[PATCH RFC] qga: fence guest-set-time if hwclock not available

[Cornelia Huck <cohuck@redhat.com>]

The Posix implementation of guest-set-time invokes hwclock to
set/retrieve the time to/from the hardware clock. If hwclock
is not available, the user is currently informed that "hwclock
failed to set hardware clock to system time", which is quite
misleading. This may happen e.g. on s390x, which has a different
timekeeping concept anyway.

Let's check for the availability of the hwclock command and
return QERR_UNSUPPORTED for guest-set-time if it is not available.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
---

Not sure if that is the correct approach, but the current error
message is really quite confusing.

Gave it a quick test with an s390x and an x86_64 guest; invoking
'virsh domtime <value>' now fails with 'not currently supported'
on s390x and continues to work as before on x86_64.

---
 qga/commands-posix.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 1c1a165daed8..bd298a38b716 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -149,6 +149,13 @@ int64_t qmp_guest_get_time(Error **errp)
    return tq.tv_sec * 1000000000LL + tq.tv_usec * 1000;
 }
 
+static int check_hwclock_available(const char *path)
+{
+    struct stat st;
+
+    return (stat(path, &st) < 0) ? 0 : 1;
+}
+
 void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
 {
     int ret;
@@ -156,6 +163,17 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
     pid_t pid;
     Error *local_err = NULL;
     struct timeval tv;
+    const char *hwclock_path = "/sbin/hwclock";
+    static int hwclock_available = -1;
+
+    if (hwclock_available < 0) {
+        hwclock_available = check_hwclock_available(hwclock_path);
+    }
+
+    if (!hwclock_available) {
+        error_setg(errp, QERR_UNSUPPORTED);
+        return;
+    }
 
     /* If user has passed a time, validate and set it. */
     if (has_time) {
@@ -195,7 +213,7 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
 
         /* Use '/sbin/hwclock -w' to set RTC from the system time,
          * or '/sbin/hwclock -s' to set the system time from RTC. */
-        execle("/sbin/hwclock", "hwclock", has_time ? "-w" : "-s",
+        execle(hwclock_path, "hwclock", has_time ? "-w" : "-s",
                NULL, environ);
         _exit(EXIT_FAILURE);
     } else if (pid < 0) {
-- 
2.21.0