scmutil: support background closing for write() Upcoming patches will add background file closer support to working copy update. This patch adds some plumbing to prepare for that.

chg: hold a lock file before connected to server This is a part of the one server per config series. In multiple-server setup, multiple clients may try to start different servers (on demand) at the same time. The old lock will not guarantee a client to connect to the server it just started, and is not crash friendly. This patch addressed above issues by using flock and does not release the lock until the client actually connects to the server or times out.

serve: allow --daemon-postexec to be 'unlink:path' or 'none' This patch changes the format of value of --daemon-postexec. Now it can be either to unlink a file, or a no-op. The following patch will make chg to use the no-op option.

serve: rename --daemon-pipefds to --daemon-postexec (BC) Initially we use --daemon-pipefds to pass file descriptors for synchronization. Later, in order to support Windows, --daemon-pipefds is changed to accept a file path to unlink instead. The name is outdated since then. chg client is designed to use flock, which will be held before starting a server and until the client actually connects to the server it started. The unlink synchronization approach is not so helpful in this case. To address the issues, this patch renames pipefds to postexec and the following patch will allow the value of --daemon-postexec to be things like 'unlink:/path/to/file' or 'none'.