREATE)) {
+		markerResolve();
+		markerResolve = null;
+	}
+});
+
+// Perform a FS operation, drop a marker, collect events up to that marker
+async function doOp(label, fn) {
+	const id = markerCounter++;
+	await fn();
+	await fs.writeFile(join(mnt, `.marker_${id}`), '');
+	await new Promise(resolve => {
+		markerName = id;
+		markerResolve = resolve;
+	});
+	// Collect all events since previous marker (excluding marker events themselves)
+	const batch = events.splice(0).filter(e => !e.name?.startsWith('/.marker_'));
+	return { label, batch };
+}
+
+function assert(label, batch, check) {
+	if (!check(batch)) {
+		console.error(`FAIL: ${label}`);
+		console.error('Batch:', JSON.stringify(batch, null, 2));
+		throw new Error(`Assertion failed: ${label}`);
+	}
+}
+
+function hasEvent(batch, flags, path, field = 'name') {
+	return batch.some(e => (e.mask & flags) === flags && e[field] === path);
+}
+
+try {
+	let op;
+
+	op = await doOp('mkdir /dir_a', () => fs.mkdir(join(mnt, 'dir_a')));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_CREATE | FAN_ONDIR, '/dir_a'));
+
+	op = await doOp('touch /file_a.txt', () => fs.writeFile(join(mnt, 'file_a.txt'), ''));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_CREATE, '/file_a.txt'));
+
+	op = await doOp('write /file_a.txt', () => fs.writeFile(join(mnt, 'file_a.txt'), 'content'));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_CLOSE_WRITE, '/file_a.txt'));
+
+	op = await doOp('mkdir /dir_b', () => fs.mkdir(join(mnt, 'dir_b')));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_CREATE | FAN_ONDIR, '/dir_b'));
+
+	op = await doOp('touch /dir_b/nested.txt', () => fs.writeFile(join(mnt, 'dir_b', 'nested.txt'), ''));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_CREATE, '/dir_b/nested.txt'));
+
+	op = await doOp('mv /file_a.txt /file_b.txt', () => fs.rename(join(mnt, 'file_a.txt'), join(mnt, 'file_b.txt')));
+	assert(op.label, op.batch, b => b.some(e => (e.mask & FAN_RENAME) && e.old === '/file_a.txt' && e.new === '/file_b.txt'));
+
+	op = await doOp('mv /dir_b /dir_a/dir_b_moved', () => fs.rename(join(mnt, 'dir_b'), join(mnt, 'dir_a', 'dir_b_moved')));
+	assert(op.label, op.batch, b => b.some(e => (e.mask & FAN_RENAME) && (e.mask & FAN_ONDIR) && e.old === '/dir_b' && e.new === '/dir_a/dir_b_moved'));
+
+	op = await doOp('chmod 600 /file_b.txt', () => fs.chmod(join(mnt, 'file_b.txt'), 0o600));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_ATTRIB, '/file_b.txt'));
+
+	op = await doOp('touch -m /file_b.txt', () => fs.utimes(join(mnt, 'file_b.txt'), new Date(), new Date()));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_ATTRIB, '/file_b.txt'));
+
+	op = await doOp('chmod 755 /dir_a', () => fs.chmod(join(mnt, 'dir_a'), 0o755));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_ATTRIB | FAN_ONDIR, '/dir_a'));
+
+	op = await doOp('rm /file_b.txt', () => fs.unlink(join(mnt, 'file_b.txt')));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_DELETE, '/file_b.txt'));
+
+	op = await doOp('rm /dir_a/dir_b_moved/nested.txt', () => fs.unlink(join(mnt, 'dir_a', 'dir_b_moved', 'nested.txt')));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_DELETE, '/dir_a/dir_b_moved/nested.txt'));
+
+	op = await doOp('rmdir /dir_a/dir_b_moved', () => fs.rmdir(join(mnt, 'dir_a', 'dir_b_moved')));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_DELETE | FAN_ONDIR, '/dir_a/dir_b_moved'));
+
+	op = await doOp('rmdir /dir_a', () => fs.rmdir(join(mnt, 'dir_a')));
+	assert(op.label, op.batch, b => hasEvent(b, FAN_DELETE | FAN_ONDIR, '/dir_a'));
+
+	console.log('PASS');
+} catch (e) {
+	console.error(e.message);
+	process.exitCode = 1;
+} finally {
+	fa2json.kill();
+	await teardown();
+}