cmd/hakurei: expose current instance identifier

This writes the 16-byte instance identifier to file descriptor specified by --identifier-fd if set, and closes the file.

This enables safely obtaining the new instance's identifier.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/outcome/process.go

@@ -33,7 +33,7 @@ const (
 func NewStore(sc *hst.Paths) *store.Store { return store.New(sc.SharePath.Append("state")) }
 
 // main carries out outcome and terminates. main does not return.
-func (k *outcome) main(msg message.Msg) {
+func (k *outcome) main(msg message.Msg, identifierFd int) {
 	if k.ctx == nil || k.sys == nil || k.state == nil {
 		panic("outcome: did not finalise")
 	}
@@ -163,6 +163,21 @@ func (k *outcome) main(msg message.Msg) {
 				continue
 			}
 
+			if f := os.NewFile(uintptr(identifierFd), "identifier"); f != nil {
+				_, err = f.Write(k.state.id.v[:])
+				if err != nil {
+					unlock()
+					// transition here to avoid the commit/revert cycle on the doomed instance
+					perrorFatal(&hst.AppError{Step: "write instance identifier", Err: err},
+						"write instance identifier", processLifecycle)
+					continue
+				}
+				msg.Verbosef("wrote identifier to %d", identifierFd)
+				if err = f.Close(); err != nil {
+					msg.Verbose(err.Error())
+				}
+			}
+
 			err = k.sys.Commit()
 			unlock()
 			if err != nil {