virtiofsd: make the debug log timestamp on stderr more human-readable

The current timestamp format doesn't help me visually notice small jumps
in time ("small" as defined on human scale, such as a few seconds or a few
ten seconds). Replace it with a local time format where such differences
stand out.

Before:

> [13316826770337] [ID: 00000004] unique: 62, opcode: RELEASEDIR (29), nodeid: 1, insize: 64, pid: 1
> [13316826778175] [ID: 00000004]    unique: 62, success, outsize: 16
> [13316826781156] [ID: 00000004] virtio_send_msg: elem 0: with 1 in desc of length 16
> [15138279317927] [ID: 00000001] virtio_loop: Got VU event
> [15138279504884] [ID: 00000001] fv_queue_set_started: qidx=1 started=0
> [15138279519034] [ID: 00000003] fv_queue_thread: kill event on queue 1 - quitting
> [15138280876463] [ID: 00000001] fv_remove_watch: TODO! fd=9
> [15138280897381] [ID: 00000001] virtio_loop: Waiting for VU event
> [15138280946834] [ID: 00000001] virtio_loop: Got VU event
> [15138281175421] [ID: 00000001] virtio_loop: Waiting for VU event
> [15138281182387] [ID: 00000001] virtio_loop: Got VU event
> [15138281189474] [ID: 00000001] virtio_loop: Waiting for VU event
> [15138309321936] [ID: 00000001] virtio_loop: Unexpected poll revents 11
> [15138309434150] [ID: 00000001] virtio_loop: Exit

(Notice how you don't (easily) notice the gap in time after
"virtio_send_msg", and especially the amount of time passed is hard to
estimate.)

After:

> [2020-12-08 06:43:22.58+0100] [ID: 00000004] unique: 51, opcode: RELEASEDIR (29), nodeid: 1, insize: 64, pid: 1
> [2020-12-08 06:43:22.58+0100] [ID: 00000004]    unique: 51, success, outsize: 16
> [2020-12-08 06:43:22.58+0100] [ID: 00000004] virtio_send_msg: elem 0: with 1 in desc of length 16
> [2020-12-08 06:43:29.34+0100] [ID: 00000001] virtio_loop: Got VU event
> [2020-12-08 06:43:29.34+0100] [ID: 00000001] fv_queue_set_started: qidx=1 started=0
> [2020-12-08 06:43:29.34+0100] [ID: 00000003] fv_queue_thread: kill event on queue 1 - quitting
> [2020-12-08 06:43:29.34+0100] [ID: 00000001] fv_remove_watch: TODO! fd=9
> [2020-12-08 06:43:29.34+0100] [ID: 00000001] virtio_loop: Waiting for VU event
> [2020-12-08 06:43:29.34+0100] [ID: 00000001] virtio_loop: Got VU event
> [2020-12-08 06:43:29.34+0100] [ID: 00000001] virtio_loop: Waiting for VU event
> [2020-12-08 06:43:29.34+0100] [ID: 00000001] virtio_loop: Got VU event
> [2020-12-08 06:43:29.34+0100] [ID: 00000001] virtio_loop: Waiting for VU event
> [2020-12-08 06:43:29.37+0100] [ID: 00000001] virtio_loop: Unexpected poll revents 11
> [2020-12-08 06:43:29.37+0100] [ID: 00000001] virtio_loop: Exit

Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201208055043.31548-1-lersek@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/passthrough_ll.c

@@ -3284,18 +3284,38 @@ 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;
+    struct timespec ts;
+    struct tm tm;
+    char sec_fmt[sizeof "2020-12-07 18:17:54"];
+    char zone_fmt[sizeof "+0100"];
 
     if (current_log_level < level) {
         return;
     }
 
     if (current_log_level == FUSE_LOG_DEBUG) {
-        if (!use_syslog) {
-            localfmt = g_strdup_printf("[%" PRId64 "] [ID: %08ld] %s",
-                                       get_clock(), syscall(__NR_gettid), fmt);
-        } else {
+        if (use_syslog) {
+            /* no timestamp needed */
             localfmt = g_strdup_printf("[ID: %08ld] %s", syscall(__NR_gettid),
                                        fmt);
+        } else {
+            /* try formatting a broken-down timestamp */
+            if (clock_gettime(CLOCK_REALTIME, &ts) != -1 &&
+                localtime_r(&ts.tv_sec, &tm) != NULL &&
+                strftime(sec_fmt, sizeof sec_fmt, "%Y-%m-%d %H:%M:%S",
+                         &tm) != 0 &&
+                strftime(zone_fmt, sizeof zone_fmt, "%z", &tm) != 0) {
+                localfmt = g_strdup_printf("[%s.%02ld%s] [ID: %08ld] %s",
+                                           sec_fmt,
+                                           ts.tv_nsec / (10L * 1000 * 1000),
+                                           zone_fmt, syscall(__NR_gettid),
+                                           fmt);
+            } else {
+                /* fall back to a flat timestamp */
+                localfmt = g_strdup_printf("[%" PRId64 "] [ID: %08ld] %s",
+                                           get_clock(), syscall(__NR_gettid),
+                                           fmt);
+            }
         }
         fmt = localfmt;
     }
@@ -3416,6 +3436,9 @@ int main(int argc, char *argv[])
     struct lo_map_elem *reserve_elem;
     int ret = -1;
 
+    /* Initialize time conversion information for localtime_r(). */
+    tzset();
+
     /* Don't mask creation mode, kernel already did that */
     umask(0);