pkg/instance: more robust instance testing

Strictly saying, we may not get the connection when
the fuzzer process exits. The accepting goroutine
may have not been scheduled yet.
For the connection for up to 10 seconds.

pkg/instance/instance.go

@@ -12,7 +12,6 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"sync/atomic"
 	"time"
 
 	"github.com/google/syzkaller/pkg/csource"
@@ -215,13 +214,13 @@ func (inst *inst) testInstance() error {
 		return fmt.Errorf("failed to open listening socket: %v", err)
 	}
 	defer ln.Close()
-	var gotConn uint32
+	acceptErr := make(chan error, 1)
 	go func() {
 		conn, err := ln.Accept()
 		if err == nil {
 			conn.Close()
-			atomic.StoreUint32(&gotConn, 1)
 		}
+		acceptErr <- err
 	}()
 	fwdAddr, err := inst.vm.Forward(ln.Addr().(*net.TCPAddr).Port)
 	if err != nil {
@@ -252,10 +251,12 @@ func (inst *inst) testInstance() error {
 			Report: rep,
 		}
 	}
-	if atomic.LoadUint32(&gotConn) == 0 {
+	select {
+	case err := <-acceptErr:
+		return err
+	case <-time.After(10 * time.Second):
 		return fmt.Errorf("test machine failed to connect to host")
 	}
-	return nil
 }
 
 func (inst *inst) testRepro() error {