Add a monitor_fuzz.py script

This script runs and monitors fuzz_opt.py for up to a given number of iterations, redirecting its output to a rotating log file and printing its progress to stdout once a minute. When it detects that the fuzzer has found a bug, it prints the iteration number and the seed so the bug can be reproduced.

This script is nicer to run in agent harnesses than raw fuzz_opt.py because agents can easily run the fuzzer for X iterations and can show the progress without overly polluting the context.

Author: tlively

.gitignore

@@ -62,4 +62,7 @@ CMakeUserPresets.json
 # files related to clangd cache
 .cache/*
 
+# Generated by scripts/monitor_fuzz.py
+/fuzz.log
+
 .venv/

scripts/monitor_fuzz.py

@@ -0,0 +1,312 @@
+#!/usr/bin/env python3
+
+# Copyright 2026 WebAssembly Community Group participants
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Run and monitor the Binaryen fuzzer (fuzz_opt.py).
+
+Monitors progress, manages log file truncation, stops at iteration limits,
+and reports bugs found.
+"""
+
+import argparse
+import collections
+import os
+import re
+import signal
+import subprocess
+import sys
+import threading
+import time
+
+try:
+    import resource
+except ImportError:
+    resource = None
+
+
+def set_stack_limit():
+    """Avoid stack overflows in interpreter by setting stack limit to unlimited."""
+    if resource is None:
+        return
+    try:
+        resource.setrlimit(
+            resource.RLIMIT_STACK,
+            (resource.RLIM_INFINITY, resource.RLIM_INFINITY),
+        )
+    except Exception:
+        try:
+            _, hard = resource.getrlimit(resource.RLIMIT_STACK)
+            resource.setrlimit(resource.RLIMIT_STACK, (hard, hard))
+        except Exception:
+            pass
+
+
+class FuzzMonitor:
+    """Monitors fuzzer output stream, manages log files, and tracks state."""
+
+    def __init__(self, log_path, max_lines, keep_lines, truncate_interval):
+        self.log_path = log_path
+        self.max_lines = max_lines
+        self.keep_lines = keep_lines
+        self.truncate_interval = truncate_interval
+
+        self.lock = threading.Lock()
+        self.latest_iteration = 0
+        self.latest_seed = 'unknown'
+        self.bug_found = False
+        self.recent_lines = collections.deque(maxlen=20)
+
+        self.deque = collections.deque(maxlen=keep_lines)
+        self.lines_written = 0
+
+        if os.path.isfile(log_path):
+            try:
+                with open(log_path, encoding='utf-8', errors='replace') as f:
+                    for line in f:
+                        self.deque.append(line)
+                        self.recent_lines.append(line)
+                        self.lines_written += 1
+                        self._parse_line(line)
+            except Exception:
+                pass
+
+    def _parse_line(self, line):
+        iter_match = re.search(r'ITERATION:\s*(\d+)', line)
+        if iter_match:
+            self.latest_iteration = int(iter_match.group(1))
+
+        seed_match = re.search(r'seed:\s*(\d+)', line)
+        if seed_match:
+            self.latest_seed = seed_match.group(1)
+
+        if re.search(r'You found a bug', line, re.IGNORECASE):
+            self.bug_found = True
+
+    def run(self, stdout_stream):
+        last_truncate = time.time()
+        try:
+            with open(self.log_path, 'a', encoding='utf-8') as f:
+                for line in stdout_stream:
+                    with self.lock:
+                        self._parse_line(line)
+                        self.deque.append(line)
+                        self.recent_lines.append(line)
+                        self.lines_written += 1
+
+                    f.write(line)
+                    f.flush()
+
+                    now = time.time()
+                    if (
+                        self.lines_written >= self.max_lines
+                        and (now - last_truncate) >= self.truncate_interval
+                    ):
+                        f.close()
+                        with open(self.log_path, 'w', encoding='utf-8') as wf:
+                            with self.lock:
+                                wf.writelines(self.deque)
+                                self.lines_written = len(self.deque)
+                        f = open(self.log_path, 'a', encoding='utf-8')
+                        last_truncate = now
+        except Exception as e:
+            print(f'Error writing to log file: {e}', file=sys.stderr)
+
+    def get_progress(self):
+        with self.lock:
+            return self.latest_iteration
+
+    def get_status(self):
+        with self.lock:
+            return (
+                self.bug_found,
+                self.latest_iteration,
+                self.latest_seed,
+                list(self.recent_lines),
+            )
+
+
+def parse_args():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        '--log-dir',
