Add support for deferred executors. (#14859)

* Add support for deferred executors.

* More docs.

* Include from quantum.h

* Cleanup.

* Parameter checks

* Comments.

* qmk format-c

* I accidentally a few words.

* API name change.

* Apply suggestions from code review

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

* Review comments.

* qmk format-c

* Review comments.

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

4 files changed, 204 insertions, 0 deletions

diff --git a/quantum/deferred_exec.c b/quantum/deferred_exec.c
new file mode 100644
index 0000000000..5b0a5b1425
--- /dev/null
+++ b/quantum/deferred_exec.c
@@ -0,0 +1,152 @@
+// Copyright 2021 Nick Brassel (@tzarc)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <stddef.h>
+#include <timer.h>
+#include <deferred_exec.h>
+
+#ifndef MAX_DEFERRED_EXECUTORS
+#    define MAX_DEFERRED_EXECUTORS 8
+#endif
+
+typedef struct deferred_executor_t {
+    deferred_token         token;
+    uint32_t               trigger_time;
+    deferred_exec_callback callback;
+    void *                 cb_arg;
+} deferred_executor_t;
+
+static deferred_token      current_token                     = 0;
+static uint32_t            last_deferred_exec_check          = 0;
+static deferred_executor_t executors[MAX_DEFERRED_EXECUTORS] = {0};
+
+static inline bool token_can_be_used(deferred_token token) {
+    if (token == INVALID_DEFERRED_TOKEN) {
+        return false;
+    }
+    for (int i = 0; i < MAX_DEFERRED_EXECUTORS; ++i) {
+        if (executors[i].token == token) {
+            return false;
+        }
+    }
+    return true;
+}
+
+static inline deferred_token allocate_token(void) {
+    deferred_token first = ++current_token;
+    while (!token_can_be_used(current_token)) {
+        ++current_token;
+        if (current_token == first) {
+            // If we've looped back around to the first, everything is already allocated (yikes!). Need to exit with a failure.
+            return INVALID_DEFERRED_TOKEN;
+        }
+    }
+    return current_token;
+}
+
+deferred_token defer_exec(uint32_t delay_ms, deferred_exec_callback callback, void *cb_arg) {
+    // Ignore queueing if it's a zero-time delay, or invalid callback
+    if (delay_ms == 0 || !callback) {
+        return INVALID_DEFERRED_TOKEN;
+    }
+
+    // Find an unused slot and claim it
+    for (int i = 0; i < MAX_DEFERRED_EXECUTORS; ++i) {
+        deferred_executor_t *entry = &executors[i];
+        if (entry->token == INVALID_DEFERRED_TOKEN) {
+            // Work out the new token value, dropping out if none were available
+            deferred_token token = allocate_token();
+            if (token == INVALID_DEFERRED_TOKEN) {
+                return false;
+            }
+
+            // Set up the executor table entry
+            entry->token        = current_token;
+            entry->trigger_time = timer_read32() + delay_ms;
+            entry->callback     = callback;
+            entry->cb_arg       = cb_arg;
+            return current_token;
+        }
+    }
+
+    // None available
+    return INVALID_DEFERRED_TOKEN;
+}
+
+bool extend_deferred_exec(deferred_token token, uint32_t delay_ms) {
+    // Ignore queueing if it's a zero-time delay, or the token is not valid
+    if (delay_ms == 0 || token == INVALID_DEFERRED_TOKEN) {
+        return false;
+    }
+
+    // Find the entry corresponding to the token
+    for (int i = 0; i < MAX_DEFERRED_EXECUTORS; ++i) {
+        deferred_executor_t *entry = &executors[i];
+        if (entry->token == token) {
+            // Found it, extend the delay
+            entry->trigger_time = timer_read32() + delay_ms;
+            return true;
+        }
+    }
+
+    // Not found
+    return false;
+}
+
+bool cancel_deferred_exec(deferred_token token) {
+    // Ignore request if the token is not valid
+    if (token == INVALID_DEFERRED_TOKEN) {
+        return false;
+    }
+
+    // Find the entry corresponding to the token
+    for (int i = 0; i < MAX_DEFERRED_EXECUTORS; ++i) {
+        deferred_executor_t *entry = &executors[i];
+        if (entry->token == token) {
+            // Found it, cancel and clear the table entry
+            entry->token        = INVALID_DEFERRED_TOKEN;
+            entry->trigger_time = 0;
+            entry->callback     = NULL;
+            entry->cb_arg       = NULL;
+            return true;
+        }
+    }
+
+    // Not found
+    return false;
+}
+
+void deferred_exec_task(void) {
+    uint32_t now = timer_read32();
+
+    // Throttle only once per millisecond
+    if (((int32_t)TIMER_DIFF_32(now, last_deferred_exec_check)) > 0) {
+        last_deferred_exec_check = now;
+
+        // Run through each of the executors
