runc run/start/exec: fix init log forwarding race

Sometimes debug.bats test cases are failing like this:

> not ok 27 global --debug to --log --log-format 'json'
> # (in test file tests/integration/debug.bats, line 77)
> #   `[[ "${output}" == *"child process in init()"* ]]' failed

It happens more when writing to disk.

This issue is caused by the fact that runc spawns log forwarding goroutine
(ForwardLogs) but does not wait for it to finish, resulting in missing
debug lines from nsexec.

ForwardLogs itself, though, never finishes, because it reads from a
reading side of a pipe which writing side is not closed. This is
especially true in case of runc create, which spawns runc init and
exits; meanwhile runc init waits on exec fifo for arbitrarily long
time before doing execve.

So, to fix the failure described above, we need to:

 1. Make runc create/run/exec wait for ForwardLogs to finish;

 2. Make runc init close its log pipe file descriptor (i.e.
    the one which value is passed in _LIBCONTAINER_LOGPIPE
    environment variable).

This is exactly what this commit does:

 1. Amend ForwardLogs to return a channel, and wait for it in start().

 2. In runc init, save the log fd and close it as late as possible.

PS I have to admit I still do not understand why an explicit close of
log pipe fd is required in e.g. (*linuxSetnsInit).Init, right before
the execve which (thanks to CLOEXEC) closes the fd anyway.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/container_linux_test.go

@@ -139,7 +139,8 @@ func (m *mockProcess) externalDescriptors() []string {
 func (m *mockProcess) setExternalDescriptors(newFds []string) {
 }
 
-func (m *mockProcess) forwardChildLogs() {
+func (m *mockProcess) forwardChildLogs() chan error {
+	return nil
 }
 
 func TestGetContainerPids(t *testing.T) {