4
\$\begingroup\$

Background

My task is to create a program to continuously download utility meter data from an external API, and cache it in a MySQL database for faster access. The performance of this program is critical as it's important we get real-time data (or as close as possible), and many of the other applications built at the organization I work at rely on this data.

I actually first wrote the daemon in PHP, however, it was not very fast, consumed a lot of CPU/memory, and only lived for a couple minutes at most before dying (presumably because of memory leaks and general strain on the server). So, I rewrote them in C. I haven't done much careful profiling, but I can tell the C daemons are several times faster when I run them side-by-side the PHP daemons. Additionally, the C daemons use about half the CPU and memory the PHP daemons did. They also live longer. I'm pretty happy with these results, but as I'm fairly new to C (this is my first real-world project), I know there are parts of the program that are a bit sloppy.

What can I do to simplify, optimize, and generally improve my code?

Program details

By default, the daemon will collect "live" (i.e. 1-2 min) resolution data. The daemon will continuously update the least up-to-date meter, requesting live data that spans from the latest reading recorded in the database to the current time. Other resolutions (15min, hour, and month) are calculated by cron jobs using the live data, so there is no need to collect this data from the API. When a new meter is added, however, the daemon can be run to collect non-live resolutions so there is no need to wait for the data to accumulate. When collecting non-live resolution data, the requested data will span from the beginning of the time period that the data is stored for (e.g. live data is stored for 2 hours, see the *_DATA_LIFESPAN definitions) to the earliest reading recorded in the database. Data are written to a CSV file and then imported into MySQL every 20 seconds by a cron job (in my tests, this is much more efficient than using INSERTs for the data, even if I batch the query).

Code

The relevant code is pasted below, which is also available on GitHub (the GitHub repo has some additional information about the database schema, cron jobs needed to support the system, and compiler flags).

/**
 * Fetches data from the BuildingOS API and caches it in the db
 *
 * @author Tim Robert-Fitzgerald
 */

#define _XOPEN_SOURCE // for strptime
#define _GNU_SOURCE // for strptime
#define PRIORITY_METER "SELECT id, org_id, url, live_last_updated FROM meters WHERE source = 'buildingos' AND live_last_updated < (UNIX_TIMESTAMP() - 300) AND (for_orb > 0 OR bos_uuid IN (SELECT DISTINCT meter_uuid FROM relative_values WHERE permission = 'orb_server' AND meter_uuid != '')) AND id NOT IN (SELECT updating_meter FROM daemons WHERE target_res = 'live') ORDER BY live_last_updated ASC LIMIT 1"
#define LIVE_TARGET_METER "SELECT id, org_id, url, live_last_updated FROM meters WHERE source = 'buildingos' AND ((gauges_using > 0 OR for_orb > 0 OR timeseries_using > 0) OR bos_uuid IN (SELECT DISTINCT meter_uuid FROM relative_values WHERE permission = 'orb_server' AND meter_uuid != '')) AND id NOT IN (SELECT updating_meter FROM daemons WHERE target_res = 'live') ORDER BY live_last_updated ASC LIMIT 1"
#define QH_TARGET_METER "SELECT id, org_id, url, quarterhour_last_updated FROM meters WHERE source = 'buildingos' AND ((gauges_using > 0 OR for_orb > 0 OR timeseries_using > 0) OR bos_uuid IN (SELECT DISTINCT meter_uuid FROM relative_values WHERE permission = 'orb_server' AND meter_uuid != '')) AND id NOT IN (SELECT updating_meter FROM daemons WHERE target_res = 'live') ORDER BY quarterhour_last_updated ASC LIMIT 1"
#define HOUR_TARGET_METER "SELECT id, org_id, url, hour_last_updated FROM meters WHERE source = 'buildingos' AND ((gauges_using > 0 OR for_orb > 0 OR timeseries_using > 0) OR bos_uuid IN (SELECT DISTINCT meter_uuid FROM relative_values WHERE permission = 'orb_server' AND meter_uuid != '')) AND id NOT IN (SELECT updating_meter FROM daemons WHERE target_res = 'live') ORDER BY hour_last_updated ASC LIMIT 1"
#define MONTH_TARGET_METER "SELECT id, org_id, url, month_last_updated FROM meters WHERE source = 'buildingos' AND ((gauges_using > 0 OR for_orb > 0 OR timeseries_using > 0) OR bos_uuid IN (SELECT DISTINCT meter_uuid FROM relative_values WHERE permission = 'orb_server' AND meter_uuid != '')) AND id NOT IN (SELECT updating_meter FROM daemons WHERE target_res = 'live') ORDER BY month_last_updated ASC LIMIT 1"
#define UPDATE_LIVE_TIMESTAMP "UPDATE meters SET live_last_updated = %d WHERE id = %d"
#define UPDATE_QH_TIMESTAMP "UPDATE meters SET quarterhour_last_updated = %d WHERE id = %d"
#define UPDATE_HOUR_TIMESTAMP "UPDATE meters SET hour_last_updated = %d WHERE id = %d"
#define UPDATE_MONTH_TIMESTAMP "UPDATE meters SET month_last_updated = %d WHERE id = %d"
#define TOKEN_URL "https://api.buildingos.com/o/token/" // where to get the token from
#define ISO8601_FORMAT "%Y-%m-%dT%H:%M:%S%z"
#define ISO8601_FORMAT_EST "%Y-%m-%dT%H:%M:%S-04:00"
#define BUFFER_FILE "/root/meter_data.csv"
#define SMALL_CONTAINER 255 // small fixed-size container for arrays
#define MED_CONTAINER 510 // just double SMALL_CONTAINER
#define LIVE_DATA_LIFESPAN 7200 // live data is stored for 2 hours i.e. 7200s
#define QH_DATA_LIFESPAN 1209600 // 2 weeks
#define HOUR_DATA_LIFESPAN 5184000 // 2 months
#define MONTH_DATA_LIFESPAN 63113904 // 2 years
#define UPDATE_CURRENT 1 // update the meters.current column with the current reading?
#define READONLY_MODE 0 // if on (i.e. 1) the daemon will not make queries that update/insert/delete data by short circuiting if stmts

