EvictPowerSrv Linux version rewritten in C.

main.c

/*
 * Shutdown the system properly when Azure evicts your Spot instance.
 *
 * Compile:
 * Requires libcurl 4 and json-c 5 headers.
 * cc -Wall -Werror -lcurl -ljson-c main.c
 *
 * Running:
 * Requires `shutdown` command and the rights to execute it.
 * Requires libcurl.so.4, libjson-c.so.5, and glibc.
 * 
 * Daemon:
 * Write yourself a systemd (or whatever service manager you are using) to run the program in background.
 *
 * License:
 * GPL v2 only.
 */

#include <string.h>
#include <curl/curl.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <json-c/json.h>

#define WAIT_ERR 5
#define WAIT_NONEED 3

// #define DEBUG 1

size_t cb(void *data, size_t size, size_t nmemb, void *ptr)
{
	size_t realsize = size * nmemb;
#ifdef DEBUG
	printf("cb(%lu)\n", realsize);
#endif
	char *str = (char*)data;
	json_object *obj = json_tokener_parse(str);
	if (obj == NULL)
	{
		fprintf(stderr, "Cannot parse JSON");
		return 0;
	}
	json_object *events = json_object_object_get(obj, "Events");
	if (events == NULL)
	{
		free(obj);
		fprintf(stderr, "Cannot obtain Events info");
		return 0;
	}
	int shutdown = 0;
	for (int i = 0; i < json_object_array_length(events); i ++)
	{
		json_object *current = json_object_array_get_idx(events, i);
		if (current == NULL)
		{
			fprintf(stderr, "Cannot get the event information of %u", i);
			free(events);
			free(obj);
			return 0;
		}
		json_object *type = json_object_object_get(current, "EventType");
		if (type == NULL)
		{
			fprintf(stderr, "Cannot get the EventType");
			free(current);
			free(events);
			free(obj);
			return 0;
		}
		if (!strcmp(json_object_get_string(type), "Preempt"))
		{
			printf("The system is being preempted. Shutting down immediatedly.\n");
			shutdown = 1;
		}
		free(type);
		free(current);
		if (shutdown)
			break;
	}
	free(events);
	free(obj);
	if (shutdown)
	{
		// TODO: There should be a better way to do so.
		if (system("shutdown now"))
		{
			fprintf(stderr, "Could not shutdown.\n");
			return 0;
		}
	}
	return realsize;
}

int main(int argc, char *argv[])
{
	CURL *curl = curl_easy_init();
	if (!curl)
	{
		fprintf(stderr, "Cannot create curl instance.\n");
		return 1;
	}

	// Set CURL opts.
	curl_easy_setopt(curl, CURLOPT_URL, "http://169.254.169.254/metadata/scheduledevents?api-version=2019-08-01");
	struct curl_slist *headers = NULL;
	headers = curl_slist_append(headers, "Metadata: true");
#ifdef DEBUG
	printf("Headers: %p\n", headers);
#endif
	if (!headers)
	{
		fprintf(stderr, "Cannot create headers\n");
		return 2;
	}
	curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
	curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, cb);

	// Begin loop
	while(1)
	{
#ifdef DEBUG
		printf("Requesting...\n");
#endif
		CURLcode res = curl_easy_perform(curl);
		if (res)
		{
			fprintf(stderr, "Cannot request scheduled events: %d\n", res);
			sleep(WAIT_ERR);
			continue;
		}
#ifdef DEBUG
		printf("No need to do anything.\n");
#endif
		sleep(WAIT_NONEED);
	}

	// TODO: Do we need to clean up?
#ifdef DEBUG
	printf("Cleaning up\n");
#endif
	curl_slist_free_all(headers);
	curl_easy_cleanup(curl);
}