From 698e4d1bae4a830eb7fc70af4d4a1e4d0a177911 Mon Sep 17 00:00:00 2001
From: Romain Vimont <rom@rom1v.com>
Date: Fri, 6 Sep 2024 23:08:08 +0200
Subject: [PATCH] Introduce non-droppable control messages

Control messages are queued from the main thread and sent to the device
from a separate thread.

When the queue is full, messages are just dropped. This avoids to
accumulate too much delay between the client and the device in case of
network issue.

However, some messages should not be dropped: for example, dropping a
UHID_CREATE message would make invalid all further UHID_INPUT messages.
Therefore, mark these messages as non-droppable.

A non-droppable event is queued anyway (resizing the queue if
necessary, unless the allocation fails).
---
 app/src/control_msg.c |  7 +++++++
 app/src/control_msg.h |  5 +++++
 app/src/controller.c  | 24 +++++++++++++++++++-----
 3 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/app/src/control_msg.c b/app/src/control_msg.c
index 9b0fab67..daa3bde7 100644
--- a/app/src/control_msg.c
+++ b/app/src/control_msg.c
@@ -278,6 +278,13 @@ sc_control_msg_log(const struct sc_control_msg *msg) {
     }
 }
 
+bool
+sc_control_msg_is_droppable(const struct sc_control_msg *msg) {
+    // Cannot drop UHID_CREATE messages, because it would cause all further
+    // UHID_INPUT messages for this device to be invalid
+    return msg->type != SC_CONTROL_MSG_TYPE_UHID_CREATE;
+}
+
 void
 sc_control_msg_destroy(struct sc_control_msg *msg) {
     switch (msg->type) {
diff --git a/app/src/control_msg.h b/app/src/control_msg.h
index 80714096..63670705 100644
--- a/app/src/control_msg.h
+++ b/app/src/control_msg.h
@@ -116,6 +116,11 @@ sc_control_msg_serialize(const struct sc_control_msg *msg, uint8_t *buf);
 void
 sc_control_msg_log(const struct sc_control_msg *msg);
 
+// Even when the buffer is "full", some messages must absolutely not be dropped
+// to avoid inconsistencies.
+bool
+sc_control_msg_is_droppable(const struct sc_control_msg *msg);
+
 void
 sc_control_msg_destroy(struct sc_control_msg *msg);
 
diff --git a/app/src/controller.c b/app/src/controller.c
index d50e1921..c3474fdd 100644
--- a/app/src/controller.c
+++ b/app/src/controller.c
@@ -4,7 +4,8 @@
 
 #include "util/log.h"
 
-#define SC_CONTROL_MSG_QUEUE_MAX 64
+// Drop droppable events above this limit
+#define SC_CONTROL_MSG_QUEUE_LIMIT 60
 
 static void
 sc_controller_receiver_on_ended(struct sc_receiver *receiver, bool error,
@@ -22,7 +23,9 @@ sc_controller_init(struct sc_controller *controller, sc_socket control_socket,
                    void *cbs_userdata) {
     sc_vecdeque_init(&controller->queue);
 
-    bool ok = sc_vecdeque_reserve(&controller->queue, SC_CONTROL_MSG_QUEUE_MAX);
+    // Add 4 to support 4 non-droppable events without re-allocation
+    bool ok = sc_vecdeque_reserve(&controller->queue,
+                                  SC_CONTROL_MSG_QUEUE_LIMIT + 4);
     if (!ok) {
         return false;
     }
@@ -93,20 +96,31 @@ sc_controller_push_msg(struct sc_controller *controller,
         sc_control_msg_log(msg);
     }
 
+    bool pushed = false;
+
     sc_mutex_lock(&controller->mutex);
-    bool full = sc_vecdeque_is_full(&controller->queue);
-    if (!full) {
+    size_t size = sc_vecdeque_size(&controller->queue);
+    if (size < SC_CONTROL_MSG_QUEUE_LIMIT) {
         bool was_empty = sc_vecdeque_is_empty(&controller->queue);
         sc_vecdeque_push_noresize(&controller->queue, *msg);
+        pushed = true;
         if (was_empty) {
             sc_cond_signal(&controller->msg_cond);
         }
+    } else if (!sc_control_msg_is_droppable(msg)) {
+        bool ok = sc_vecdeque_push(&controller->queue, *msg);
+        if (ok) {
+            pushed = true;
+        } else {
+            // A non-droppable event must be dropped anyway
+            LOG_OOM();
+        }
     }
     // Otherwise (if the queue is full), the msg is discarded
 
     sc_mutex_unlock(&controller->mutex);
 
-    return !full;
+    return pushed;
 }
 
 static bool