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <time.h>
#include <mysql.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/prctl.h>
#include <stdlib.h>
#include <signal.h>
#include <syslog.h>
#include <sys/stat.h>
#include <curl/curl.h> // install with `apt-get install libcurl4-openssl-dev`
#include <curl/easy.h>
#include "./lib/cJSON/cJSON.h"
#include "db.h"

static pid_t buildingosd_pid;
// Stores page downloaded by http_request()
struct MemoryStruct {
    char *memory;
    size_t size;
};

/**
 * Utility function copied from https://stackoverflow.com/a/779960/2624391
 */
char *str_replace(char *orig, char *rep, char *with) {
    char *result; // the return string
    char *ins;    // the next insert point
    char *tmp;    // varies
    int len_rep;  // length of rep (the string to remove)
    int len_with; // length of with (the string to replace rep with)
    int len_front; // distance between rep and end of last rep
    int count;    // number of replacements

    // sanity checks and initialization
    if (!orig || !rep) {
        return NULL;
    }
    len_rep = strlen(rep);
    if (len_rep == 0) {
        return NULL; // empty rep causes infinite loop during count
    }
    if (!with) {
        with = "";
    }
    len_with = strlen(with);

    // count the number of replacements needed
    ins = orig;
    for (count = 0; (tmp = strstr(ins, rep)); ++count) {
        ins = tmp + len_rep;
    }

    tmp = result = malloc(strlen(orig) + (len_with - len_rep) * count + 1);

    if (!result) {
        return NULL;
    }

    // first time through the loop, all the variable are set correctly
    // from here on,
    //    tmp points to the end of the result string
    //    ins points to the next occurrence of rep in orig
    //    orig points to the remainder of orig after "end of rep"
    while (count--) {
        ins = strstr(orig, rep);
        len_front = ins - orig;
        tmp = strncpy(tmp, orig, len_front) + len_front;
        tmp = strcpy(tmp, with) + len_with;
        orig += len_front + len_rep; // move to next "end of rep"
    }
    strcpy(tmp, orig);
    return result;
}

/**
 * Signal handler
 * @param signo [description]
 */
static void catch_signal(int signo) {
    int success = system("/var/www/html/oberlin/daemons/buildingosd -d"); // lol
    if (success == -1) {
        syslog(LOG_ERR, "Unable to relaunch self");
    }
    syslog(LOG_ERR, "Caught pipe #%d; exiting", signo);
}

