Add an evdev_frame wrapper struct

The kernel only ever gives us a frame of events in one go (it flushes on
SYN_REPORT). We then need to look at that frame as a single state change
(keyboards excepted for historical reasons) so let's push this into a
proper struct we can pass around.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1215>

src/evdev-frame.h

@@ -0,0 +1,251 @@
+
+/*
+ * Copyright © 2025 Red Hat, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#pragma once
+
+#include "config.h"
+
+#include "util-mem.h"
+#include "util-input-event.h"
+
+#include <stdbool.h>
+#include <linux/input.h>
+
+/**
+ * A wrapper around a SYN_REPORT-terminated set of input events.
+ *
+ * This struct always has a count of >= 1 (the SYN_REPORT)
+ * and the timestamp of the SYN_REPORT is always that of the
+ * most recently appended event (if nonzero)
+ *
+ * The event frame is of a fixed size given in
+ * evdev_frame_new() and cannot be resized via helpers.
+ *
+ * The struct should be considered opaque, use the helpers
+ * to access the various fields.
+ */
+struct evdev_frame {
+	int refcount;
+	size_t max_size;
+	size_t count;
+	struct input_event events[];
+};
+
+static inline struct evdev_frame *
+evdev_frame_ref(struct evdev_frame *frame)
+{
+	assert(frame->refcount > 0);
+	++frame->refcount;
+	return frame;
+}
+
+static inline struct evdev_frame *
+evdev_frame_unref(struct evdev_frame *frame)
+{
+	if (frame) {
+		assert(frame->refcount > 0);
+		if (--frame->refcount == 0) {
+			frame->max_size = 0;
+			frame->count = 0;
+			free(frame);
+		}
+	}
+	return NULL;
+}
+
+DEFINE_UNREF_CLEANUP_FUNC(evdev_frame);
+
+static inline bool
+evdev_frame_is_empty(const struct evdev_frame *frame)
+{
+	return frame->count == 1;
+}
+
+static inline size_t
+evdev_frame_get_count(const struct evdev_frame *frame)
+{
+	return frame->count;
+}
+
+static inline struct input_event *
+evdev_frame_get_events(struct evdev_frame *frame, size_t *nevents)
+{
+	if (nevents)
+		*nevents = frame->count;
+
+	return frame->events;
+}
+
+/**
+ * Set the timestamp for all events in this event frame.
+ */
+static inline void
+evdev_frame_set_time(struct evdev_frame *frame, uint64_t time)
+{
+	assert(frame->count > 0);
+
+	for (size_t i = 0; i < frame->count; i++)
+		input_event_set_time(&frame->events[i], time);
+}
+
+static inline uint64_t
+evdev_frame_get_time(const struct evdev_frame *frame)
+{
+	assert(frame->count > 0);
+
+	return input_event_time(&frame->events[frame->count - 1]);
+}
+
+static inline int
+evdev_frame_reset(struct evdev_frame *frame)
+{
+	memset(frame->events, 0, frame->max_size * sizeof(struct input_event));
+	frame->count = 1; /* SYN_REPORT is always there */
+
+	return 0;
+}
+
+static inline struct evdev_frame *
+evdev_frame_new(size_t max_size)
+{
+	struct evdev_frame *frame = zalloc(max_size * sizeof(struct input_event) + sizeof(*frame));
+
+	frame->refcount = 1;
+	frame->max_size = max_size;
+	frame->count = 1; /* SYN_REPORT is always there */
+
+	return frame;
+}
+
+static inline struct evdev_frame *
+evdev_frame_new_on_stack(size_t max_size)
+{
+	assert(max_size <= 64);
+	struct evdev_frame *frame = alloca(max_size * sizeof(struct input_event) + sizeof(*frame));