+        default=os.environ.get('LOG_DIR', '.'),
+        help='Directory to save fuzz.log (default: current directory or $LOG_DIR)',
+    )
+    parser.add_argument(
+        '--max-iters',
+        type=int,
+        default=int(os.environ.get('MAX_ITERS', '0')),
+        help='Stop after N iterations (0 for infinite, default: $MAX_ITERS or 0)',
+    )
+    parser.add_argument(
+        '--truncate-interval',
+        type=float,
+        default=30.0,
+        help='Seconds between log truncation checks (default: 30)',
+    )
+    parser.add_argument(
+        '--max-lines',
+        type=int,
+        default=10000,
+        help='Maximum lines in log before truncation (default: 10000)',
+    )
+    parser.add_argument(
+        '--keep-lines',
+        type=int,
+        default=5000,
+        help='Lines to keep when truncating (default: 5000)',
+    )
+    parser.add_argument(
+        'cmd',
+        nargs=argparse.REMAINDER,
+        help='Fuzzer command to run (default: ./scripts/fuzz_opt.py)',
+    )
+    return parser.parse_args()
+
+
+def main():
+    args = parse_args()
+
+    cmd = list(args.cmd)
+    if cmd and cmd[0] == '--':
+        cmd.pop(0)
+    if not cmd:
+        default_fuzzer = os.path.join(
+            os.path.dirname(os.path.abspath(__file__)), 'fuzz_opt.py',
+        )
+        cmd = [sys.executable, default_fuzzer]
+
+    os.makedirs(args.log_dir, exist_ok=True)
+    log_file_path = os.path.join(args.log_dir, 'fuzz.log')
+
+    set_stack_limit()
+
+    monitor = FuzzMonitor(
+        log_path=log_file_path,
+        max_lines=args.max_lines,
+        keep_lines=args.keep_lines,
+        truncate_interval=args.truncate_interval,
+    )
+
+    env = os.environ.copy()
+    env['PYTHONUNBUFFERED'] = '1'
+
+    proc = subprocess.Popen(
+        cmd,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+        text=True,
+        bufsize=1,
+        env=env,
+        errors='replace',
+        start_new_session=True,
+    )
+
+    print(f'Fuzzer started with PID {proc.pid}. Monitoring...', flush=True)
+
+    reader_thread = threading.Thread(
+        target=monitor.run,
+        args=(proc.stdout,),
+        daemon=True,
+    )
+    reader_thread.start()
+
+    def stop_child():
+        if proc.poll() is None:
+            try:
+                os.killpg(proc.pid, signal.SIGTERM)
+            except ProcessLookupError:
+                pass
+            try:
+                proc.wait(timeout=5)
+            except subprocess.TimeoutExpired:
+                try:
+                    os.killpg(proc.pid, signal.SIGKILL)
+                except ProcessLookupError:
+                    pass
+                proc.wait()
+
+    def signal_handler(signum, _frame):
+        stop_child()
+        reader_thread.join(timeout=2.0)
+        sys.exit(128 + signum)
+
+    signal.signal(signal.SIGINT, signal_handler)
+    signal.signal(signal.SIGTERM, signal_handler)
+
+    start_time = time.time()
+    last_report = 0
+    limit_reached = False
+
+    try:
+        while reader_thread.is_alive() or proc.poll() is None:
+            reader_thread.join(timeout=1.0)
+            now = time.time()
+            elapsed = int(now - start_time)
+
+            minute = elapsed // 60
+            latest_iter = monitor.get_progress()
+
+            if minute > last_report:
+                last_report = minute
+                timestamp = time.strftime('%H:%M:%S')
+                print(
+                    f'[{timestamp}] Runtime: {last_report} min, Latest'
+                    f' Iteration: {latest_iter}',
+                    flush=True,
+                )
+
+            if args.max_iters > 0 and latest_iter >= args.max_iters:
+                print(
+                    f'Reached max iterations ({args.max_iters}). Stopping'
+                    ' fuzzer...',
+                    flush=True,
+                )
+                limit_reached = True
+                stop_child()
+                break
+    finally:
+        stop_child()
+        reader_thread.join(timeout=5.0)
+
+    exit_code = proc.returncode
+
+    if limit_reached:
+        print(
+            f'SUCCESS: Reached max iterations ({args.max_iters}) without finding'
+            ' a bug.',
+        )
+        return 0
+
+    bug_found, iteration, seed, recent_lines = monitor.get_status()
+
+    if bug_found:
+        print('SUCCESS: Bug found!')
+        print(f'Iteration: {iteration}')
+        print(f'Seed: {seed}')
+        print(f'Exit code: {exit_code}')
+        return 0
+
+    print('FAILURE: Fuzzer stopped unexpectedly without finding a bug.')
+    print(f'Exit code: {exit_code}')
+    if recent_lines:
+        print('Last 20 lines of log:')
+        for line in recent_lines:
+            print(line.rstrip('\r\n'))
+    return 1
+
+
+if __name__ == '__main__':
+    sys.exit(main())