[PATCH libinput 1/2] util: introduce ratelimit helpers

[David Herrmann]

This adds "struct ratelimit" and "ratelimit_test()". It's a very simple
rate-limit helper modeled after Linux' lib/ratelimit.c by Dave Young.

This comes in handy to limit log-messages in possible busy loops etc..

Signed-off-by: David Herrmann <dh.herrmann at gmail.com>
---
 src/libinput-util.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/libinput-util.h | 19 +++++++++++++++++++
 test/misc.c         | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 104 insertions(+)

diff --git a/src/libinput-util.c b/src/libinput-util.c
index eeb9786..19594e3 100644
--- a/src/libinput-util.c
+++ b/src/libinput-util.c
@@ -65,3 +65,51 @@ list_empty(const struct list *list)
 {
 	return list->next == list;
 }
+
+/*
+ * Perform rate-limit test. Returns true if the rate-limited action is still
+ * allowed, false if it should be suppressed.
+ *
+ * The ratelimit object must be initialized via RATELIMIT_INIT().
+ *
+ * Modelled after Linux' lib/ratelimit.c by Dave Young
+ * <hidave.darkstar at gmail.com>, which is licensed GPLv2.
+ */
+bool ratelimit_test(struct ratelimit *r)
+{
+	struct timespec ts;
+	uint64_t utime;
+
+	if (r->interval <= 0 || r->burst <= 0)
+		return true;
+
+	clock_gettime(CLOCK_MONOTONIC, &ts);
+	utime = ts.tv_sec * 1000 * 1000 + ts.tv_nsec / 1000;
+
+	if (r->begin <= 0 || r->begin + r->interval < utime) {
+		/* reset counter */
+		r->begin = utime;
+		r->num = 1;
+		return true;
+	} else if (r->num < r->burst) {
+		/* continue burst */
+		r->num++;
+		return true;
+	}
+
+	/* rate-limit with overflow check */
+	if (r->num + 1 > r->num)
+		++r->num;
+
+	return false;
+}
+
+/*
+ * Return true if the ratelimit counter just crossed the cutoff value. That is,
+ * this function returns true iff the last call to ratelimit_test() was the
+ * first call to exceed the burst value in this interval.
+ */
+bool ratelimit_cutoff(struct ratelimit *r)
+{
+	return r->num == r->burst + 1;
+}
diff --git a/src/libinput-util.h b/src/libinput-util.h
index 51759e8..8ff8778 100644
--- a/src/libinput-util.h
+++ b/src/libinput-util.h
@@ -25,6 +25,7 @@
 
 #include <unistd.h>
 #include <math.h>
+#include <stdbool.h>
 #include <string.h>
 #include <time.h>
 
@@ -280,4 +281,22 @@ matrix_to_farray6(const struct matrix *m, float out[6])
 	out[5] = m->val[1][2];
 }
 
+struct ratelimit {
+	uint64_t interval;
+	uint64_t begin;
+	unsigned burst;
+	unsigned num;
+} RateLimit;
+
+#define RATELIMIT_INIT(_interval, _burst)		\
+	((struct ratelimit){				\
+		.interval = (_interval),		\
+		.begin = 0,				\
+		.burst = (_burst),			\
+		.num = 0,				\
+	})
+
+bool ratelimit_test(struct ratelimit *r);
+bool ratelimit_cutoff(struct ratelimit *r);
+
 #endif /* LIBINPUT_UTIL_H */
diff --git a/test/misc.c b/test/misc.c
index 1512180..70b3e57 100644
--- a/test/misc.c
+++ b/test/misc.c
@@ -508,6 +508,42 @@ START_TEST(matrix_helpers)
 }
 END_TEST
 
+START_TEST(ratelimit_helpers)
+{
+	/* 10 attempts every 10ms */
+	struct ratelimit rl = RATELIMIT_INIT(10000, 10);
+	unsigned int i, j;
+
+	for (j = 0; j < 100; ++j) {
+		/* a burst of 10 attempts must succeed */
+		for (i = 0; i < 10; ++i) {
+			ck_assert(ratelimit_test(&rl));
+			ck_assert(!ratelimit_cutoff(&rl));
+		}
+
+		/* ..then further attempts must fail.. */
+		ck_assert(!ratelimit_test(&rl));
+		ck_assert(ratelimit_cutoff(&rl));
+
+		/* ..regardless of how often we try. */
+		for (i = 0; i < 100; ++i) {
+			ck_assert(!ratelimit_test(&rl));
+			ck_assert(!ratelimit_cutoff(&rl));
+		}
+
+		/* ..even after waiting 5ms */
+		usleep(5000);
+		for (i = 0; i < 100; ++i) {
+			ck_assert(!ratelimit_test(&rl));
+			ck_assert(!ratelimit_cutoff(&rl));
+		}
+
+		/* but after 10ms the counter is reset */
+		usleep(6000); /* +1ms to account for time drifts */
+	}
+}
+END_TEST
+
 int main (int argc, char **argv) {
 	litest_add_no_device("events:conversion", event_conversion_device_notify);
 	litest_add_no_device("events:conversion", event_conversion_pointer);
@@ -519,5 +555,6 @@ int main (int argc, char **argv) {
 	litest_add_no_device("config:status string", config_status_string);
 
 	litest_add_no_device("misc:matrix", matrix_helpers);
+	litest_add_no_device("misc:ratelimit", ratelimit_helpers);
 	return litest_run(argc, argv);
 }
-- 
2.1.3