+
+	frame->refcount = 1;
+	frame->max_size = max_size;
+	frame->count = 1; /* SYN_REPORT is always there */
+	memset(frame->events, 0, max_size * sizeof(struct input_event));
+
+	return frame;
+}
+
+/**
+ * Append events to the event frame. nevents must be larger than 0
+ * and specifies the number of elements in events. If any events in
+ * the given events is a EV_SYN/SYN_REPORT event, that event is the last
+ * one appended even if nevents states a higher number of events (roughly
+ * equivalent to having a \0 inside a string).
+ *
+ * This function guarantees the frame is terminated with a SYN_REPORT event.
+ * Appending SYN_REPORTS to a frame does not increase the count of events in the
+ * frame - the new SYN_REPORT will simply replace the existing SYN_REPORT.
+ *
+ * The timestamp of the SYN_REPORT (if any) is used for this event
+ * frame. If the appended sequence does not contain a SYN_REPORT, the highest
+ * timestamp of any event appended is used. This timestamp will overwrite the
+ * frame's timestamp even if the timestamp of the frame is higher.
+ *
+ * If all to-be-appended events (including the SYN_REPORT) have a timestamp of
+ * 0, the existing frame's timestamp is left as-is.
+ *
+ * The caller SHOULD terminate the events with a SYN_REPORT event with a
+ * valid timestamp to ensure correct behavior.
+ *
+ * Returns 0 on success, or a negative errno on failure
+ */
+static inline int
+evdev_frame_append(struct evdev_frame *frame,
+		   const struct input_event *events,
+		   size_t nevents)
+{
+	assert(nevents > 0);
+
+	uint64_t time = 0;
+
+	for (size_t i = 0; i < nevents; i++) {
+		if (events[i].type == EV_SYN && events[i].code == SYN_REPORT) {
+			nevents = i;
+			time = input_event_time(&events[i]);
+			break;
+		}
+		time = max(time, input_event_time(&events[i]));
+	}
+
+	if (nevents > 0) {
+		if (frame->count + nevents > frame->max_size)
+			return -ENOMEM;
+
+		memcpy(frame->events + frame->count - 1, events, nevents * sizeof(struct input_event));
+		frame->count += nevents;
+	}
+	if (time)
+		evdev_frame_set_time(frame, time);
+
+	return 0;
+}
+
+/**
+ * Behaves like evdev_frame_append() but resets the frame before appending.
+ *
+ * On error the frame is left as-is.
+ *
+ * Returns 0 on success, or a negative errno on failure
+ */
+static inline int
+evdev_frame_set(struct evdev_frame *frame,
+		const struct input_event *events,
+		size_t nevents)
+{
+	assert(nevents > 0);
+
+	size_t count = nevents;
+
+	for (size_t i = 0; i < nevents; i++) {
+		if (events[i].type == EV_SYN && events[i].code == SYN_REPORT) {
+			count = i;
+			break;
+		}
+	}
+
+	if (count > frame->max_size - 1)
+		return -ENOMEM;
+
+	evdev_frame_reset(frame);
+	return evdev_frame_append(frame, events, nevents);
+}
+
+static inline struct evdev_frame *
+evdev_frame_clone(struct evdev_frame *frame)
+{
+	size_t nevents;
+	struct input_event *events = evdev_frame_get_events(frame, &nevents);
+	struct evdev_frame *clone = evdev_frame_new(nevents);
+
+	evdev_frame_append(clone, events, nevents);
+	evdev_frame_set_time(clone, evdev_frame_get_time(frame));
+
+	return clone;
+}