+        for (int i = 0; i < MAX_DEFERRED_EXECUTORS; ++i) {
+            deferred_executor_t *entry = &executors[i];
+
+            // Check if we're supposed to execute this entry
+            if (entry->token != INVALID_DEFERRED_TOKEN && ((int32_t)TIMER_DIFF_32(entry->trigger_time, now)) <= 0) {
+                // Invoke the callback and work work out if we should be requeued
+                uint32_t delay_ms = entry->callback(entry->trigger_time, entry->cb_arg);
+
+                // Update the trigger time if we have to repeat, otherwise clear it out
+                if (delay_ms > 0) {
+                    // Intentionally add just the delay to the existing trigger time -- this ensures the next
+                    // invocation is with respect to the previous trigger, rather than when it got to execution. Under
+                    // normal circumstances this won't cause issue, but if another executor is invoked that takes a
+                    // considerable length of time, then this ensures best-effort timing between invocations.
+                    entry->trigger_time += delay_ms;
+                } else {
+                    // If it was zero, then the callback is cancelling repeated execution. Free up the slot.
+                    entry->token        = INVALID_DEFERRED_TOKEN;
+                    entry->trigger_time = 0;
+                    entry->callback     = NULL;
+                    entry->cb_arg       = NULL;
+                }
+            }
+        }
+    }
+}
diff --git a/quantum/deferred_exec.h b/quantum/deferred_exec.h
new file mode 100644
index 0000000000..f80d353169
--- /dev/null
+++ b/quantum/deferred_exec.h
@@ -0,0 +1,38 @@
+// Copyright 2021 Nick Brassel (@tzarc)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+// A token that can be used to cancel an existing deferred execution.
+typedef uint8_t deferred_token;
+#define INVALID_DEFERRED_TOKEN 0
+
+// Callback to execute.
+//  -- Parameter trigger_time: the intended trigger time to execute the callback -- equivalent time-space as timer_read32()
+//               cb_arg: the callback argument specified when enqueueing the deferred executor
+//  -- Return value: Non-zero re-queues the callback to execute after the returned number of milliseconds. Zero cancels repeated execution.
+typedef uint32_t (*deferred_exec_callback)(uint32_t trigger_time, void *cb_arg);
+
+// Configures the supplied deferred executor to be executed after the required number of milliseconds.
+//  -- Parameter delay_ms: the number of milliseconds before executing the callback
+//  --           callback: the executor to invoke
+//  --           cb_arg: the argument to pass to the executor, may be NULL if unused by the executor
+//  -- Return value: a token usable for cancellation, or INVALID_DEFERRED_TOKEN if an error occurred
+deferred_token defer_exec(uint32_t delay_ms, deferred_exec_callback callback, void *cb_arg);
+
+// Allows for extending the timeframe before an existing deferred execution is invoked.
+//  -- Parameter token: the returned value from defer_exec for the deferred execution you wish to extend.
+//  --           delay_ms: the new delay (with respect to the current time)
+//  -- Return value: if the token was found, and the delay was extended
+bool extend_deferred_exec(deferred_token token, uint32_t delay_ms);
+
+// Allows for cancellation of an existing deferred execution.
+//  -- Parameter token: the returned value from defer_exec for the deferred execution you wish to cancel.
+//  -- Return value: if the token was found, and the executor was cancelled
+bool cancel_deferred_exec(deferred_token token);
+
+// Forward declaration for the main loop in order to execute any deferred executors. Should not be invoked by keyboard/user code.
+void deferred_exec_task(void);
diff --git a/quantum/main.c b/quantum/main.c
index 3814d371c1..6ed6b95741 100644
--- a/quantum/main.c
+++ b/quantum/main.c
@@ -43,6 +43,10 @@ void protocol_task(void) {
     protocol_post_task();
 }
 
+#ifdef DEFERRED_EXEC_ENABLE
+void deferred_exec_task(void);
+#endif  // DEFERRED_EXEC_ENABLE
+
 /** \brief Main
  *
  * FIXME: Needs doc
@@ -58,6 +62,12 @@ int main(void) {
     /* Main loop */
     while (true) {
         protocol_task();
+
+#ifdef DEFERRED_EXEC_ENABLE
+        // Run deferred executions
+        deferred_exec_task();
+#endif  // DEFERRED_EXEC_ENABLE
+
         housekeeping_task();
     }
 }
diff --git a/quantum/quantum.h b/quantum/quantum.h
index e6015adbe9..b34ff6ec51 100644
--- a/quantum/quantum.h
+++ b/quantum/quantum.h
@@ -54,6 +54,10 @@
 #include <stddef.h>
 #include <stdlib.h>
 
+#ifdef DEFERRED_EXEC_ENABLE
+#    include "deferred_exec.h"
+#endif
+
 extern layer_state_t default_layer_state;
 
 #ifndef NO_ACTION_LAYER