/**
 * Helper for http_request()
 */
static size_t WriteMemoryCallback(void *contents, size_t size, size_t nmemb, void *userp) {
    size_t realsize = size * nmemb;
    struct MemoryStruct *mem = (struct MemoryStruct *)userp;
    mem->memory = realloc(mem->memory, mem->size + realsize + 1);
    if (mem->memory == NULL) {
        fprintf(stderr, "not enough memory (realloc returned NULL)\n");
        return 0;
    }
    memcpy(&(mem->memory[mem->size]), contents, realsize);
    mem->size += realsize;
    mem->memory[mem->size] = 0;
    return realsize;
}

/**
 * See https://curl.haxx.se/libcurl/c/postinmemory.html
 * @param url           http://www.example.org/
 * @param post          e.g. Field=1&Field=2&Field=3
 * @param custom_header 1 for a custom header, 0 for default
 * @param method        1 if POST, 0 if GET
 */
struct MemoryStruct http_request(char *url, char *post, int custom_header, int method, char *api_token) {
    char header[SMALL_CONTAINER];
    if (custom_header) {
        snprintf(header, sizeof(header), "Authorization: Bearer %s", api_token);
    }
    CURL *curl;
    CURLcode res;
    struct MemoryStruct chunk;
    chunk.memory = malloc(1);  /* will be grown as needed by realloc above */ 
    chunk.size = 0;    /* no data at this point */ 
    curl_global_init(CURL_GLOBAL_ALL);
    curl = curl_easy_init();
    if (curl) {
        if (custom_header) {
            struct curl_slist *chunk = NULL; // https://curl.haxx.se/libcurl/c/httpcustomheader.html
            /* Add a custom header */ 
            chunk = curl_slist_append(chunk, header);
            res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);
        }
        if (method == 1) {
            curl_easy_setopt(curl, CURLOPT_URL, url);
            curl_easy_setopt(curl, CURLOPT_POSTFIELDS, post); // if we don't provide POSTFIELDSIZE, libcurl will strlen() by itself
            curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long)strlen(post)); // Perform the request, res will get the return code
        } else {
            char full_url[SMALL_CONTAINER];
            strcpy(full_url, url);
            strcat(full_url, "?");
            strcat(full_url, post);
            curl_easy_setopt(curl, CURLOPT_URL, full_url);
            curl_easy_setopt(curl, CURLOPT_HTTPGET, 1L);
        }
        // curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L); // https://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYPEER.html
        /* send all data to this function  */ 
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
        /* we pass our 'chunk' struct to the callback function */ 
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *)&chunk);
        /* some servers don't like requests that are made without a user-agent
             field, so we provide one */ 
        curl_easy_setopt(curl, CURLOPT_USERAGENT, "libcurl-agent/1.0");
        res = curl_easy_perform(curl);
        /* Check for errors */ 
        if (res != CURLE_OK) {
            syslog(LOG_ERR, "curl_easy_perform() failed: %s", curl_easy_strerror(res));
            exit(1);
        }
        curl_easy_cleanup(curl);
        curl_global_cleanup();
    }
    return chunk;//chunk.memory;
    // free(chunk.memory);
}

/**
 * Execute before program termination
 */
void cleanup(MYSQL *conn) {
    char query[SMALL_CONTAINER];
    snprintf(query, sizeof(query), "DELETE FROM daemons WHERE pid = %d", buildingosd_pid);
    if (READONLY_MODE == 0 && mysql_query(conn, query)) {
        syslog(LOG_ERR, "%s", mysql_error(conn));
    }
    closelog();
    mysql_close(conn);
    exit(1); // this might just kill the child, but since the mysql conn is closed, daemon will die
}

/**
 * Handle errors
 */
void error(const char *msg, MYSQL *conn) {
    syslog(LOG_ERR, "%s", msg);
    cleanup(conn);
}

/**
 * Fetches a single record, terminating the program if there are no results
 */
MYSQL_ROW fetch_row(MYSQL *conn, char *query) {
    MYSQL_RES *res;
    MYSQL_ROW row;
    if (mysql_query(conn, query)) {
        error(mysql_error(conn), conn);
    }
    res = mysql_store_result(conn);
    row = mysql_fetch_row(res);
    // mysql_free_result(res);
    if (row == NULL) {
        syslog(LOG_ERR, "QUERY '%s' RETURNED 0 ROWS\n", query);
        cleanup(conn);
    }
    return row;
}