src/util-input-event.h

@@ -26,6 +26,9 @@
 #include "config.h"
 
 #include "util-time.h"
+#include "util-mem.h"
+#include <string.h>
+#include <stdbool.h>
 #include <linux/input.h>
 
 static inline struct input_event

test/test-utils.c

@@ -31,6 +31,7 @@
 
 #include "litest.h"
 #include "litest-runner.h"
+#include "evdev-frame.h"
 #include "util-files.h"
 #include "util-list.h"
 #include "util-strings.h"
@@ -2559,6 +2560,150 @@ START_TEST(macros_expand)
 }
 END_TEST
 
+START_TEST(evdev_frames)
+{
+	{
+		evdev_frame_unref(NULL); /* unref on NULL is permitted */
+	}
+	{
+		_unref_(evdev_frame) *frame = evdev_frame_new(3);
+		litest_assert_int_eq(evdev_frame_get_count(frame), 1U); /* SYN_REPORT */
+
+		litest_assert_ptr_eq(evdev_frame_ref(frame), frame);
+		litest_assert_ptr_eq(evdev_frame_unref(frame), NULL);
+	}
+	{
+		_unref_(evdev_frame) *frame = evdev_frame_new(3);
+		struct input_event toobig[] = {
+			{ .type = EV_ABS, .code = ABS_X, .value = 1, },
+			{ .type = EV_ABS, .code = ABS_Y, .value = 2, },
+			{ .type = EV_ABS, .code = ABS_Z, .value = 3, },
+			{ .type = EV_SYN, .code = SYN_REPORT, .value = 0, },
+		};
+
+		int rc = evdev_frame_set(frame, toobig, ARRAY_LENGTH(toobig));
+		litest_assert_int_eq(rc, -ENOMEM);
+	}
+	{
+		struct input_event events[] = {
+			{ .type = EV_ABS, .code = ABS_X, .value = 1, },
+			{ .type = EV_ABS, .code = ABS_Y, .value = 2, },
+			{ .type = EV_SYN, .code = SYN_REPORT, .value = 0, },
+		};
+
+		_unref_(evdev_frame) *frame = evdev_frame_new(3);
+		int rc = evdev_frame_set(frame, events, ARRAY_LENGTH(events));
+		litest_assert_neg_errno_success(rc);
+		litest_assert_int_eq(evdev_frame_get_count(frame), ARRAY_LENGTH(events));
+		litest_assert_int_eq(frame->max_size, ARRAY_LENGTH(events));
+
+		size_t nevents;
+		rc = memcmp(evdev_frame_get_events(frame, &nevents), events, sizeof(events));
+		litest_assert_int_eq(rc, 0);
+		litest_assert_int_eq(nevents, ARRAY_LENGTH(events));
+
+		/* Already full, can't append */
+		rc = evdev_frame_append(frame, events, 1);
+		litest_assert_int_eq(rc, -ENOMEM);
+	}
+	{
+		struct input_event events[] = {
+			{ .type = EV_ABS, .code = ABS_X, .value = 1, },
+			{ .type = EV_ABS, .code = ABS_Y, .value = 2, },
+			{ .type = EV_SYN, .code = SYN_REPORT, .value = 0, },
+		};
+
+		_unref_(evdev_frame) *frame = evdev_frame_new(3);
+		int rc = evdev_frame_set(frame, events, 1);
+		litest_assert_neg_errno_success(rc);
+		litest_assert_int_eq(evdev_frame_get_count(frame), 2U); /* we appended SYN_REPORT */
+		rc = evdev_frame_append(frame, events + 1, 1);
+		litest_assert_neg_errno_success(rc);
+		litest_assert_int_eq(evdev_frame_get_count(frame), 3U); /* we appended SYN_REPORT */
+		rc = evdev_frame_append(frame, events + 2, 1);
+		litest_assert_neg_errno_success(rc);
+		litest_assert_int_eq(evdev_frame_get_count(frame), 3U); /* SYN_REPORT already there */
+	}
+	{
+		struct input_event interrupted[] = {
+			{ .type = EV_ABS, .code = ABS_X, .value = 1, },
+			{ .type = EV_ABS, .code = ABS_Y, .value = 2, },
