RRULE parsing in Postgresql

Question

The goal: to generate a list of timestamps, each representing an event occurrance, from an RRULE string (i.e. FREQ=WEEKLY;INTERVAL=2;COUNT=8;WKST=SU;BYDAY=TU,TH) and a starting timestamp. Since these times will be changed at the whim of a user, postgresql is desired for (1) it's quality of strict business logic and (2) triggers (auto-update rows when a value changes)

Alternative Solution:

I ended up using plpythonu (python language for postgresql). The dateutil library has a great rrule parser.

mydatabase=# CREATE FUNCTION parse_occurrences(rule text, start timestamp) RETURNS timestamp[] AS
mydatabase-# 'from dateutil.rrule import *
mydatabase'# from dateutil.parser import *
mydatabase'# import datetime
mydatabase'# dates = list(rrulestr(rule, dtstart=parse(start)))
mydatabase'# return [str(a)[0:10] for a in dates]'
mydatabase-# LANGUAGE plpythonu;
CREATE FUNCTION
mydatabase=# SELECT parse_occurrences('FREQ=WEEKLY;INTERVAL=2;COUNT=8;WKST=SU;BYDAY=TU,TH'::text, now()::timestamp);                                                                  
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 {"2013-02-14 00:00:00","2013-02-26 00:00:00","2013-02-28 00:00:00","2013-03-12 00:00:00","2013-03-14 00:00:00","2013-03-26 00:00:00","2013-03-28 00:00:00","2013-04-09 00:00:00"}

The (original) endeavor: to connect the C-library libical with Postgresql C-Extensions. This requires a specialized C program to do the following: (1) convert postgresql data types to C-data types, (2) execute all necessary C-library functions, and (3) return the data in postgresql formats using the header file "postgres.h".

The Connector File: ical_recur.c

#include "postgres.h"
#include "icalrecur.h"
#include <time.h> /* for time() */
#include "fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

PG_FUNCTION_INFO_V1(get_occurrences);

Datum
get_occurrences(PG_FUNCTION_ARGS)
{
        //char*        rule; /* rule string */
        time_t      start; /* start time */
        int         count;
        char        *rrule;

       // *rule = PG_GETARG_CHAR(0);
        start = (time_t) PG_GETARG_INT32(1);
        count = (int) PG_GETARG_INT32(2);
        *rrule = PG_GETARG_CHAR(0);

        time_t   *result[count]; /* output array */

        icalrecur_expand_recurrence(rrule, start, count, *result);

        PG_RETURN_INT32(*result);
}

Preparing the Connector File:

Step 1: Compile to the object file, create the shared object, copy to the location postgresql looks for C-extensions

sudo gcc -I/usr/local/libical/lib -lical -I/usr/include/postgresql/9.2/server -fpic -c ical_recur.c
sudo gcc -shared -L/usr/local/libical/lib -lical -o ical_recur.so ical_recur.o
sudo cp ical_recur.so /usr/lib/postgresql/9.2/lib/

Step 2: Add the libical lib folder to be found by C & reload the configuration

sudo echo "/usr/local/libical/lib" >> /etc/ld.so.conf.d/libc.conf
sudo ldconfig

Testing the Connector File:

Step 1: Load up psql and CREATE the FUNCTION

psql
mydatabase=# CREATE FUNCTION get_occurrences(text, integer, integer) RETURNS int[]
mydatabase=- AS '$libdir/ical_recur', 'get_occurrences'
mydatabase=- LANGUAGE C STRICT;
CREATE FUNCTION
mydatabase=# 

Current Roadblock:

The C-function has crashed the postgresql server.

psql (9.2.3)
Type "help" for help.

mydatabase=# SELECT get_occurrences('FREQ=WEEKLY;INTERVAL=2;COUNT=8;WKST=SU;BYDAY=TU,TH', now()::timestamp, 5);
The connection to the server was lost. Attempting reset: Failed.
!> 

The logs...

