runtime/v2/runc: fix leaking socket path

When runC shimv2 starts, the StartShim interface will re-exec itself as
long-running process, which will read the `address` during initializing.

```happycase
Process

containerd-shim-runc-v1/v2 start             containerd-shim-runc-v1/v2

	initializing socket

	reexec containerd-shim-runc-v1/v2

	write address into file

						initializing

							read address

	write back to containerd daemon

						serving

						...

						remove address in Shutdown call
```

However, there is no synchronization after reexec. Then the data race is
like:

```leaking-case
Process

containerd-shim-runc-v1/v2 start             containerd-shim-runc-v1/v2

	initializing socket

	reexec containerd-shim-runc-v1/v2

						initializing

							read address

	write address into file

	write back to containerd daemon

						serving

						...

						fail to remove address
						because of empty address
```

The `address` should be writen into file first before reexec.

And if shutdown the whole service before cleanup temporary
resource (like socket file), the Shutdown caller will receive `ttrpc: closed`
sometime, which depends on go runtime scheduler. Then it also causes leaking
socket files.

Since the shimV2-Delete binary API must be called to cleanup shim temporary
resource and shimV2-runC-v1 doesn't support grouping multi containers in one,
it is safe to remove the socket file in the binary call for shimV2-runC-v1.
But for the shimV2-runC-v2 shim, we still cleanup socket in Shutdown.
Hopefully we can find a way to cleanup socket in shimV2-Delete binary
call.

Fix: #5173

Signed-off-by: Wei Fu <fuweid89@gmail.com>

runtime/v2/shim/shim.go

@@ -239,6 +239,13 @@ func run(id string, initFunc Init, config Config) error {
 				return err
 			}
 		}
+
+		// NOTE: If the shim server is down(like oom killer), the address
+		// socket might be leaking.
+		if address, err := ReadAddress("address"); err == nil {
+			_ = RemoveSocket(address)
+		}
+
 		select {
 		case <-publisher.Done():
 			return nil
@@ -299,15 +306,11 @@ func serve(ctx context.Context, server *ttrpc.Server, path string) error {
 		return err
 	}
 	go func() {
+		defer l.Close()
 		if err := server.Serve(ctx, l); err != nil &&
 			!strings.Contains(err.Error(), "use of closed network connection") {
 			logrus.WithError(err).Fatal("containerd-shim: ttrpc server failure")
 		}
-		l.Close()
-		if address, err := ReadAddress("address"); err == nil {
-			_ = RemoveSocket(address)
-		}
-
 	}()
 	return nil
 }