command: prevent early exit from graceful shutdown (#26023)

While waiting for the agent to leave during a graceful shutdown
the wait can be interrupted immediately if another signal is
received. It is common that while waiting a `SIGPIPE` is received
from journald causing the wait to end early. This results in the
agent not finishing the leave process and reporting an error when
the process has stopped. Instead of allowing any signal to interrupt
the wait, the signal is checked for a `SIGPIPE` and if matched will
continue waiting.

.changelog/26023.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+agent: Fixed bug where agent would exit early from graceful shutdown when managed by systemd
+```

command/agent/command.go

@@ -1007,15 +1007,27 @@ func (c *Command) terminateGracefully(signalCh chan os.Signal, sdSock io.Writer)
 
 	delay := time.NewTimer(timeout)
 
-	// Wait for leave or another signal
+	// Wait for leave or another signal to be received
+	for {
 		select {
-	case <-signalCh:
+		case sig := <-signalCh:
+			// If a SIGPIPE is received, ignore it and
+			// continue waiting
+			if sig == syscall.SIGPIPE {
+				c.agent.logger.Trace("caught SIGPIPE during graceful shutdown, ignoring")
+				continue
+			}
+			c.agent.logger.Trace("caught signal during graceful shutdown", "signal", sig)
+
 			return 1
 		case <-delay.C:
 			return 1
 		case <-gracefulCh:
 		}
 
+		break
+	}
+
 	return 0
 }