/**
 * Sets the API token, fetching a new one if necessary
 * @param conn
 * @param org_id to get API credentials for
 */
char *set_api_token(MYSQL *conn, char *org_id) {
    char query[SMALL_CONTAINER];
    MYSQL_ROW row;
    snprintf(query, sizeof(query), "SELECT api_id FROM orgs WHERE id = %s", org_id);
    int api_id = atoi(fetch_row(conn, query)[0]);
    snprintf(query, sizeof(query), "SELECT token, token_updated FROM api WHERE id = %d", api_id);
    row = fetch_row(conn, query);
    int update_token_at = atoi(row[1]) + 3595;
    time_t epoch = time(NULL);
    struct tm *tm = localtime(&epoch);
    int time = (int) mktime(tm);
    if (update_token_at > time) { // token still not expired
        return row[0]; // Invalid read of size 8
    } else { // amortized cost; need to get new API token
        snprintf(query, sizeof(query), "SELECT client_id, client_secret, username, password FROM api WHERE id = '%d'", api_id);
        row = fetch_row(conn, query);
        char post_data[MED_CONTAINER];
        snprintf(post_data, sizeof(post_data), "client_id=%s&client_secret=%s&username=%s&password=%s&grant_type=password", row[0], row[1], row[2], row[3]);
        struct MemoryStruct response = http_request(TOKEN_URL, post_data, 0, 1, "");
        cJSON *root = cJSON_Parse(response.memory);
        cJSON *access_token = cJSON_GetObjectItem(root, "access_token");
        char *api_token = access_token->valuestring;
        snprintf(query, sizeof(query), "UPDATE api SET token = '%s', token_updated = %d WHERE id = %d", api_token, time, api_id);
        if (mysql_query(conn, query)) { // do this even if READONLY_MODE is on bc it cant hurt to update the api token
            error(mysql_error(conn), conn);
        }
        free(response.memory);
        cJSON_Delete(root);
        return api_token;
    }
}

/**
 * Updates a meter
 * this function has way too many parameters, but it's better than globals
 * @param conn
 * @param meter_id
 * @param meter_url
 * @param resolution
 * @param start_time    the earlier date
 * @param end_time      the later date
 */
void update_meter(MYSQL *conn, int meter_id, char *meter_url, char *api_token, char *resolution, time_t start_time, time_t end_time, int verbose) {
    struct tm *ts;
    char iso8601_end_time[30];
    char iso8601_start_time[30];
    char query[SMALL_CONTAINER];
    ts = localtime(&end_time);
    strftime(iso8601_end_time, sizeof(iso8601_end_time), ISO8601_FORMAT_EST, ts);
    ts = localtime(&start_time);
    strftime(iso8601_start_time, sizeof(iso8601_start_time), ISO8601_FORMAT_EST, ts);
    // Make call to the API for meter data
    char post_data[SMALL_CONTAINER];
    char *encoded_iso8601_start_time = str_replace(iso8601_start_time, ":", "%3A");
    char *encoded_iso8601_end_time = str_replace(iso8601_end_time, ":", "%3A");
    snprintf(post_data, sizeof(post_data), "resolution=%s&start=%s&end=%s", resolution, encoded_iso8601_start_time, encoded_iso8601_end_time);
    free(encoded_iso8601_start_time);
    free(encoded_iso8601_end_time);
    struct MemoryStruct response = http_request(meter_url, post_data, 1, 0, api_token);
    cJSON *root = cJSON_Parse(response.memory);
    if (!cJSON_HasObjectItem(root, "data")) {
        error(response.memory, conn);
    }
    cJSON *data = cJSON_GetObjectItem(root, "data");
    // save new data
    FILE *buffer = fopen(BUFFER_FILE, "a");
    if (buffer == NULL) {
        error("Error opening meter_data buffer", conn);
    }
    int data_size = cJSON_GetArraySize(data);
    double last_non_null = -9999.0; // error value
    for (int i = 0; i < data_size; i++) {
        cJSON *data_point = cJSON_GetArrayItem(data, i);
        cJSON *data_point_val = cJSON_GetObjectItem(data_point, "value");
        cJSON *data_point_time = cJSON_GetObjectItem(data_point, "localtime");
        char val[10];
        if (data_point_val->type == 4) {
            val[0] = '\\'; val[1] = 'N'; val[2] = '\0'; // https://stackoverflow.com/a/2675493
        } else {
            last_non_null = data_point_val->valuedouble;
            snprintf(val, sizeof(val), "%.3f", last_non_null);
        }
        // https://stackoverflow.com/a/1002631/2624391
        struct tm tm = {0};
        time_t epoch = 0;
        if (strptime(data_point_time->valuestring, ISO8601_FORMAT, &tm) != NULL) {
            tm.tm_isdst = -1; // Is DST on? 1 = yes, 0 = no, -1 = unknown
            epoch = mktime(&tm);
        } else {
            error("Unable to parse date", conn);
        }
        fprintf(buffer, "%d,%s,%d,\"%s\"\n", meter_id, val, (int) epoch, resolution);
        if (verbose) {
            printf("%d,%s,%d,\"%s\"\n", meter_id, val, (int) epoch, resolution);
        }
    }
    fclose(buffer);
    free(response.memory);
    cJSON_Delete(root);
    #if UPDATE_CURRENT == 1
    if (last_non_null != -9999.0 && strcmp(resolution, "live") == 0) {
        query[0] = '\0';
        snprintf(query, sizeof(query), "UPDATE meters SET current = %.3f WHERE id = %d", last_non_null, meter_id);
        if (READONLY_MODE == 0 && mysql_query(conn, query)) {
            error(mysql_error(conn), conn);
        }
    }
    #endif
}

