From bb92e3ada9ddbf315a50f2ec4b8bdafb05df7d3d Mon Sep 17 00:00:00 2001
From: Ophestra <cat@gensokyo.uk>
Date: Mon, 10 Nov 2025 07:51:00 +0900
Subject: 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/main.go    |  8 ++++----
 internal/outcome/process.go | 17 ++++++++++++++++-
 2 files changed, 20 insertions(+), 5 deletions(-)

(limited to 'internal')

diff --git a/internal/outcome/main.go b/internal/outcome/main.go
index feffb661..a32d220a 100644
--- a/internal/outcome/main.go
+++ b/internal/outcome/main.go
@@ -10,21 +10,21 @@ import (
 )
 
 // Main runs an app according to [hst.Config] and terminates. Main does not return.
-func Main(ctx context.Context, msg message.Msg, config *hst.Config) {
+func Main(ctx context.Context, msg message.Msg, config *hst.Config, fd int) {
 	var id hst.ID
 	if err := hst.NewInstanceID(&id); err != nil {
 		log.Fatal(err.Error())
 	}
 
-	seal := outcome{syscallDispatcher: direct{msg}}
+	k := outcome{syscallDispatcher: direct{msg}}
 
 	finaliseTime := time.Now()
-	if err := seal.finalise(ctx, msg, &id, config); err != nil {
+	if err := k.finalise(ctx, msg, &id, config); err != nil {
 		printMessageError(msg.GetLogger().Fatalln, "cannot seal app:", err)
 		panic("unreachable")
 	}
 	msg.Verbosef("finalise took %.2f ms", float64(time.Since(finaliseTime).Nanoseconds())/1e6)
 
-	seal.main(msg)
+	k.main(msg, fd)
 	panic("unreachable")
 }
diff --git a/internal/outcome/process.go b/internal/outcome/process.go
index 78570d26..3aa4d0e2 100644
--- a/internal/outcome/process.go
+++ b/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 {
-- 
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