Allow waiting for LinkEndpoint worker goroutines to finish.

Previously, the only safe way to use an fdbased endpoint was to leak the FD.
This change makes it possible to safely close the FD.

This is the first step towards having stoppable stacks.

Updates #837

PiperOrigin-RevId: 270346582

1 files changed, 18 insertions, 2 deletions

diff --git a/pkg/tcpip/link/fdbased/endpoint.go b/pkg/tcpip/link/fdbased/endpoint.go
index adcf21371..584db710e 100644
--- a/pkg/tcpip/link/fdbased/endpoint.go
+++ b/pkg/tcpip/link/fdbased/endpoint.go
@@ -41,6 +41,7 @@ package fdbased
 
 import (
 	"fmt"
+	"sync"
 	"syscall"
 
 	"golang.org/x/sys/unix"
@@ -81,6 +82,7 @@ const (
 	PacketMMap
 )
 
+// An endpoint implements the link-layer using a message-oriented file descriptor.
 type endpoint struct {
 	// fds is the set of file descriptors each identifying one inbound/outbound
 	// channel. The endpoint will dispatch from all inbound channels as well as
@@ -114,6 +116,9 @@ type endpoint struct {
 	// gsoMaxSize is the maximum GSO packet size. It is zero if GSO is
 	// disabled.
 	gsoMaxSize uint32
+
+	// wg keeps track of running goroutines.
+	wg sync.WaitGroup
 }
 
 // Options specify the details about the fd-based endpoint to be created.
@@ -164,7 +169,8 @@ type Options struct {
 // New creates a new fd-based endpoint.
 //
 // Makes fd non-blocking, but does not take ownership of fd, which must remain
-// open for the lifetime of the returned endpoint.
+// open for the lifetime of the returned endpoint (until after the endpoint has
+// stopped being using and Wait returns).
 func New(opts *Options) (stack.LinkEndpoint, error) {
 	caps := stack.LinkEndpointCapabilities(0)
 	if opts.RXChecksumOffload {
@@ -290,7 +296,11 @@ func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
 	// saved, they stop sending outgoing packets and all incoming packets
 	// are rejected.
 	for i := range e.inboundDispatchers {
-		go e.dispatchLoop(e.inboundDispatchers[i]) // S/R-SAFE: See above.
+		e.wg.Add(1)
+		go func(i int) { // S/R-SAFE: See above.
+			e.dispatchLoop(e.inboundDispatchers[i])
+			e.wg.Done()
+		}(i)
 	}
 }
 
@@ -320,6 +330,12 @@ func (e *endpoint) LinkAddress() tcpip.LinkAddress {
 	return e.addr
 }
 
+// Wait implements stack.LinkEndpoint.Wait. It waits for the endpoint to stop
+// reading from its FD.
+func (e *endpoint) Wait() {
+	e.wg.Wait()
+}
+
 // virtioNetHdr is declared in linux/virtio_net.h.
 type virtioNetHdr struct {
 	flags      uint8