2013-02-11 22:03:33 UTC LOG:  server process (PID 22733) was terminated by signal 11: Segmentation fault

2013-02-11 22:03:33 UTC DETAIL:  Failed process was running: SELECT get_occurrences('FREQ=WEEKLY;INTERVAL=2;COUNT=8;WKST=SU;BYDAY=TU,TH', now()::timestamp, 5);
2013-02-11 22:03:33 UTC LOG:  terminating any other active server processes
2013-02-11 22:03:33 UTC WARNING:  terminating connection because of crash of another server process
2013-02-11 22:03:33 UTC DETAIL:  The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
2013-02-11 22:03:33 UTC HINT:  In a moment you should be able to reconnect to the database and repeat your command.
2013-02-11 22:03:33 UTC LOG:  all server processes terminated; reinitializing
2013-02-11 22:03:33 UTC FATAL:  the database system is in recovery mode
2013-02-11 22:03:33 UTC LOG:  database system was interrupted; last known up at 2013-02-11 21:47:26 UTC
2013-02-11 22:03:33 UTC LOG:  database system was not properly shut down; automatic recovery in progress
2013-02-11 22:03:33 UTC LOG:  redo starts at 0/1903A0C
2013-02-11 22:03:33 UTC LOG:  record with zero length at 0/190E1E0
2013-02-11 22:03:33 UTC LOG:  redo done at 0/190E1B8
2013-02-11 22:03:33 UTC LOG:  last completed transaction was at log time 2013-02-11 22:03:29.641161+00
2013-02-11 22:03:33 UTC LOG:  database system is ready to accept connections
2013-02-11 22:03:33 UTC LOG:  autovacuum launcher started

UPDATED:

I've updated the method to address some suggestions.

#include "postgres.h"
#include "icalrecur.h"
#include <time.h> /* for time() */
#include "fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

PG_FUNCTION_INFO_V1(get_occurrences);

Datum
get_occurrences(PG_FUNCTION_ARGS)
{
        time_t start = (time_t) PG_GETARG_INT32(1); /* convert int to time_t */
        int count = PG_GETARG_INT32(2);
        char rrule = PG_GETARG_CHAR(0);

        char *_rrule = &rrule; /* icalrecur wants a pointer to the rrule */

        time_t  result[count]; /* instantiate the output array */

        int success = icalrecur_expand_recurrence(_rrule, start, count, result);

        /* convert time_t values to int */
        int *output = malloc(sizeof(result));

        int i;
        for(i = 0; i < (int) (sizeof(result) / sizeof(result[0]) - 1); i++){
                output[i] = (int) result[i];
        }

        if(success != 1){
                PG_RETURN_INT32(0);
        } else {
                PG_RETURN_INT32(output);
        }
}

Building with..

sudo gcc -Wall -Wextra -l/usr/local/libical/lib/ical -I/usr/local/libical/include/libical -I/usr/include/postgresql/9.2/server -fpic -c ical_recur.c
sudo gcc -Wall -shared -static -L/usr/local/libical/lib -lical -o ical_recur.so ical_recur.o
sudo cp libical.so /usr/lib/postgresql/9.2/lib/

Enter database & execute with..

mydb=# CREATE FUNCTION get_occurrences(text, integer, integer) RETURNS int[]
AS '$libdir/ical_recur', 'get_occurrences'
LANGUAGE C STRICT;
CREATE FUNCTION
mydb=# SELECT get_occurrences('FREQ=WEEKLY;INTERVAL=2;COUNT=8;WKST=SU;BYDAY=TU,TH', 1360690024, 5);
The connection to the server was lost. Attempting reset: Failed.
!> \q

Same log output. The documentation notes the parameters:

int icalrecur_expand_recurrence(char * rule, time_t start, int count, time_t * array)

Question: How can we get this to work without postgres crashing?

Answer 1

There are two obvious pointer-related flaws in the code shown:

First:

    *rrule = PG_GETARG_CHAR(0);

At this point of execution, rrule is a non-initialized pointer, and this instruction tries to write a char into the random location it points to. This generally causes the kind of crash you're seeing.

Second:

time_t   *result[count]; /* output array */

icalrecur_expand_recurrence(rrule, start, count, *result);

result should be an array of time_t, not an array of time_t*, and it should be passed as result, not *result which again is not initialized at this point.

Answer 2

Now you can use this extension for postgresql.

Example usage:

SELECT * FROM
     unnest(
         rrule_get_occurrences('FREQ=WEEKLY;INTERVAL=1;WKST=MO;UNTIL=20200101T045102Z;BYDAY=SA;BYHOUR=10;BYMINUTE=51;BYSECOND=2'::rrule,
             '2019-12-07 10:51:02+00'::timestamp with time zone)
     );

          unnest
 ------------------------
  2019-12-07 10:51:02+00
  2019-12-14 10:51:02+00
  2019-12-21 10:51:02+00
  2019-12-28 10:51:02+00
 (4 rows)