/*
 * Teleport
 * Copyright (C) 2023  Gravitational, Inc.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

package utils

import (
	"context"
	"errors"
	"io"
	"net"
	"strings"
	"syscall"

	"github.com/gravitational/trace"

	"github.com/gravitational/teleport/api/constants"
)

// IsUseOfClosedNetworkError returns true if the specified error
// indicates the use of a closed network connection.
func IsUseOfClosedNetworkError(err error) bool {
	if err == nil {
		return false
	}

	return errors.Is(err, net.ErrClosed) || strings.Contains(err.Error(), constants.UseOfClosedNetworkConnection)
}

// IsFailedToSendCloseNotifyError returns true if the provided error is the
// "tls: failed to send closeNotify".
func IsFailedToSendCloseNotifyError(err error) bool {
	if err == nil {
		return false
	}
	return strings.Contains(err.Error(), constants.FailedToSendCloseNotify)
}

// IsOKNetworkError returns true if the provided error received from a network
// operation is one of those that usually indicate normal connection close. If
// the error is a trace.Aggregate, all the errors must be OK network errors.
func IsOKNetworkError(err error) bool {
	// trace.Aggregate contains at least one error and all the errors are
	// non-nil
	var a trace.Aggregate
	if errors.As(trace.Unwrap(err), &a) {
		for _, err := range a.Errors() {
			if !IsOKNetworkError(err) {
				return false
			}
		}
		return true
	}
	return errors.Is(err, io.EOF) || IsUseOfClosedNetworkError(err) || IsFailedToSendCloseNotifyError(err)
}

// IsConnectionRefused returns true if the given err is "connection refused" error.
func IsConnectionRefused(err error) bool {
	var errno syscall.Errno
	if errors.As(err, &errno) {
		return errors.Is(errno, syscall.ECONNREFUSED)
	}
	return false
}

// IsUntrustedCertErr checks if an error is an untrusted cert error.
func IsUntrustedCertErr(err error) bool {
	if err == nil {
		return false
	}
	errMsg := err.Error()
	return strings.Contains(errMsg, "x509: certificate is valid for") ||
		strings.Contains(errMsg, "certificate is not trusted")
}

// CanExplainNetworkError returns a simple to understand error message that can
// be used to debug common network and/or protocol errors.
func CanExplainNetworkError(err error) (string, bool) {
	var derr *net.DNSError

	switch {
	// Connection refused errors can be reproduced by attempting to connect to a
	// host:port that no process is listening on. The raw error typically looks
	// like the following:
	//
	// dial tcp 127.0.0.1:8000: connect: connection refused
	case errors.Is(err, syscall.ECONNREFUSED):
		return `Connection Refused

Teleport was unable to connect to the requested host, possibly because the server is not running. Ensure the server is running and listening on the correct port.

Use "nc -vz HOST PORT" to help debug this issue.`, true
	// Host unreachable errors can be reproduced by running
	// "ip route add unreachable HOST" to update the routing table to make
	// the host unreachable. Packets will be discarded and an ICMP message
	// will be returned. The raw error typically looks like the following:
	//
	// dial tcp 10.10.10.10:8000: connect: no route to host
	case errors.Is(err, syscall.EHOSTUNREACH):
		return `No Route to Host

Teleport could not connect to the requested host, likely because there is no valid network path to reach it. Check the network routing table to ensure a valid path to the host exists.

Use "ping HOST" and "ip route get HOST" to help debug this issue.`, true
	// Connection reset errors can be reproduced by creating a HTTP server that
	// accepts requests but closes the connection before writing a response. The
	// raw error typically looks like the following:
	//
	// read tcp 127.0.0.1:49764->127.0.0.1:8000: read: connection reset by peer
	case errors.Is(err, syscall.ECONNRESET):
		return `Connection Reset by Peer

Teleport could not complete the request because the server abruptly closed the connection before the response was received. To resolve this issue, ensure the server (or load balancer) does not have a timeout terminating the connection early and verify that the server is not crash looping.

Use protocol-specific tools (e.g., curl, psql) to help debug this issue.`, true
	// Slow responses can be reprodued by creating a HTTP server that does a
	// time.Sleep before responding. The raw error typically looks like the following:
	//
	// context deadline exceeded
	case errors.Is(err, context.DeadlineExceeded):
		return `Context Deadline Exceeded

Teleport did not receive a response within the timeout period, likely due to the system being overloaded, network congestion, or a firewall blocking traffic. To resolve this issue, connect to the host directly and ensure it is responding promptly.

Use protocol-specific tools (e.g., curl, psql) to assist in debugging this issue.`, true
	// No such host errors can be reproduced by attempting to resolve a invalid
	// domain name. The raw error typically looks like the following:
	//
	// dial tcp: lookup qweqweqwe.com: no such host
	case errors.As(err, &derr) && derr.IsNotFound:
		return `No Such Host

Teleport was unable to resolve the provided domain name, likely because the domain does not exist. To resolve this issue, verify the domain is correct and ensure the DNS resolver is properly resolving it.

Use "dig +short HOST" to help debug this issue.`, true
	}

	return "", false
}

const (
	// SelfSignedCertsMsg is a helper message to point users towards helpful documentation.
	SelfSignedCertsMsg = "Your proxy certificate is not trusted or expired. " +
		"Please update the certificate or follow this guide for self-signed certs: https://goteleport.com/docs/admin-guides/management/admin/self-signed-certs/"
)