safely handle existing net namespace in default network manager

When a client restarts, the network_hook's prerun will call
`CreateNetwork`. Drivers that don't implement their own network manager will
fall back to the default network manager, which doesn't handle the case where
the network namespace is being recreated safely. This results in an error and
the task being restarted for `exec` tasks with `network` blocks (this also
impacts the community `containerd` and probably other community task drivers).

If we get an error when attempting to create the namespace and that error is
because the file already exists and is locked by its process, then we'll
return a `nil` error with the `created` flag set to false, just as we do with
the `docker` driver.

client/allocrunner/network_manager_linux.go

@@ -2,7 +2,10 @@ package allocrunner
 
 import (
 	"fmt"
+	"os"
+	"path"
 	"strings"
+	"syscall"
 
 	hclog "github.com/hashicorp/go-hclog"
 	clientconfig "github.com/hashicorp/nomad/client/config"
@@ -92,6 +95,15 @@ type defaultNetworkManager struct{}
 func (*defaultNetworkManager) CreateNetwork(allocID string) (*drivers.NetworkIsolationSpec, bool, error) {
 	netns, err := nsutil.NewNS(allocID)
 	if err != nil {
+		// when a client restarts, the namespace will already exist and
+		// there will be a namespace file in use by the task process
+		if e, ok := err.(*os.PathError); ok && e.Err == syscall.EPERM {
+			nsPath := path.Join(nsutil.NetNSRunDir, allocID)
+			_, err := os.Stat(nsPath)
+			if err == nil {
+				return nil, false, nil
+			}
+		}
 		return nil, false, err
 	}