int main(int argc, char *argv[]) {
    int argv0size = strlen(argv[0]);
    // data fetched spans from start_time to end_time
    time_t end_time;
    time_t start_time;
    int opt;
    int data_lifespan;
    int move_back_amount;
    int secs_in_res;
    char *target_meter;
    char *update_timestamp_col;
    char tmp[SMALL_CONTAINER];
    // If the -o flag is set, the program will update a single meter instead of looping
    int o_flag = 0;
    // if the -d flag is set, the program will become a true daemon, disconnecting itself from the shell it was started in
    int d_flag = 0;
    // -v flag prints debugging information
    int v_flag = 0;
    // if the -r flag is set with one of "live", "quarterhour", "hour", or "month" the program will fetch the specified resolution
    // when fetching "live" data, the program will always fetch the newest data i.e. data spanning from the last recorded date in our db to now
    // when fetching other resolutions, the program will be checking that all of that data is there, because normally it is calculated based of min data by crons. if all the data is not there, it will fetch it
    char *r_flag = NULL;
    while ((opt = getopt (argc, argv, "r:odv")) != -1) {
        switch (opt) {
            case 'r': // "resolution"
                r_flag = optarg;
                break;
            case 'o': // run "once"
                o_flag = 1;
                break;
            case 'd': // "daemon"
                d_flag = 1;
                break;
            case 'v': // "verbose"
                v_flag = 1;
                break;
        }
    }
    // connect to db
    MYSQL *conn;
    conn = mysql_init(NULL);
    // Connect to database
    if (!mysql_real_connect(conn, DB_SERVER,
    DB_USER, DB_PASS, DB_NAME, 0, NULL, 0)) {
        error(mysql_error(conn), conn);
    }
    // interpret command line input
    if (d_flag) {
        if (v_flag) {
            printf("Can't use -d and -v at same time; ignoring -v flag\n");
            v_flag = 0;
        }
        if (daemon(1, 0) == -1) { // http://man7.org/linux/man-pages/man3/daemon.3.html
            error("Failed to daemonize", conn);
        }
    }
    if (r_flag == NULL) {
        r_flag = "live";
    }
    int live_res = 0;
    if (strcmp(r_flag, "live") == 0) {
        target_meter = LIVE_TARGET_METER;
        update_timestamp_col = UPDATE_LIVE_TIMESTAMP;
        data_lifespan = LIVE_DATA_LIFESPAN;
        move_back_amount = 180; // meant to move meters back in the queue of what's being updated by update_meter() so they don't hold up everything if update_meter() keeps failing for some reason. note that if update_meter() does finish, it pushes the meter to the end of the queue by updating the last_updated_col to the current time otherwise the last_updated_col remains the current time minus this amount.
        secs_in_res = 60;
        live_res = 1;
    } else if (strcmp(r_flag, "quarterhour") == 0) {
        target_meter = QH_TARGET_METER;
        update_timestamp_col = UPDATE_QH_TIMESTAMP;
        data_lifespan = QH_DATA_LIFESPAN;
        move_back_amount = 480;
        secs_in_res = 900;
    } else if (strcmp(r_flag, "hour") == 0) {
        target_meter = HOUR_TARGET_METER;
        update_timestamp_col = UPDATE_HOUR_TIMESTAMP;
        data_lifespan = HOUR_DATA_LIFESPAN;
        move_back_amount = 900;
        secs_in_res = 3600;
    } else if (strcmp(r_flag, "month") == 0) {
        target_meter = MONTH_TARGET_METER;
        update_timestamp_col = UPDATE_MONTH_TIMESTAMP;
        data_lifespan = MONTH_DATA_LIFESPAN;
        move_back_amount = 86400;
        secs_in_res = 2592000;
    } else {
        printf("Please provide a proper resolution via the -r flag\n");
        return 1;
    }
    buildingosd_pid = getpid(); // save this in a global so the children know
    // Insert record of daemon
    char query[SMALL_CONTAINER];
    snprintf(query, sizeof(query), "INSERT INTO daemons (pid, enabled, target_res) VALUES (%d, %d, '%s')", buildingosd_pid, 1, r_flag);
    if (READONLY_MODE == 0 && mysql_query(conn, query)) { // short circuit
        error(mysql_error(conn), conn);
    }
    openlog("buildingosd", LOG_PID, LOG_DAEMON);
    signal(SIGPIPE, catch_signal);
    snprintf(query, sizeof(query), "SELECT enabled FROM daemons WHERE pid = %d", buildingosd_pid); // dont modify query variable again
    while (1) {
        MYSQL_RES *res;
        MYSQL_ROW row;
        MYSQL_ROW meter;
        time_t now = time(NULL);
        if (READONLY_MODE == 0) {
            // if the daemon is 'enabled' in the db
            if (mysql_query(conn, query)) { // this line triggers a SIGPIPE?
                error(mysql_error(conn), conn);
            }
            res = mysql_use_result(conn);
            row = mysql_fetch_row(res);
            if (row == NULL) { // record of daemon does not exist
                error("I should not exist", conn);
            } else if (row[0][0] != '1') {
                // if enabled column turned off, exit
                if (d_flag) {
