Fix race when counting pins open for tests

The unit tests check that pins properly get cleaned up, but there was a
race condition. This fixes this by using atomics.

Since keeping open pin counts was intended as a test-only feature and
there's a chance of the use of atomics creating compilation issues,
demote the pin count handling to the stub backend only.

Author: fhunleth

c_src/gpio_nif.c

@@ -20,7 +20,6 @@ static void release_gpio_pin(struct gpio_priv *priv, struct gpio_pin *pin)
 {
     if (pin->fd >= 0) {
         hal_close_gpio(pin);
-        priv->pins_open--;
         pin->fd = -1;
     }
 }
@@ -104,7 +103,6 @@ static int load(ErlNifEnv *env, void **priv_data, ERL_NIF_TERM info)
         return 1;
     }
 
-    priv->pins_open = 0;
     priv->gpio_pin_rt = enif_open_resource_type_x(env, "gpio_pin", &gpio_pin_init, ERL_NIF_RT_CREATE, NULL);
 
     if (hal_load(&priv->hal_priv) < 0) {
@@ -331,8 +329,6 @@ static ERL_NIF_TERM open_gpio(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[
     ERL_NIF_TERM pin_resource = enif_make_resource(env, pin);
     enif_release_resource(pin);
 
-    priv->pins_open++;
-
     return make_ok_tuple(env, pin_resource);
 }
 
@@ -356,8 +352,6 @@ static ERL_NIF_TERM gpio_info(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[
     struct gpio_priv *priv = enif_priv_data(env);
     ERL_NIF_TERM info = enif_make_new_map(env);
 
-    enif_make_map_put(env, info, enif_make_atom(env, "pins_open"), enif_make_int(env, priv->pins_open), &info);
-
     return hal_info(env, priv->hal_priv, info);
 }
 

c_src/gpio_nif.h

@@ -46,8 +46,6 @@ enum pull_mode {
 struct gpio_priv {
     ErlNifResourceType *gpio_pin_rt;
 
-    int pins_open;
-
     uint32_t hal_priv[1];
 };
 

c_src/hal_stub.c

@@ -3,6 +3,8 @@
 // SPDX-License-Identifier: Apache-2.0
 
 #include "gpio_nif.h"
+
+#include <stdatomic.h>
 #include <string.h>
 
 #define NUM_GPIOS 64
@@ -13,16 +15,19 @@
  */
 
 struct stub_priv {
+    atomic_int pins_open;
     int value[NUM_GPIOS / 2];
     ErlNifPid pid[NUM_GPIOS];
     enum trigger_mode mode[NUM_GPIOS];
 };
 
 ERL_NIF_TERM hal_info(ErlNifEnv *env, void *hal_priv, ERL_NIF_TERM info)
 {
-    (void) hal_priv;
+    struct stub_priv *stub_priv = (struct stub_priv *) hal_priv;
+    int pins_open = atomic_load(&stub_priv->pins_open);
 
     enif_make_map_put(env, info, enif_make_atom(env, "name"), enif_make_atom(env, "stub"), &info);
+    enif_make_map_put(env, info, enif_make_atom(env, "pins_open"), enif_make_int(env, pins_open), &info);
     return info;
 }
 
@@ -33,7 +38,11 @@ size_t hal_priv_size(void)
 
 int hal_load(void *hal_priv)
 {
-    memset(hal_priv, 0, sizeof(struct stub_priv));
+    struct stub_priv *stub_priv = (struct stub_priv *) hal_priv;
+
+    memset(stub_priv, 0, sizeof(struct stub_priv));
+    stub_priv->pins_open = 0;
+
     return 0;
 }
 
@@ -49,7 +58,10 @@ int hal_open_gpio(struct gpio_pin *pin,
     (void) env;
     // For test purposes, pins 0-63 work and everything else fails
     if (pin->pin_number >= 0 && pin->pin_number < NUM_GPIOS) {
+        struct stub_priv *stub_priv = pin->hal_priv;
+
         pin->fd = pin->pin_number;
+        atomic_fetch_add(&stub_priv->pins_open, 1);
 
         if (pin->config.is_output && pin->config.initial_value != -1)
             hal_write_gpio(pin, pin->config.initial_value, env);
@@ -67,6 +79,7 @@ void hal_close_gpio(struct gpio_pin *pin)
     if (pin->fd >= 0) {
         struct stub_priv *hal_priv = pin->hal_priv;
         hal_priv->mode[pin->pin_number] = TRIGGER_NONE;
+        atomic_fetch_sub(&hal_priv->pins_open, 1);
         pin->fd = -1;
     }
 }