BUG #16143: PGTYPEStimestamp_fmt_asc() returns the incorrect month when the format specifier %b is used.

The following bug has been logged on the website:

Bug reference: 16143
Logged by: Paul Spencer
Email address: paul(at)intekon(dot)com
PostgreSQL version: 11.5
Operating system: Redhat and Debian
Description:

PGTYPEStimestamp_fmt_asc() returns the incorrect month when the format
specifier %b is used. The returned month is one greater then the expected
month. If the expected month is “Dec”, the application may crash with a
segment fault. The format specifier %B has a similar issue.

** Investigation Notes
- The month is increased by one at line 143 in timestamp2tm() defined in
timestamp.c 
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/interfaces/ecpg/pgtypeslib/timestamp.c;h=810dd06ee68b9e39bfbb8d1fb4b58b8205f24246;hb=HEAD
- The month number is converted to the abbreviation at line 337 in
dttofmtasc_replace() defined in timestamp.c 
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/interfaces/ecpg/pgtypeslib/timestamp.c;h=810dd06ee68b9e39bfbb8d1fb4b58b8205f24246;hb=HEAD
- Month abbreviations are defined at line 499 in dt_common.c 

https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/interfaces/ecpg/pgtypeslib/dt_common.c;h=c1a3a3e2cb7e2d4f375a3b1a2e858f7347a867ea;hb=HEAD

***
* Use Case
***
pi(at)raspberrypi4:~/projects/postgres_month_date $ ./pg_month_date
PostgreSQL timestamp_fmt_asc()
PGTYPEStimestamp_to_asc returns 1999-01-08 04:06:06
Format string = %Y-%m-%d %H:%M:%S, Formated Date = 1999-01-08 04:06:06
Format string = %Y-%b-%d %H:%M:%S, Formated Date = 1999-Feb-08 04:06:06
pi(at)raspberrypi4:~/projects/postgres_month_date $

***
* Source code for the use case
***
/*
* pg_month_date.c
*/
#include <stdio.h>
#include <string.h>

#include "pgtypes_timestamp.h"

int main(int argc, char **argv)
{
char formatString[255] = "";
char stringBuffer[255] = "";
timestamp testTimestamp;
char * endPtr = NULL;

printf("%s\n","PostgreSQL timestamp_fmt_asc()");

testTimestamp = PGTYPEStimestamp_from_asc("1999-01-08 04:06:06",
&endPtr);
printf("PGTYPEStimestamp_to_asc returns %s\n",
PGTYPEStimestamp_to_asc(testTimestamp));

strncpy(formatString, "%Y-%m-%d %H:%M:%S", sizeof(formatString));
PGTYPEStimestamp_fmt_asc(&testTimestamp, stringBuffer,
sizeof(stringBuffer),formatString);
printf( "Format string = %s, Formated Date = %s\n", formatString ,
stringBuffer);

strncpy(formatString, "%Y-%b-%d %H:%M:%S", sizeof(formatString));
PGTYPEStimestamp_fmt_asc(&testTimestamp, stringBuffer,
sizeof(stringBuffer),formatString);
printf( "Format string = %s, Formated Date = %s\n", formatString ,
stringBuffer);
return 0;
}

Paul Spencer