                    error("Enabled column switched off", conn);
                } else {
                    puts("Enabled column switched off");
                    cleanup(conn);
                }
            }
            mysql_free_result(res);
        }
        // if (live_res) { // make sure the priority meters (i.e. the orbs) are always up to date
        //  if (mysql_query(conn, PRIORITY_METER)) {
        //      error(mysql_error(conn), conn);
        //  }
        //  res = mysql_store_result(conn);
        //  meter = mysql_fetch_row(res);
        //  mysql_free_result(res);
        // }
        // if (live_res == 0 || meter == NULL) { // if the orbs are up to date or we're collecting non-minute resolution data
        meter = fetch_row(conn, target_meter);
        // }
        char meter_url[SMALL_CONTAINER];
        meter_url[0] = '\0';
        int meter_id = atoi(meter[0]);
        char *org_id = meter[1];
        strcat(meter_url, meter[2]);
        strcat(meter_url, "/data");
        int last_updated = atoi(meter[3]);
        snprintf(tmp, sizeof(tmp), "UPDATE daemons SET updating_meter = %d WHERE pid = %d", meter_id, buildingosd_pid);
        if (READONLY_MODE == 0) {
            if (mysql_query(conn, tmp)) {
                error(mysql_error(conn), conn);
            }
        }
        if (last_updated > (now - secs_in_res)) { // if the least up to date meter was last updated secs_in_res seconds ago
            int sleep_time = (int) (secs_in_res / 10);
            now += sleep_time;
            if (live_res) {
                sleep(sleep_time);
            } else {
                return EXIT_SUCCESS;
            }
        }
        // Set start/end time
        if (live_res) {
            // if live res, fetch data spanning from the latest point recorded in the db to now
            end_time = now;
            snprintf(tmp, sizeof(tmp), "SELECT recorded FROM meter_data WHERE meter_id = %d AND resolution = '%s' ORDER BY recorded DESC LIMIT 1", meter_id, r_flag);
            if (mysql_query(conn, tmp)) {
                error(mysql_error(conn), conn);
            }
            res = mysql_store_result(conn);
            row = mysql_fetch_row(res);
            if (row == NULL) { // no data exists for this meter
                start_time = end_time - (time_t) data_lifespan;
            } else {
                start_time = (time_t) atoi(row[0]);
            }
            mysql_free_result(res);
        } else {
            // if other res, only make sure data goes back as far as it's supposed to
            // i.e. fetch data spanning from data_lifespan to the earliest point recorded in the db
            start_time = now - (time_t) data_lifespan;
            snprintf(tmp, sizeof(tmp), "SELECT recorded FROM meter_data WHERE meter_id = %d AND resolution = '%s' ORDER BY recorded ASC LIMIT 1", meter_id, r_flag);
            if (mysql_query(conn, tmp)) {
                error(mysql_error(conn), conn);
            }
            res = mysql_store_result(conn);
            row = mysql_fetch_row(res);
            if (row == NULL) { // no data exists for this meter
                end_time = now;
                mysql_free_result(res);
            } else {
                end_time = (time_t) atoi(row[0]);
                mysql_free_result(res);
                if (end_time < ((now - data_lifespan) + secs_in_res)) { // if the end time goes as far back as we store data for, mark meter as updated and continue
                    snprintf(tmp, sizeof(tmp), update_timestamp_col, (int) now, meter_id);
                    if (READONLY_MODE == 0 && mysql_query(conn, tmp)) {
                        error(mysql_error(conn), conn);
                    }
                    continue;
                }
            }
        }
        pid_t childpid = fork();
        if (childpid == -1) {
            error("Failed to fork", conn);
        } 
        else if (childpid > 0) {
            int status;
            waitpid(childpid, &status, 0);
        } else { // we are the child
            strncpy(argv[0], "bosd_child", argv0size);
            prctl(PR_SET_NAME, "bosd_child", NULL, NULL, NULL);
            signal(SIGPIPE, catch_signal);
            snprintf(tmp, sizeof(tmp), update_timestamp_col, (int) now - move_back_amount, meter_id);
            if (READONLY_MODE == 0 && mysql_query(conn, tmp)) {
                error(mysql_error(conn), conn);
            }
            update_meter(conn, meter_id, meter_url, set_api_token(conn, org_id), r_flag, start_time, end_time, v_flag);
            snprintf(tmp, sizeof(tmp), update_timestamp_col, (int) now, meter_id);
            if (READONLY_MODE == 0 && mysql_query(conn, tmp)) {
                error(mysql_error(conn), conn);
            }
            if (d_flag == 0) {
                printf("Updated meter %d (fetched data from %d to %d)\n", meter_id, (int) start_time, (int) end_time);
            }
            exit(1);
        }
        if (o_flag == 1) {
            break;
        }
    }
    cleanup(conn);
    mysql_close(conn);
    return EXIT_SUCCESS;
}

