Rework 'wexecvp' replacing legacy 'select' in favor of 'poll' #4001

This commit addresses several issues with 'wexecvp':
  - Fix invalid double call to 'close' in case fd was higher than
    'FD_SETSIZE'. This could lead to the invalidation of an unrelated
    fd resulting in asserts, as in issue #4001 and other instabilities.
  - Fix previous limitations of the legacy 'select 'impl that rendered
    the RESTAPI unusable when ProxySQL had more then 'FD_SETSIZE' fds
    opened.
  - Other minor improvements in function logic and interface.

include/proxysql_utils.h

@@ -128,27 +128,27 @@ cfmt_t cstr_format(char (&out_buf)[N], const char* fmt, ...) {
 /**
  * @brief Simple struct that holds the 'timeout options' for 'wexecvp'.
  */
-struct to_opts {
+struct to_opts_t {
 	/**
 	 * @brief Timeout for the script execution to be completed, in case of being
 	 *   exceeded, the script will be terminated.
 	 */
 	unsigned int timeout_us;
 	/**
-	 * @brief Timeout used for 'select()' non blocking calls.
+	 * @brief Timeout used for 'poll()' non blocking calls.
 	 */
-	suseconds_t select_to_us;
+	suseconds_t poll_to_us;
 	/**
 	 * @brief The duration of the sleeps between the checks being performed
 	 *   on the child process waiting it to exit after being signaled to terminate,
 	 *   before issuing 'SIGKILL'.
 	 */
-	unsigned int it_delay_us;
+	unsigned int waitpid_delay_us;
 	/**
 	 * @brief The timeout to be waited on the child process after being signaled
 	 *   with 'SIGTERM' before being forcely terminated by 'SIGKILL'.
 	 */
-	unsigned int sigkill_timeout_us;
+	unsigned int sigkill_to_us;
 };
 
 /**
@@ -165,17 +165,16 @@ struct to_opts {
  * @return 0 In case of success or one of the following error codes:
  *   - '-1' in case any 'pipe()' creation failed.
  *   - '-2' in case 'fork()' call failed.
- *   - '-3' in case 'fcntl()' call failed .
- *   - '-4' in case of resource exhaustion, file descriptors being used exceeds 'FD_SETSIZE'.
- *   - '-5' in case 'select()' call failed.
- *   - '-6' in case 'read()' from pipes failed with a non-expected error.
+ *   - '-3' in case 'fcntl()' call failed.
+ *   - '-4' in case 'poll()' call failed.
+ *   - '-5' in case 'read()' from pipes failed with a non-expected error.
  *   - 'ETIME' in case the executable has exceeded the timeout supplied in 'opts.timeout_us'.
  *  In all this cases 'errno' is set to the error reported by the failing 'system call'.
  */
 int wexecvp(
 	const std::string& file,
 	const std::vector<const char*>& argv,
-	const to_opts* opts,
+	const to_opts_t& opts,
 	std::string& s_stdout,
 	std::string& s_stderr
 );

lib/ProxySQL_RESTAPI_Server.cpp

@@ -97,17 +97,17 @@ class sync_resource : public http_resource {
 		if (nullptr!=result)
 			return result;
 
-		to_opts wexecvp_opts {};
+		to_opts_t wexecvp_opts {};
 		wexecvp_opts.timeout_us = (interval_ms * 1000);
-		wexecvp_opts.select_to_us = 100*1000;
-		wexecvp_opts.it_delay_us = 500*1000;
-		wexecvp_opts.sigkill_timeout_us = 3000 * 1000;
+		wexecvp_opts.poll_to_us = 100*1000;
+		wexecvp_opts.waitpid_delay_us = 500*1000;
+		wexecvp_opts.sigkill_to_us = 3000 * 1000;
 
 		std::string script_stdout {""};
 		std::string script_stderr {""};
 
 		const std::vector<const char*> args { const_cast<const char*>(params.c_str()), NULL};
-		int script_err = wexecvp(script.c_str(), args, &wexecvp_opts, script_stdout, script_stderr);
+		int script_err = wexecvp(script.c_str(), args, wexecvp_opts, script_stdout, script_stderr);
 		int script_errno = errno;
 
 		std::string str_response_err {};