forkAndMountat forks a process to chdir then mount layers. Signals are
blocked (using runtime_beforeFork) during fork.
There is a race condition that the child process finishes before the
parent process is scheduled and can unblock signal handling. The SIGCHLD
signal sent from the finished process may have been delivered to the
shim process's reaper thread and caused the parent process fail with
ECHLD error.
This patch sets up a pipe for communication between child and parent
instead of waiting for child exit status.
Fixes#4009.
Signed-off-by: Haitao Li <hli@atlassian.com>
35c14c6b56