P.S.

One specific question I have is whether it is a good idea to fork before calling update_meter(). The reasoning for this is that any memory leaks caused by update_meter() (the bulk of the program) will cleaned up when the child exits. Because the daemon aims to live forever, memory leaks are especially problematic.

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You went to some trouble to ensure a daemon is always running, even in the face of failures. This is an oft-reinvented wheel, addressed by several packages, including DJB's daemontools. The simplest approach is a bash script:

while true; do buildingosd; done

This tends to invite disaster, though. If an unusual config or a bug causes rapid failure, it tends to look like a fork bomb. So it is prudent to rate limit forking:

while true
do
    sleep 1
    buildingosd
done

You're not doing the Gentle Reader a favor with how you've formatted those long #define's for PRIORITY_METER and similar. They seem to have a fair amount of copy-n-paste duplication that could be refactored out.

Rather than e.g. seeing 1209600, I'd prefer to see something like:

#define DAY 86400   /* or 24 * 3600, or 24 * 60 * 60, as you wish */
#define QH_DATA_LIFESPAN (14 * DAY)

This is clearly the wrong identifier name: ISO8601_FORMAT_EST. EST is GMT-5 and EDT is GMT-4. It suggests that update_meter() wants to know what the current local offset is.

The choice of a text-based live / quarterhour / hour / month ENUM seems odd. Using a number would have been more natural, and would accommodate the inevitable "fortnight" and "three-month" feature requests down the road.

Putting DB credentials in db.h is one choice. Reading them from a text file at runtime is an attractive alternative.

As I explained above, you might consider making catch_signal() do sleep(1) before it does anything else. Then again, you already have operational experience, so perhaps I'm being too paranoid.

In WriteMemoryCallback(), bytes (or num_bytes) would be a more natural name than realsize. It is usual to wrap malloc() with a routine like safe_malloc() that upon getting null will log a fatal error and call exit(). That makes reasoning about the correctness of code further up the stack much simpler, which matters here since you believe you suffer from leaks. You may want to use a debug malloc library that maintains guard sentinels (canaries) before and after allocated regions, or use the Longbow library which enforces object ownership within a layer.

