util: make spawndetached() handle subprocess early terminations
The file-based synchronization introduced by
e22695b4472f hangs when the child
process fails before terminating the handshake, which the previous pipe-based
version handled correctly. To fix this, the parent polling loop was fixed to
detect premature terminations of the child process.
% fail
abort: inotify-server: cannot start: .hg/inotify.sock is a broken symlink
inotify-client: could not start inotify server: child process failed to start
abort: inotify-server: cannot start: .hg/inotify.sock is a broken symlink
% inserve
% status
? hg.pid