trace2: add macOS process ancestry tracing

In 353d3d7 (trace2: collect Windows-specific process information)
Windows-specific process ancestry information was added as a data_json
event to TRACE2. Furthermore in 2f732bf (tr2: log parent process name)
similar functionality was added for Linux-based systems, using procfs.

Teach Git to also log process ancestry on macOS using the sysctl with
KERN_PROC to get process information (PPID and process name).
Like the Linux implementation, we use the cmd_ancestry TRACE2 event
rather than using a data_json event and creating another custom data
point.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: mjcheetham

compat/darwin/procinfo.c

@@ -0,0 +1,99 @@
+#define USE_THE_REPOSITORY_VARIABLE
+
+#include "git-compat-util.h"
+#include "strbuf.h"
+#include "strvec.h"
+#include "trace2.h"
+#include <sys/sysctl.h>
+
+/*
+ * An arbitrarily chosen value to limit the depth of the ancestor chain.
+ */
+#define NR_PIDS_LIMIT 10
+
+/*
+ * Get the process name and parent PID for a given PID using sysctl().
+ * Returns 0 on success, -1 on failure.
+ */
+static int get_proc_info(pid_t pid, struct strbuf *name, pid_t *ppid)
+{
+	int mib[4];
+	struct kinfo_proc proc;
+	size_t size = sizeof(proc);
+
+	mib[0] = CTL_KERN;
+	mib[1] = KERN_PROC;
+	mib[2] = KERN_PROC_PID;
+	mib[3] = pid;
+
+	if (sysctl(mib, 4, &proc, &size, NULL, 0) < 0)
+		return -1;
+
+	if (size == 0)
+		return -1;
+
+	strbuf_addstr(name, proc.kp_proc.p_comm);
+	*ppid = proc.kp_eproc.e_ppid;
+
+	return 0;
+}
+
+/*
+ * Recursively push process names onto the ancestry array.
+ * We guard against cycles by limiting the depth to NR_PIDS_LIMIT.
+ */
+static void push_ancestry_name(struct strvec *names, pid_t pid, int depth)
+{
+	struct strbuf name = STRBUF_INIT;
+	pid_t ppid;
+
+	if (depth >= NR_PIDS_LIMIT)
+		return;
+
+	if (pid <= 0)
+		return;
+
+	if (get_proc_info(pid, &name, &ppid) < 0)
+		goto cleanup;
+
+	strvec_push(names, name.buf);
+
+	/*
+	 * Recurse to the parent process. Stop if ppid is 0 or 1
+	 * (init/launchd) or if we've reached ourselves (cycle).
+	 */
+	if (ppid > 1 && ppid != pid)
+		push_ancestry_name(names, ppid, depth + 1);
+
+cleanup:
+	strbuf_release(&name);
+}
+
+void trace2_collect_process_info(enum trace2_process_info_reason reason)
+{
+	struct strvec names = STRVEC_INIT;
+
+	if (!trace2_is_enabled())
+		return;
+
+	switch (reason) {
+	case TRACE2_PROCESS_INFO_STARTUP:
+		push_ancestry_name(&names, getppid(), 0);
+		if (names.nr)
+			trace2_cmd_ancestry(names.v);
+
+		strvec_clear(&names);
+		break;
+
+	case TRACE2_PROCESS_INFO_EXIT:
+		/*
+		 * The Windows version of this calls its
+		 * get_peak_memory_info() here. We may want to insert
+		 * similar process-end statistics here in the future.
+		 */
+		break;
+
+	default:
+		BUG("trace2_collect_process_info: unknown reason '%d'", reason);
+	}
+}