feat(linux): provide getrandom implementation for macOS

This makes getrandom(2) usable when compiling with IDF_TARGET=linux
on a macOS host.

components/linux/CMakeLists.txt

@@ -3,5 +3,10 @@ if(NOT "${target}" STREQUAL "linux")
     return()
 endif()
 
+if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
+    list(APPEND srcs getrandom.c)
+endif()
+
 idf_component_register(INCLUDE_DIRS include
-                       REQUIRED_IDF_TARGETS linux)
+                       REQUIRED_IDF_TARGETS linux
+                       SRCS ${srcs})

components/linux/getrandom.c

@@ -0,0 +1,28 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "sys/random.h"
+#include <errno.h>
+#include <fcntl.h>
+#include <stdint.h>
+#include <unistd.h>
+
+
+// getrandom() is not available on macOS, so we read from /dev/urandom instead.
+
+int getrandom(void *buf, size_t buflen, unsigned int flags)
+{
+    int fd = open("/dev/urandom", O_RDONLY);
+    if (fd < 0) {
+        return -1;
+    }
+    ssize_t ret = read(fd, buf, buflen);
+    close(fd);
+    if (ret < 0) {
+        return -1;
+    }
+    return 0;
+}

components/linux/include/sys/random.h

@@ -0,0 +1,15 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#pragma once
+
+#include_next "sys/random.h"
+
+#if __APPLE__
+#include <stddef.h>
+
+int getrandom(void *buf, size_t buflen, unsigned int flags);
+
+#endif  // __APPLE__