From c809c046be509226d0cad44a47ae78f6a428d500 Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Thu, 17 Feb 2022 18:47:26 +0900
Subject: [PATCH] udev-util: introduce udev_available() helper function

(cherry picked from commit f92c5bb18cee41d48b95d61a2a2dc613203844ca)
---
 src/shared/udev-util.c | 15 +++++++++++++++
 src/shared/udev-util.h |  2 ++
 2 files changed, 17 insertions(+)

diff --git a/src/shared/udev-util.c b/src/shared/udev-util.c
index 56c28773ce..69c3255967 100644
--- a/src/shared/udev-util.c
+++ b/src/shared/udev-util.c
@@ -19,6 +19,7 @@
 #include "path-util.h"
 #include "signal-util.h"
 #include "socket-util.h"
+#include "stat-util.h"
 #include "string-table.h"
 #include "string-util.h"
 #include "strxcpyx.h"
@@ -718,3 +719,17 @@ int on_ac_power(void) {
 
         return found_online || !found_offline;
 }
+
+bool udev_available(void) {
+        static int cache = -1;
+
+        /* The service systemd-udevd is started only when /sys is read write.
+         * See systemd-udevd.service: ConditionPathIsReadWrite=/sys
+         * Also, our container interface (http://systemd.io/CONTAINER_INTERFACE/) states that /sys must
+         * be mounted in read-only mode in containers. */
+
+        if (cache >= 0)
+                return cache;
+
+        return (cache = path_is_read_only_fs("/sys/") <= 0);
+}
diff --git a/src/shared/udev-util.h b/src/shared/udev-util.h
index 8d21dc4364..a48beb95ea 100644
--- a/src/shared/udev-util.h
+++ b/src/shared/udev-util.h
@@ -55,6 +55,8 @@ int udev_queue_init(void);
 
 int on_ac_power(void);
 
+bool udev_available(void);
+
 #if HAVE_SYS_SDT_H
 
 /* Each trace point can have different number of additional arguments. Note that when the macro is used only
-- 
2.25.1