Merge branch 'fix/freertos_sim_nonblock_select' into 'master'

fix(freertos): Made select function non-blocking on Linux target (GitHub PR)

Closes IDFGH-13569 and IDFGH-13498

See merge request espressif/esp-idf!33331

components/freertos/esp_additions/FreeRTOSSimulator_wrappers.c

@@ -1,56 +1,95 @@
 /*
- * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2023-2024 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
-#include <pthread.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/task.h>
 #include <dlfcn.h>
-#include <sys/types.h>
-#include "esp_err.h"
-#include "errno.h"
+#include <assert.h>
+#include <sys/select.h>
+#include <errno.h>
 
-/** This module addresses the FreeRTOS simulator's coexistence with linux system calls from user apps.
- *  It's only included when building without lwIP, so we need to use linux system's select() which would receive
- *  EINTR event on every FreeRTOS interrupt; we workaround this problem by wrapping select()
- *  to bypass and silence these events.
+/** This module addresses the FreeRTOS simulator's coexistence with Linux system calls from user apps.
+ *  It wraps select so that it doesn't block the FreeRTOS task calling it, so that the
+ *  scheduler will allow lower priority tasks to run.
+ *  Without the wrapper, most components such as ESP-MQTT block lower priority tasks from running at all.
  */
-typedef int (*select_func_t) (int fd, fd_set * rfds, fd_set * wfds, fd_set *efds, struct timeval *tval);
+typedef int (*select_func_t)(int fd, fd_set *rfds, fd_set *wfds, fd_set *efds, struct timeval *tval);
 
-static inline int64_t get_us(void)
+int select(int fd, fd_set *rfds, fd_set *wfds, fd_set *efds, struct timeval *tval)
 {
-    struct timespec spec;
-    clock_gettime(CLOCK_REALTIME, &spec);
-    return spec.tv_nsec / 1000 + spec.tv_sec * 1000000;
-}
+    static select_func_t s_real_select = NULL;
+    TickType_t end_ticks = portMAX_DELAY;
+    fd_set o_rfds, o_wfds, o_efds;
 
-int select (int fd, fd_set * rfds, fd_set * wfds, fd_set *efds, struct timeval *tval)
-{
-    int ret;
-    struct timeval *tv = tval;
-    struct timeval timeval_local = {};
-    int64_t start = 0;
-    int64_t timeout_us = 0;
-    select_func_t real_select = (select_func_t) dlsym(RTLD_NEXT, "select");
-    if (tv != NULL) {
-        start = get_us();
-        timeout_us = tval->tv_sec * 1000000 + tval->tv_usec;
-        timeval_local.tv_sec = tval->tv_sec;
-        timeval_local.tv_usec = tval->tv_usec;
-        tv = &timeval_local;  // this (tv != NULL) indicates that we should handle timeouts
+    // Lookup the select symbol
+    if (s_real_select == NULL) {
+        s_real_select = (select_func_t)dlsym(RTLD_NEXT, "select");
+        assert(s_real_select);  // abort() if we cannot locate the symbol
     }
-    while ((ret = real_select(fd, rfds, wfds, efds, tv)) < 0 && errno == EINTR) {
-        if (tv != NULL) {
-            int64_t now = get_us();
-            timeout_us -= now - start;
-            if (timeout_us < 0) {
-                errno = 0;
-                ret = 0;
-                break;
-            }
-            start = now;
-            tv->tv_usec = timeout_us % 1000000;
-            tv->tv_sec = timeout_us / 1000000;
+
+    // Calculate the end_ticks if a timeout is provided
+    if (tval != NULL) {
+        end_ticks = xTaskGetTickCount() + pdMS_TO_TICKS(tval->tv_sec * 1000 + tval->tv_usec / 1000);
+    }
+
+    // Preserve the original FD sets as select call will change them
+    if (rfds) {
+        o_rfds = *rfds;
+    }
+    if (wfds) {
+        o_wfds = *wfds;
+    }
+    if (efds) {
+        o_efds = *efds;
+    }
+
+    while (1) {
+        // Restore original FD sets before the select call
+        if (rfds) {
+            *rfds = o_rfds;
         }
+        if (wfds) {
+            *wfds = o_wfds;
+        }
+        if (efds) {
+            *efds = o_efds;
+        }
+
+        // Call select with a zero timeout to avoid blocking
+        struct timeval zero_tv = {0, 0};
+        int ret = s_real_select(fd, rfds, wfds, efds, &zero_tv);
+
+        // Return on success
+        if (ret > 0) {
+            return ret;
+        }
+
+        // Return on any error except EINTR
+        if (ret == -1 && errno != EINTR) {
+            return ret;
+        }
+
+        /**
+         * Sleep for maximum 10 tick(s) to allow other tasks to run.
+         * This can be any value greater than zero.
+         * 10 is a good trade-off between CPU time usage and timeout resolution.
+         */
+        const TickType_t max_sleep_ticks = 10;
+        TickType_t sleep_ticks = max_sleep_ticks;
+
+        if (tval != NULL) {
+            TickType_t now_ticks = xTaskGetTickCount();
+            if (now_ticks >= end_ticks) {
+                errno = 0;
+                return 0;
+            }
+            // Sleep for the remaining time or a maximum of 10 tick
+            TickType_t remaining_ticks = end_ticks - now_ticks;
+            sleep_ticks = (remaining_ticks < max_sleep_ticks) ? remaining_ticks : max_sleep_ticks;
+        }
+
+        vTaskDelay(sleep_ticks);
     }
-    return ret;
 }