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Agent Runs Out of File Descriptors — Too Many Open Connections

Symptom

• Agent crashes with OSError: [Errno 24] Too many open files
• Error appears after hours of running — not at startup
• lsof -p <pid> | wc -l shows 65,000+ open file descriptors before crash
• New HTTP connections fail: socket.error: [Errno 24]
• Docker container hits the default 1,048,576 FD limit in Kubernetes
• Temporary fix: restart the container — but it crashes again in a few hours

Root Cause

File descriptors (FDs) are consumed by: open files, network sockets, database connections, pipes, and subprocesses. Each connection or file opened without being closed leaks one FD. Common leaks: httpx.AsyncClient() created per-request without closing, SQLite connect() in a loop without close(), subprocesses with close_fds=False inheriting parent FDs, and log file handlers that accumulate. Linux defaults to 1,024 FDs per process (or 65,536 in containers). A slow leak hits this limit after hours.

Fix

Option 1: Always use context managers for HTTP clients

import httpx
import asyncio

# WRONG — creates a new client (and socket) per request, never closed
async def fetch_data_wrong(url: str) -> dict:
    client = httpx.AsyncClient()  # FD leak — client never closed
    response = await client.get(url)
    return response.json()

# WRONG — explicit but may leak on exception
async def fetch_data_still_wrong(url: str) -> dict:
    client = httpx.AsyncClient()
    response = await client.get(url)
    await client.aclose()  # Never reached if exception occurs
    return response.json()

# RIGHT — context manager guarantees cleanup on any exit
async def fetch_data_correct(url: str) -> dict:
    async with httpx.AsyncClient(timeout=30.0) as client:
        response = await client.get(url)
        return response.json()

# BEST — one long-lived client shared across all requests
# Created once, reused, closed on shutdown
_shared_client: httpx.AsyncClient | None = None

async def get_shared_client() -> httpx.AsyncClient:
    global _shared_client
    if _shared_client is None or _shared_client.is_closed:
        _shared_client = httpx.AsyncClient(
            timeout=httpx.Timeout(connect=5.0, read=60.0, write=10.0),
            limits=httpx.Limits(max_keepalive_connections=20, max_connections=50)
        )
    return _shared_client

async def fetch_with_shared_client(url: str) -> dict:
    client = await get_shared_client()
    response = await client.get(url)
    return response.json()

async def shutdown():
    global _shared_client
    if _shared_client and not _shared_client.is_closed:
        await _shared_client.aclose()

Option 2: Monitor and alert on FD usage

import os
import psutil
import asyncio
import logging

logger = logging.getLogger(__name__)

class FDMonitor:
    """
    Monitor file descriptor usage — alert before exhaustion.
    """

    def __init__(
        self,
        alert_threshold: float = 0.80,  # Alert at 80% of limit
        check_interval: float = 60.0    # Check every minute
    ):
        self.alert_threshold = alert_threshold
        self.check_interval = check_interval
        self._proc = psutil.Process(os.getpid())

    def get_fd_stats(self) -> dict:
        """Get current FD usage"""
        try:
            open_fds = self._proc.num_fds()
            # Get limit
            import resource
            soft_limit, hard_limit = resource.getrlimit(resource.RLIMIT_NOFILE)

            utilization = open_fds / soft_limit if soft_limit > 0 else 0

            return {
                "open_fds": open_fds,
                "soft_limit": soft_limit,
                "hard_limit": hard_limit,
                "utilization": utilization,
                "available": soft_limit - open_fds,
                "status": (
                    "critical" if utilization > 0.90 else
                    "warning" if utilization > self.alert_threshold else
                    "ok"
                )
            }
        except Exception as e:
            return {"error": str(e)}

    def get_fd_breakdown(self, top_n: int = 20) -> list[dict]:
        """Show what files/sockets are consuming FDs"""
        try:
            connections = []
            for conn in self._proc.connections():
                connections.append({
                    "type": conn.type.name if hasattr(conn, 'type') else "unknown",
                    "status": conn.status if hasattr(conn, 'status') else "",
                    "raddr": f"{conn.raddr.ip}:{conn.raddr.port}" if conn.raddr else ""
                })

            open_files = []
            for f in self._proc.open_files()[:top_n]:
                open_files.append({"path": f.path, "fd": f.fd})

            return {"connections": connections[:top_n], "files": open_files}
        except Exception as e:
            return [{"error": str(e)}]

    async def monitor_loop(self):
        """Background monitoring — logs warnings before exhaustion"""
        while True:
            await asyncio.sleep(self.check_interval)
            stats = self.get_fd_stats()

            if stats.get("status") == "critical":
                logger.critical(
                    "CRITICAL: File descriptor exhaustion imminent",
                    extra={
                        "open_fds": stats["open_fds"],
                        "soft_limit": stats["soft_limit"],
                        "utilization": f"{stats['utilization']*100:.0f}%"
                    }
                )
                breakdown = self.get_fd_breakdown()
                logger.critical(f"FD breakdown: {breakdown}")

            elif stats.get("status") == "warning":
                logger.warning(
                    f"FD usage at {stats['utilization']*100:.0f}% "
                    f"({stats['open_fds']}/{stats['soft_limit']})"
                )

fd_monitor = FDMonitor(alert_threshold=0.80)
asyncio.create_task(fd_monitor.monitor_loop())