+			{ .type = EV_SYN, .code = SYN_REPORT, .value = 0, },
+			{ .type = EV_ABS, .code = ABS_RX, .value = 1, },
+			{ .type = EV_ABS, .code = ABS_RY, .value = 2, },
+			{ .type = EV_SYN, .code = SYN_REPORT, .value = 0, },
+		};
+
+		_unref_(evdev_frame) *frame = evdev_frame_new(5);
+		int rc = evdev_frame_set(frame, interrupted, ARRAY_LENGTH(interrupted));
+		litest_assert_neg_errno_success(rc);
+		litest_assert_int_eq(evdev_frame_get_count(frame), 3U);
+
+		rc = evdev_frame_set(frame, &interrupted[2], 1);
+		litest_assert_neg_errno_success(rc);
+		litest_assert_int_eq(evdev_frame_get_count(frame), 1U);
+
+		rc = evdev_frame_set(frame, &interrupted[1], ARRAY_LENGTH(interrupted) - 1);
+		litest_assert_neg_errno_success(rc);
+		litest_assert_int_eq(evdev_frame_get_count(frame), 2U);
+
+		/* We never appended a timestamp */
+		litest_assert_int_eq(evdev_frame_get_time(frame), 0U);
+	}
+	{
+		struct input_event e = {
+			.type = EV_ABS,
+			.code = ABS_X,
+			.value = 1,
+			.input_event_sec = 1234,
+			.input_event_usec = 567,
+
+		};
+
+		_unref_(evdev_frame) *frame = evdev_frame_new(3);
+		litest_assert_int_eq(evdev_frame_get_time(frame), 0U);
+
+		evdev_frame_append(frame, &e, 1);
+		litest_assert_int_eq(evdev_frame_get_time(frame), 1234000567U);
+		evdev_frame_append(frame, &e, 1);
+		litest_assert_int_eq(evdev_frame_get_time(frame), 1234000567U);
+
+		struct input_event syn = {
+			.type = EV_SYN,
+			.code = SYN_REPORT,
+			.value = 0,
+			.input_event_sec = 111,
+			.input_event_usec = 333,
+
+		};
+
+		litest_assert_neg_errno_success(evdev_frame_append(frame, &syn, 1));
+		litest_assert_int_eq(evdev_frame_get_time(frame), 111000333U);
+
+		/* SYN_REPORT overwrites lower timestamp */
+		syn.input_event_usec = 111;
+		litest_assert_neg_errno_success(evdev_frame_append(frame, &syn, 1));
+		litest_assert_int_eq(evdev_frame_get_time(frame), 111000111U);
+	}
+	{
+		/* Expect highest timestamp */
+		_unref_(evdev_frame) *frame = evdev_frame_new(4);
+		struct input_event mixed_times[] = {
+			{ .type = EV_ABS, .code = ABS_X, .value = 1, .input_event_sec = 12, .input_event_usec = 700, },
+			{ .type = EV_ABS, .code = ABS_Y, .value = 2, .input_event_sec = 56, .input_event_usec = 800, },
+			{ .type = EV_ABS, .code = ABS_Z, .value = 3, .input_event_sec = 34, .input_event_usec = 900, },
+			{ .type = EV_SYN, .code = SYN_REPORT, .value = 0, .input_event_sec = 0, .input_event_usec = 1, },
+		};
+		evdev_frame_set(frame, mixed_times, 3);
+		litest_assert_int_eq(evdev_frame_get_time(frame), 56000800U);
+
+		/* but SYN_REPORT overwrites any other timestamp */
+		evdev_frame_set(frame, mixed_times, 4);
+		litest_assert_int_eq(evdev_frame_get_time(frame), 1U);
+	}
+}
+END_TEST
+
 int main(void)
 {
 	struct litest_runner *runner = litest_runner_new();
@@ -2640,6 +2785,8 @@ int main(void)
 	ADD_TEST(attribute_cleanup);
 	ADD_TEST(macros_expand);
 
+	ADD_TEST(evdev_frames);
+
 	enum litest_runner_result result = litest_runner_run_tests(runner);
 	litest_runner_destroy(runner);