trace2: clarify UTC datetime formatting

Update tr2_tbuf_utc_datetime to generate extended UTC format.
Update tr2_tgt_event target to use extended format in 'time' columns.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-trace2.txt

@@ -78,11 +78,11 @@ git version 2.20.1.155.g426c96fcdb
 +
 ------------
 $ cat ~/log.event
-{"event":"version","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.620713","file":"common-main.c","line":38,"evt":"1","exe":"2.20.1.155.g426c96fcdb"}
+{"event":"version","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16T17:28:42.620713Z","file":"common-main.c","line":38,"evt":"1","exe":"2.20.1.155.g426c96fcdb"}
-{"event":"start","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621027","file":"common-main.c","line":39,"t_abs":0.001173,"argv":["git","version"]}
+{"event":"start","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16T17:28:42.621027Z","file":"common-main.c","line":39,"t_abs":0.001173,"argv":["git","version"]}
-{"event":"cmd_name","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621122","file":"git.c","line":432,"name":"version","hierarchy":"version"}
+{"event":"cmd_name","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16T17:28:42.621122Z","file":"git.c","line":432,"name":"version","hierarchy":"version"}
-{"event":"exit","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621236","file":"git.c","line":662,"t_abs":0.001227,"code":0}
+{"event":"exit","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16T17:28:42.621236Z","file":"git.c","line":662,"t_abs":0.001227,"code":0}
-{"event":"atexit","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621268","file":"trace2/tr2_tgt_event.c","line":163,"t_abs":0.001265,"code":0}
+{"event":"atexit","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16T17:28:42.621268Z","file":"trace2/tr2_tgt_event.c","line":163,"t_abs":0.001265,"code":0}
 ------------
 
 == Enabling a Target
@@ -542,7 +542,7 @@ The following key/value pairs are common to all events:
 	"event":"version",
 	"sid":"1547659722619736-11614",
 	"thread":"main",
-	"time":"2019-01-16 17:28:42.620713",
+	"time":"2019-01-16T17:28:42.620713Z",
 	"file":"common-main.c",
 	"line":38,
 	...

trace2/tr2_tbuf.c

@@ -15,7 +15,7 @@ void tr2_tbuf_local_time(struct tr2_tbuf *tb)
 		  tm.tm_min, tm.tm_sec, (long)tv.tv_usec);
 }
 
-void tr2_tbuf_utc_time(struct tr2_tbuf *tb)
+void tr2_tbuf_utc_datetime_extended(struct tr2_tbuf *tb)
 {
 	struct timeval tv;
 	struct tm tm;
@@ -26,7 +26,7 @@ void tr2_tbuf_utc_time(struct tr2_tbuf *tb)
 	gmtime_r(&secs, &tm);
 
 	xsnprintf(tb->buf, sizeof(tb->buf),
-		  "%4d-%02d-%02d %02d:%02d:%02d.%06ld", tm.tm_year + 1900,
+		  "%4d-%02d-%02dT%02d:%02d:%02d.%06ldZ", tm.tm_year + 1900,
 		  tm.tm_mon + 1, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec,
 		  (long)tv.tv_usec);
 }

trace2/tr2_tbuf.h

@@ -16,8 +16,8 @@ struct tr2_tbuf {
 void tr2_tbuf_local_time(struct tr2_tbuf *tb);
 
 /*
- * Fill buffer with formatted UTC time string.
+ * Fill buffer with formatted UTC datatime string.
  */
-void tr2_tbuf_utc_time(struct tr2_tbuf *tb);
+void tr2_tbuf_utc_datetime_extended(struct tr2_tbuf *tb);
 
 #endif /* TR2_TBUF_H */

trace2/tr2_tgt_event.c

@@ -94,7 +94,7 @@ static void event_fmt_prepare(const char *event_name, const char *file,
 	 */
 	if (!tr2env_event_be_brief || !strcmp(event_name, "version") ||
 	    !strcmp(event_name, "atexit")) {
-		tr2_tbuf_utc_time(&tb_now);
+		tr2_tbuf_utc_datetime_extended(&tb_now);
 		jw_object_string(jw, "time", tb_now.buf);
 	}