Option 3: Increase FD limits in Docker and Kubernetes

# docker-compose.yml — increase FD limits for the agent container
services:
  agent:
    image: my-agent:latest
    ulimits:
      nofile:
        soft: 65536
        hard: 65536

# Kubernetes pod spec — increase FD limits
apiVersion: apps/v1
kind: Deployment
spec:
  template:
    spec:
      containers:
      - name: agent
        image: my-agent:latest
        securityContext:
          # Allow setting higher limits
          allowPrivilegeEscalation: false
        resources:
          limits:
            memory: "2Gi"
            cpu: "1"
        # Note: Kubernetes inherits host's fs.file-max
        # Set ulimits via container runtime or init container

# Dockerfile — set limits in entrypoint
# docker-entrypoint.sh:
#!/bin/bash
set -e

# Check and log current FD limits
echo "FD limits: $(ulimit -n)"

# Increase if running as root (development)
if [ "$(id -u)" = "0" ]; then
    ulimit -n 65536 2>/dev/null || true
fi

exec "$@"

# Python — set limits programmatically at startup
import resource
import os

def configure_fd_limits(target: int = 65536):
    """
    Attempt to raise file descriptor limit.
    Call at agent startup before any connections are made.
    """
    try:
        soft, hard = resource.getrlimit(resource.RLIMIT_NOFILE)
        new_soft = min(target, hard)

        if new_soft > soft:
            resource.setrlimit(resource.RLIMIT_NOFILE, (new_soft, hard))
            print(f"FD limit raised: {soft} → {new_soft} (hard limit: {hard})")
        else:
            print(f"FD limit: {soft} (already at or above target {target})")

    except (ValueError, resource.error) as e:
        print(f"Could not raise FD limit: {e}")
        print("Set ulimits in docker-compose.yml or Kubernetes securityContext")

configure_fd_limits(65536)

Option 4: Subprocess FD inheritance — prevent leaks to child processes

import subprocess
import os

def run_subprocess_safe(cmd: list[str], **kwargs) -> subprocess.CompletedProcess:
    """
    Run subprocess without leaking parent FDs.
    By default, Python subprocesses inherit all parent file descriptors.
    This can exhaust FDs if many subprocesses are spawned.
    """
    return subprocess.run(
        cmd,
        close_fds=True,        # Close all inherited FDs (Python 3 default on Unix)
        capture_output=True,
        text=True,
        timeout=60,
        **kwargs
    )

async def run_async_subprocess_safe(cmd: list[str]) -> tuple[str, str]:
    """
    Async subprocess that doesn't leak FDs.
    """
    proc = await asyncio.create_subprocess_exec(
        *cmd,
        stdout=asyncio.subprocess.PIPE,
        stderr=asyncio.subprocess.PIPE,
        close_fds=True   # Prevent FD inheritance
    )

    try:
        stdout, stderr = await asyncio.wait_for(proc.communicate(), timeout=60.0)
        return stdout.decode(), stderr.decode()
    except asyncio.TimeoutError:
        proc.kill()
        await proc.wait()
        raise
    # Process streams are closed automatically when proc goes out of scope

# Check subprocess FD inheritance:
def diagnose_subprocess_fds():
    """
    Check how many FDs a subprocess inherits.
    Run this in development to detect leaks.
    """
    import subprocess
    result = subprocess.run(
        ["bash", "-c", "ls /proc/$$/fd | wc -l"],
        capture_output=True, text=True, close_fds=True
    )
    child_fds = int(result.stdout.strip())

    current_fds = len(os.listdir(f"/proc/{os.getpid()}/fd"))

    print(f"Parent FDs: {current_fds}, Child (subprocess) FDs: {child_fds}")
    if child_fds > 20:
        print(f"WARNING: Subprocess inheriting {child_fds} FDs — check for close_fds=True")

Option 5: Database connection FD tracking

import sqlite3
import weakref
from contextlib import contextmanager

class TrackedSQLitePool:
    """
    SQLite connection pool with FD tracking.
    Alerts on unclosed connections — prevents FD leaks.
    """

    def __init__(self, db_path: str, pool_size: int = 5):
        self.db_path = db_path
        self._pool: list[sqlite3.Connection] = []
        self._in_use: set[int] = set()  # ids of connections in use
        self._total_created = 0
        self._total_leaked = 0

        # Pre-create pool
        for _ in range(pool_size):
            conn = sqlite3.connect(db_path, check_same_thread=False)
            conn.execute("PRAGMA journal_mode=WAL")
            self._pool.append(conn)
            self._total_created += 1