In http_request() you have a pair of strcat's rescanning the url, where snprintf() could avoid that. You might want to have code that calls exit(1) call your error() routine instead, for consistent logging / debugging.

In cleanup() you seem to believe both processes will die. You could choose to signal the other process and then wait4().

In fetch_row(), not freeing the result seems worrying. Perhaps you want to keep looping until mysql reports no more rows, even when you expect just a single row. I worry that perhaps you're not giving the mysql client code a chance to tidy things up.

In set_api_token() I'm trying to understand the lifetime of row. It looks like the mysql driver allocated it? I don't see it ever being freed.

In update_meter() I have trouble believing we can't know whether we're in DST. Usually you only care upon output. Values on the network and in a DB should typically be timezone free, stored as seconds since midnight GMT 1-Jan-1970.

Your main() is quite long - surely some of it could be placed in helper function(s). Definitely the while (1) part. The strcat's to meter_url could be done by snprintf(). It's not a problem here, it's just an antipattern, since it's too easy for strcat to wind up in a loop so a linear algorithm accidentally becomes quadratic.

Kudos! I see a call to mysql_free_result(). First one. I was wondering when that was going to happen. Oh good, there's a couple of calls. Ok, it looks like your fetch_row() offers an interface that makes it hard for the caller to clean up. Consider having caller allocate storage, and copying into that storage.

You specifically asked about fork(). For the current (leaky) code, it appears it is necessary. Ideally you would address the leak and remove the fork. Sharing the mysqld socket connection between parent and throwaway child doesn't seem ideal.

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How I would simplify the code and make it more maintainable in my opinion:


You have data sets within code. Code should ideally be independent from the data. We can see that target_meter, update_timestamp_col, data_lifespan, move_back_amount, secs_in_res all correlate.

I would extract the data from the functions:

struct meter_data {
  const char * select_sql_string;
  const char * update_sql_string;
  unsigned data_lifespan;
  unsigned move_back_amount;
  unsigned secs_in_res;
}

struct meter_data meter_data_table[] = {
 { LIVE_TARGET_METER, UPDATE_LIVE_TIMESTAMP,    7200, 180,  60 },
 { QH_TARGET_METER,   UPDATE_QH_TIMESTAMP,   1209600, 480, 900 },
 [...]
}

This way changes of the data do not change the executing code, also the data is actually grouped together - you have a chance to spot erroneous data just by pattern recognition (or rather that something disturbs the pattern).

Then we see that the data also correlates with the input parameter. We can include this, too:

struct meter_data {
  const char * r_flag_string;
  const char * select_sql_string;
  const char * update_sql_string;
  unsigned data_lifespan;
  unsigned move_back_amount;
  unsigned secs_in_res;
}

struct meter_data meter_data_table[] = {
 { "live",         LIVE_TARGET_METER, UPDATE_LIVE_TIMESTAMP,    7200, 180,  60 },
 { "quarterhour",  QH_TARGET_METER,   UPDATE_QH_TIMESTAMP,   1209600, 480, 900 },
 [...]
}

Now we can simplify part of the main() function:

int index;
[...]
for(index = 0; index < sizeof(meter_data_table) / sizeof(struct meter_data); index++)
{
  if(strcmp(r_flag, meter_data_table[index].r_flag_string) == 0) {
   break;
  }
}
/* Return 1 if no parameter set for r_flag found */
if(index >= sizeof(meter_data_table) / sizeof(struct meter_data)) return 1;

[...]

meter = fetch_row(conn, meter_data_table[index].select_sql_string);

With this change it's a breeze to adapt data and also to add new resolutions without accidently messing up code that worked before.


this function has way too many parameters, but it's better than globals

Here I would establish something that I call context state. Your functions and daemon run in a certain context that is established.

struct daemon_context
{
  MYSQL *conn;

  int idxMeterData;

  bool runOnce;
  bool debugOutput;
  bool isDaemon;
};

The context is passed (as pointer) to each function so it knows the circumstances it operates under. This also groups relevant data together.

The second context you operate under is a specific meter.

struct meter_context
{
  int  id;
  char url[SMALL_CONTAINER];
  char api_token;
}


void update_meter(struct daemon_context *dctx, struct meter_context *mctx, time_t start_time, time_t end_time);
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