trace2: create new combined trace facility

Create a new unified tracing facility for git.  The eventual intent is to
replace the current trace_printf* and trace_performance* routines with a
unified set of git_trace2* routines.

In addition to the usual printf-style API, trace2 provides higer-level
event verbs with fixed-fields allowing structured data to be written.
This makes post-processing and analysis easier for external tools.

Trace2 defines 3 output targets.  These are set using the environment
variables "GIT_TR2", "GIT_TR2_PERF", and "GIT_TR2_EVENT".  These may be
set to "1" or to an absolute pathname (just like the current GIT_TRACE).

* GIT_TR2 is intended to be a replacement for GIT_TRACE and logs command
  summary data.

* GIT_TR2_PERF is intended as a replacement for GIT_TRACE_PERFORMANCE.
  It extends the output with columns for the command process, thread,
  repo, absolute and relative elapsed times.  It reports events for
  child process start/stop, thread start/stop, and per-thread function
  nesting.

* GIT_TR2_EVENT is a new structured format. It writes event data as a
  series of JSON records.

Calls to trace2 functions log to any of the 3 output targets enabled
without the need to call different trace_printf* or trace_performance*
routines.

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

1 files changed, 67 insertions, 0 deletions

diff --git a/trace2/tr2_sid.c b/trace2/tr2_sid.c
new file mode 100644
index 0000000..984524a
--- /dev/null
+++ b/trace2/tr2_sid.c
@@ -0,0 +1,67 @@
+#include "cache.h"
+#include "trace2/tr2_sid.h"
+
+#define TR2_ENVVAR_PARENT_SID "GIT_TR2_PARENT_SID"
+
+static struct strbuf tr2sid_buf = STRBUF_INIT;
+static int tr2sid_nr_git_parents;
+
+/*
+ * Compute a "unique" session id (SID) for the current process.  This allows
+ * all events from this process to have a single label (much like a PID).
+ *
+ * Export this into our environment so that all child processes inherit it.
+ *
+ * If we were started by another git instance, use our parent's SID as a
+ * prefix.  (This lets us track parent/child relationships even if there
+ * is an intermediate shell process.)
+ *
+ * Additionally, count the number of nested git processes.
+ */
+static void tr2_sid_compute(void)
+{
+	uint64_t us_now;
+	const char *parent_sid;
+
+	if (tr2sid_buf.len)
+		return;
+
+	parent_sid = getenv(TR2_ENVVAR_PARENT_SID);
+	if (parent_sid && *parent_sid) {
+		const char *p;
+		for (p = parent_sid; *p; p++)
+			if (*p == '/')
+				tr2sid_nr_git_parents++;
+
+		strbuf_addstr(&tr2sid_buf, parent_sid);
+		strbuf_addch(&tr2sid_buf, '/');
+		tr2sid_nr_git_parents++;
+	}
+
+	us_now = getnanotime() / 1000;
+	strbuf_addf(&tr2sid_buf, "%" PRIuMAX "-%" PRIdMAX, (uintmax_t)us_now,
+		    (intmax_t)getpid());
+
+	setenv(TR2_ENVVAR_PARENT_SID, tr2sid_buf.buf, 1);
+}
+
+const char *tr2_sid_get(void)
+{
+	if (!tr2sid_buf.len)
+		tr2_sid_compute();
+
+	return tr2sid_buf.buf;
+}
+
+int tr2_sid_depth(void)
+{
+	if (!tr2sid_buf.len)
+		tr2_sid_compute();
+
+	return tr2sid_nr_git_parents;
+}
+
+void tr2_sid_release(void)
+{
+	strbuf_release(&tr2sid_buf);
+}