        # Leak detector via weakrefs
        self._weakrefs: list[weakref.ref] = []

    @contextmanager
    def acquire(self):
        """Acquire connection — always released via context manager"""
        if not self._pool:
            raise RuntimeError("Connection pool exhausted — increase pool_size or check for leaks")

        conn = self._pool.pop()
        conn_id = id(conn)
        self._in_use.add(conn_id)

        try:
            yield conn
            conn.commit()
        except Exception:
            conn.rollback()
            raise
        finally:
            self._in_use.discard(conn_id)
            self._pool.append(conn)  # Return to pool

    def close_all(self):
        """Close all connections — call at shutdown"""
        for conn in self._pool:
            conn.close()
        self._pool.clear()
        print(f"SQLite pool closed: {self._total_created} connections released")

    @property
    def stats(self) -> dict:
        return {
            "pool_size": len(self._pool),
            "in_use": len(self._in_use),
            "total_created": self._total_created,
        }

db_pool = TrackedSQLitePool("/data/agent.db", pool_size=5)

# Usage — connection always returned to pool:
with db_pool.acquire() as conn:
    rows = conn.execute("SELECT * FROM tasks WHERE status='pending'").fetchall()
# Connection automatically returned — no FD leak

Option 6: FD audit script — find leaks in development

import os
import psutil
from collections import Counter

def audit_file_descriptors() -> dict:
    """
    Audit open file descriptors — identify what's consuming them.
    Run in development to find leaks before they hit production.
    """
    proc = psutil.Process(os.getpid())

    # Categorize open FDs
    fd_types = Counter()
    socket_states = Counter()
    remote_addresses = Counter()
    file_extensions = Counter()

    for conn in proc.connections():
        fd_types["socket"] += 1
        socket_states[conn.status] += 1
        if conn.raddr:
            remote_addresses[f"{conn.raddr.ip}:{conn.raddr.port}"] += 1

    for f in proc.open_files():
        fd_types["file"] += 1
        ext = os.path.splitext(f.path)[1] or "no-ext"
        file_extensions[ext] += 1

    # Check for pipe FDs
    try:
        fd_dir = f"/proc/{os.getpid()}/fd"
        for fd in os.listdir(fd_dir):
            fd_path = os.readlink(f"{fd_dir}/{fd}")
            if "pipe" in fd_path:
                fd_types["pipe"] += 1
            elif "socket" not in fd_path and fd not in ["/dev/null", "/dev/urandom"]:
                pass
    except (PermissionError, NotADirectoryError):
        pass

    import resource
    soft_limit, _ = resource.getrlimit(resource.RLIMIT_NOFILE)

    return {
        "total_open": proc.num_fds(),
        "soft_limit": soft_limit,
        "utilization": f"{proc.num_fds()/soft_limit*100:.1f}%",
        "by_type": dict(fd_types),
        "socket_states": dict(socket_states.most_common(5)),
        "top_remote_addrs": dict(remote_addresses.most_common(10)),
        "file_types": dict(file_extensions.most_common(10)),
        "recommendation": (
            "Check CLOSE_WAIT sockets — connections not properly closed"
            if socket_states.get("CLOSE_WAIT", 0) > 10 else
            "Check pipe count — may indicate subprocess FD leaks"
            if fd_types.get("pipe", 0) > 50 else
            "FD usage looks healthy"
        )
    }

# Run periodically in development:
audit = audit_file_descriptors()
print(f"FD audit: {audit['total_open']}/{audit['soft_limit']} ({audit['utilization']})")
print(f"By type: {audit['by_type']}")
print(f"Recommendation: {audit['recommendation']}")

Common FD Leak Sources

Source	Leak Pattern	Fix
httpx.AsyncClient() per request	New socket per call	Share one client instance
sqlite3.connect() in loop	New connection, no close	Connection pool with context manager
open() without context manager	File handle not closed	Always use with open(...)
Subprocess without close_fds	Inherits all parent FDs	close_fds=True (default on Unix)
asyncio.Queue with abandoned tasks	Pipe FDs for cancelled tasks	Cancel tasks properly on shutdown
Log file handlers	New handler on each log init	Create handlers once at startup

FD Limits by Environment

Environment	Default FD Limit	Recommended
Linux default	1,024	Raise to 65,536
Docker container	1,048,576 (from host)	Set via ulimits in compose
Kubernetes pod	Inherits node limit	Set securityContext or init container
macOS	256 (soft), unlimited (hard)	ulimit -n 10240 in dev

Expected Token Savings

Agent crashes with EMFILE → restart → re-explain task → resume: ~15,000 tokens per crash Proper FD management → agent runs indefinitely: 0 crash-recovery overhead

Environment

• Any long-running agent deployed in production; critical for agents making many HTTP requests, database connections, or spawning subprocesses over their lifetime
• Source: direct experience; FD exhaustion is the most common resource leak in agents that pass 24-hour soak tests during QA but fail in production after days of operation

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