The "%f" specifier in g_date_time_format() is only available in glib
2.65.2 or later. If combined with older glib, the function returns null
and the timestamp displayed as "(null)".

For backward compatibility, g_date_time_get_microsecond should be used
to retrieve subsecond.

In this patch the g_date_time_format() leaves subsecond field as "%06d"
and let next snprintf to format with g_date_time_get_microsecond.

Signed-off-by: Yusuke Okada <okada.yusuke@jp.fujitsu.com>
---
 tools/virtiofsd/passthrough_ll.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 371a7bead6..20f0f41f99 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -4185,6 +4185,7 @@ static void setup_nofile_rlimit(unsigned long rlimit_nofile)
 static void log_func(enum fuse_log_level level, const char *fmt, va_list ap)
 {
     g_autofree char *localfmt = NULL;
+    char buf[64];
 
     if (current_log_level < level) {
         return;
@@ -4197,9 +4198,11 @@ static void log_func(enum fuse_log_level level, const char *fmt, va_list ap)
                                        fmt);
         } else {
             g_autoptr(GDateTime) now = g_date_time_new_now_utc();
-            g_autofree char *nowstr = g_date_time_format(now, "%Y-%m-%d %H:%M:%S.%f%z");
+            g_autofree char *nowstr = g_date_time_format(now,
+                                       "%Y-%m-%d %H:%M:%S.%%06d%z");
+            snprintf(buf, 64, nowstr, g_date_time_get_microsecond(now));
             localfmt = g_strdup_printf("[%s] [ID: %08ld] %s",
-                                       nowstr, syscall(__NR_gettid), fmt);
+                                       buf, syscall(__NR_gettid), fmt);
         }
         fmt = localfmt;
     }
-- 
2.31.1

It works. I tested on RHEL8
Before this fix:
```
# /root/qemu/build/tools/virtiofsd/virtiofsd --socket-path=/tmp/sock -o 
source=/home/test -d

[(null)] [ID: 00133152] virtio_session_mount: Waiting for vhost-user 
socket connection...


```

After applying this patch
```
# /root/qemu/build/tools/virtiofsd/virtiofsd --socket-path=/tmp/sock -o 
source=/home/test -d

[2022-08-19 01:45:41.981608+0000] [ID: 00134587] virtio_session_mount: 
Waiting for vhost-user socket connection...

```	


On 8/19/22 01:46, Yusuke Okada wrote:
> The "%f" specifier in g_date_time_format() is only available in glib
> 2.65.2 or later. If combined with older glib, the function returns null
> and the timestamp displayed as "(null)".
> 
> For backward compatibility, g_date_time_get_microsecond should be used
> to retrieve subsecond.
> 
> In this patch the g_date_time_format() leaves subsecond field as "%06d"
> and let next snprintf to format with g_date_time_get_microsecond.
> 
> Signed-off-by: Yusuke Okada <okada.yusuke@jp.fujitsu.com>
> ---
>   tools/virtiofsd/passthrough_ll.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> index 371a7bead6..20f0f41f99 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -4185,6 +4185,7 @@ static void setup_nofile_rlimit(unsigned long rlimit_nofile)
>   static void log_func(enum fuse_log_level level, const char *fmt, va_list ap)
>   {
>       g_autofree char *localfmt = NULL;
> +    char buf[64];
>   
>       if (current_log_level < level) {
>           return;
> @@ -4197,9 +4198,11 @@ static void log_func(enum fuse_log_level level, const char *fmt, va_list ap)
>                                          fmt);
>           } else {
>               g_autoptr(GDateTime) now = g_date_time_new_now_utc();
> -            g_autofree char *nowstr = g_date_time_format(now, "%Y-%m-%d %H:%M:%S.%f%z");
> +            g_autofree char *nowstr = g_date_time_format(now,
> +                                       "%Y-%m-%d %H:%M:%S.%%06d%z");
> +            snprintf(buf, 64, nowstr, g_date_time_get_microsecond(now));
>               localfmt = g_strdup_printf("[%s] [ID: %08ld] %s",
> -                                       nowstr, syscall(__NR_gettid), fmt);
> +                                       buf, syscall(__NR_gettid), fmt);
>           }
>           fmt = localfmt;
>       }

-- 
Thanks,
Yiding