Biggest change is no longer using standard library's tls.Config.getCertificate function to get a certificate during TLS handshake. Implemented our own cache which can be changed dynamically at runtime, even during TLS handshakes. As such, restarts are no longer required after certificate renewals or OCSP updates.
We also allow loading multiple certificates and keys per host, even by specifying a directory (tls got a new 'load' command for that).
Renamed the letsencrypt package to https in a gradual effort to become more generic; and https is more fitting for what the package does now.
There are still some known bugs, e.g. reloading where a new certificate is required but port 80 isn't currently listening, will cause the challenge to fail. There's still plenty of cleanup to do and tests to write. It is especially confusing right now how we enable "on-demand" TLS during setup and keep track of that. But this change should basically work so far.
Added a -grace flag to customize graceful shutdown period, fixed bugs related to closing file descriptors (and dup'ed fds), improved healthcheck signaling to parent, fixed a race condition with the graceful listener, etc. These improvements mainly provide better support for frequent reloading or unusual use cases of Start and Stop after a Restart (POSIX systems). This forum thread was valuable help in debugging: https://forum.golangbridge.org/t/bind-address-already-in-use-even-after-listener-closed/1510?u=matt
Fixed pidfile writing problem where a pidfile would be written even if child failed, also cleaned up restarts a bit and fixed a few bugs, it's more robust now in case of failures and with logging.
Whether the original parent process or a child process as part of a restart, the pidfile will not be written/changed until that process has started successfully. It is written every time caddy.Start() succeeds (may be reundant, but that's probably okay).
The error channel used when starting all the servers must be buffered so that, even if there are no errors at startup, the returns that insert into the error channel will not be blocked, since after startup, nobody is reading that channel anymore.
Fixed bug where manually specifying port 443 disabled TLS (whoops); otherHostHasScheme was the culprit, since it would return true even if it was the same config that had that scheme.
Also, an error at startup (if not a restart) is now fatal, rather than keeping a half-alive zombie server.
The file path of the originally-loaded Caddyfile must be piped to the forked process; previously it was using stdin after the first fork, which wouldn't load the newest Caddyfile from disk, which is the point of SIGUSR1.
OCSP status is checked at a regular interval, and if the OCSP status changes for any of the certificates, the change callback is executed (restarts the server, updating the OCSP staple).
