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tailscale/portlist/poller.go

186 lines
4.7 KiB
Go

// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This file contains the code related to the Poller type and its methods.
// The hot loop to keep efficient is Poller.Run.
package portlist
import (
"context"
"errors"
"runtime"
"sync"
"time"
"golang.org/x/exp/slices"
"tailscale.com/envknob"
)
var pollInterval = 5 * time.Second // default; changed by some OS-specific init funcs
var debugDisablePortlist = envknob.RegisterBool("TS_DEBUG_DISABLE_PORTLIST")
// Poller scans the systems for listening ports periodically and sends
// the results to C.
type Poller struct {
c chan List // unbuffered
// os, if non-nil, is an OS-specific implementation of the portlist getting
// code. When non-nil, it's responsible for getting the complete list of
// cached ports complete with the process name. That is, when set,
// addProcesses is not used.
// A nil values means we don't have code for getting the list on the current
// operating system.
os osImpl
osOnce sync.Once // guards init of os
// closeCtx is the context that's canceled on Close.
closeCtx context.Context
closeCtxCancel context.CancelFunc
runDone chan struct{} // closed when Run completes
// scatch is memory for Poller.getList to reuse between calls.
scratch []Port
prev List // most recent data, not aliasing scratch
}
// osImpl is the OS-specific implementation of getting the open listening ports.
type osImpl interface {
Close() error
// AppendListeningPorts appends to base (which must have length 0 but
// optional capacity) the list of listening ports. The Port struct should be
// populated as completely as possible. Another pass will not add anything
// to it.
//
// The appended ports should be in a sorted (or at least stable) order so
// the caller can cheaply detect when there are no changes.
AppendListeningPorts(base []Port) ([]Port, error)
}
// newOSImpl, if non-nil, constructs a new osImpl.
var newOSImpl func() osImpl
var errUnimplemented = errors.New("portlist poller not implemented on " + runtime.GOOS)
// NewPoller returns a new portlist Poller. It returns an error
// if the portlist couldn't be obtained.
func NewPoller() (*Poller, error) {
if debugDisablePortlist() {
return nil, errors.New("portlist disabled by envknob")
}
p := &Poller{
c: make(chan List),
runDone: make(chan struct{}),
}
p.closeCtx, p.closeCtxCancel = context.WithCancel(context.Background())
p.osOnce.Do(p.initOSField)
if p.os == nil {
return nil, errUnimplemented
}
// Do one initial poll synchronously so we can return an error
// early.
if pl, err := p.getList(); err != nil {
return nil, err
} else {
p.setPrev(pl)
}
return p, nil
}
func (p *Poller) setPrev(pl List) {
// Make a copy, as the pass in pl slice aliases pl.scratch and we don't want
// that to except to the caller.
p.prev = slices.Clone(pl)
}
func (p *Poller) initOSField() {
if newOSImpl != nil {
p.os = newOSImpl()
}
}
// Updates return the channel that receives port list updates.
//
// The channel is closed when the Poller is closed.
func (p *Poller) Updates() <-chan List { return p.c }
// Close closes the Poller.
// Run will return with a nil error.
func (p *Poller) Close() error {
p.closeCtxCancel()
<-p.runDone
if p.os != nil {
p.os.Close()
}
return nil
}
// send sends pl to p.c and returns whether it was successfully sent.
func (p *Poller) send(ctx context.Context, pl List) (sent bool, err error) {
select {
case p.c <- pl:
return true, nil
case <-ctx.Done():
return false, ctx.Err()
case <-p.closeCtx.Done():
return false, nil
}
}
// Run runs the Poller periodically until either the context
// is done, or the Close is called.
//
// Run may only be called once.
func (p *Poller) Run(ctx context.Context) error {
tick := time.NewTicker(pollInterval)
defer tick.Stop()
return p.runWithTickChan(ctx, tick.C)
}
func (p *Poller) runWithTickChan(ctx context.Context, tickChan <-chan time.Time) error {
defer close(p.runDone)
defer close(p.c)
// Send out the pre-generated initial value.
if sent, err := p.send(ctx, p.prev); !sent {
return err
}
for {
select {
case <-tickChan:
pl, err := p.getList()
if err != nil {
return err
}
if pl.equal(p.prev) {
continue
}
p.setPrev(pl)
if sent, err := p.send(ctx, p.prev); !sent {
return err
}
case <-ctx.Done():
return ctx.Err()
case <-p.closeCtx.Done():
return nil
}
}
}
func (p *Poller) getList() (List, error) {
if debugDisablePortlist() {
return nil, nil
}
p.osOnce.Do(p.initOSField)
var err error
p.scratch, err = p.os.AppendListeningPorts(p.scratch[:0])
return p.scratch, err
}