1 files changed, 123 insertions, 0 deletions
diff --git a/builtin-log.c b/builtin-log.c
index 197f6eec03..e1f1cf6714 100644
--- a/builtin-log.c
+++ b/builtin-log.c
@@ -77,11 +77,134 @@ static void cmd_log_init(int argc, const char **argv, const char *prefix,
 	}
 }
 
+/*
+ * This gives a rough estimate for how many commits we
+ * will print out in the list.
+ */
+static int estimate_commit_count(struct rev_info *rev, struct commit_list *list)
+{
+	int n = 0;
+
+	while (list) {
+		struct commit *commit = list->item;
+		unsigned int flags = commit->object.flags;
+		list = list->next;
+		if (!(flags & (TREESAME | UNINTERESTING)))
+			n++;
+	}
+	return n;
+}
+
+static void show_early_header(struct rev_info *rev, const char *stage, int nr)
+{
+	if (rev->shown_one) {
+		rev->shown_one = 0;
+		if (rev->commit_format != CMIT_FMT_ONELINE)
+			putchar(rev->diffopt.line_termination);
+	}
+	printf("Final output: %d %s\n", nr, stage);
+}
+
+struct itimerval early_output_timer;
+
+static void log_show_early(struct rev_info *revs, struct commit_list *list)
+{
+	int i = revs->early_output;
+	int show_header = 1;
+
+	sort_in_topological_order(&list, revs->lifo);
+	while (list && i) {
+		struct commit *commit = list->item;
+		switch (simplify_commit(revs, commit)) {
+		case commit_show:
+			if (show_header) {
+				int n = estimate_commit_count(revs, list);
+				show_early_header(revs, "incomplete", n);
+				show_header = 0;
+			}
+			log_tree_commit(revs, commit);
+			i--;
+			break;
+		case commit_ignore:
+			break;
+		case commit_error:
+			return;
+		}
+		list = list->next;
+	}
+
+	/* Did we already get enough commits for the early output? */
+	if (!i)
+		return;
+
+	/*
+	 * ..if no, then repeat it twice a second until we
+	 * do.
+	 *
+	 * NOTE! We don't use "it_interval", because if the
+	 * reader isn't listening, we want our output to be
+	 * throttled by the writing, and not have the timer
+	 * trigger every second even if we're blocked on a
+	 * reader!
+	 */
+	early_output_timer.it_value.tv_sec = 0;
+	early_output_timer.it_value.tv_usec = 500000;
+	setitimer(ITIMER_REAL, &early_output_timer, NULL);
+}
+
+static void early_output(int signal)
+{
+	show_early_output = log_show_early;
+}
+
+static void setup_early_output(struct rev_info *rev)
+{
+	struct sigaction sa;
+
+	/*
+	 * Set up the signal handler, minimally intrusively:
+	 * we only set a single volatile integer word (not
+	 * using sigatomic_t - trying to avoid unnecessary
+	 * system dependencies and headers), and using
+	 * SA_RESTART.
+	 */
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_handler = early_output;
+	sigemptyset(&sa.sa_mask);
+	sa.sa_flags = SA_RESTART;
+	sigaction(SIGALRM, &sa, NULL);
+
+	/*
+	 * If we can get the whole output in less than a
+	 * tenth of a second, don't even bother doing the
+	 * early-output thing..
+	 *
+	 * This is a one-time-only trigger.
+	 */
+	early_output_timer.it_value.tv_sec = 0;
+	early_output_timer.it_value.tv_usec = 100000;
+	setitimer(ITIMER_REAL, &early_output_timer, NULL);
+}
+
+static void finish_early_output(struct rev_info *rev)
+{
+	int n = estimate_commit_count(rev, rev->commits);
+	signal(SIGALRM, SIG_IGN);
+	show_early_header(rev, "done", n);
+}
+
 static int cmd_log_walk(struct rev_info *rev)
 {
 	struct commit *commit;
 
+	if (rev->early_output)
+		setup_early_output(rev);
+
 	prepare_revision_walk(rev);
+
+	if (rev->early_output)
+		finish_early_output(rev);
+
 	while ((commit = get_revision(rev)) != NULL) {
 		log_tree_commit(rev, commit);
 		if (!rev->reflog_info) {