From d28af58cbbf955f2270babb5c7101b19412919ff Mon Sep 17 00:00:00 2001
From: Arian van Putten <arian.vanputten@gmail.com>
Date: Mon, 14 Apr 2025 17:38:56 +0200
Subject: [PATCH] agent: implement sd-notify reload correctly (#25636)
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First of all, we should not send the unix time, but the monotonic time.
Second of all, RELOADING= and MONOTONIC_USEC fields should be sent in
*single* message not two separate messages.

From the man page of [systemd.service](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Type=)

> notification message via sd_notify(3) that contains the "RELOADING=1" field in
> combination with "MONOTONIC_USEC=" set to the current monotonic time (i.e.
> CLOCK_MONOTONIC in clock_gettime(2)) in μs, formatted as decimal string.

[sd_notify](https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html)
now has code samples of the protocol to clarify.

Without these changes, if you'd set
Type=notify-reload on the agen'ts systemd unit, systemd
would kill the service due to the service not responding to reload
correctly.
---
 .changelog/25636.txt              |  3 +++
 command/agent/command.go          | 13 +------------
 command/agent/sdnotify.go         | 12 ++++++++++++
 command/agent/sdnotify_default.go |  2 ++
 command/agent/sdnotify_linux.go   |  9 +++++++++
 5 files changed, 27 insertions(+), 12 deletions(-)
 create mode 100644 .changelog/25636.txt
 create mode 100644 command/agent/sdnotify.go

diff --git a/.changelog/25636.txt b/.changelog/25636.txt
new file mode 100644
index 000000000..fa01e0516
--- /dev/null
+++ b/.changelog/25636.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+agent: Fixed a bug where reloading the agent with systemd notification enabled would cause the agent to be killed by system
+```
diff --git a/command/agent/command.go b/command/agent/command.go
index 4d72029b0..77054b324 100644
--- a/command/agent/command.go
+++ b/command/agent/command.go
@@ -978,16 +978,6 @@ func (c *Command) handleRetryJoin(config *Config) error {
 	return nil
 }
 
-// These constants are for readiness signalling via the systemd notify protocol.
-// The functions we send these messages to are no-op on non-Linux systems. See
-// also https://www.man7.org/linux/man-pages/man3/sd_notify.3.html
-const (
-	sdReady     = "READY=1"
-	sdReloading = "RELOADING=1"
-	sdStopping  = "STOPPING=1"
-	sdMonotonic = "MONOTONIC_USEC=%d"
-)
-
 // handleSignals blocks until we get an exit-causing signal
 func (c *Command) handleSignals() int {
 	signalCh := make(chan os.Signal, 4)
@@ -1026,8 +1016,7 @@ WAIT:
 
 	// Check if this is a SIGHUP
 	if sig == syscall.SIGHUP {
-		sdNotify(sdSock, sdReloading)
-		sdNotify(sdSock, fmt.Sprintf(sdMonotonic, time.Now().UnixMicro()))
+		sdNotifyReloading(sdSock)
 		c.handleReload()
 		sdNotify(sdSock, sdReady)
 		goto WAIT
diff --git a/command/agent/sdnotify.go b/command/agent/sdnotify.go
new file mode 100644
index 000000000..b977139f6
--- /dev/null
+++ b/command/agent/sdnotify.go
@@ -0,0 +1,12 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: BUSL-1.1
+
+package agent
+
+// These constants are for readiness signalling via the systemd notify protocol.
+// The functions we send these messages to are no-op on non-Linux systems. See
+// also https://www.man7.org/linux/man-pages/man3/sd_notify.3.html
+const (
+	sdReady    = "READY=1"
+	sdStopping = "STOPPING=1"
+)
diff --git a/command/agent/sdnotify_default.go b/command/agent/sdnotify_default.go
index 7ce1f03ee..eb394c750 100644
--- a/command/agent/sdnotify_default.go
+++ b/command/agent/sdnotify_default.go
@@ -14,3 +14,5 @@ func openNotify() (io.WriteCloser, error) {
 }
 
 func sdNotify(_ io.Writer, _ string) {}
+
+func sdNotifyReloading(_ io.Writer) {}
diff --git a/command/agent/sdnotify_linux.go b/command/agent/sdnotify_linux.go
index 707ec15f0..e46c158d7 100644
--- a/command/agent/sdnotify_linux.go
+++ b/command/agent/sdnotify_linux.go
@@ -6,10 +6,13 @@
 package agent
 
 import (
+	"fmt"
 	"io"
 	"net"
 	"os"
 	"time"
+
+	"golang.org/x/sys/unix"
 )
 
 const sdNotifySocketEnvVar = "NOTIFY_SOCKET"
@@ -38,3 +41,9 @@ func sdNotify(w io.Writer, msg string) {
 	}
 	w.Write([]byte(msg))
 }
+
+func sdNotifyReloading(w io.Writer) {
+	var ts unix.Timespec
+	unix.ClockGettime(unix.CLOCK_MONOTONIC, &ts)
+	sdNotify(w, fmt.Sprintf("RELOADING=1\nMONOTONIC_USEC=%d", ts.Nano()/1000))
+}