@ -15,6 +15,7 @@
# include <fcntl.h>
# include <sys/stat.h>
# include <time.h>
# include <unistd.h>
# include "pgtz.h"
@ -126,12 +127,19 @@ pg_load_tz(const char *name)
* On most systems , we rely on trying to match the observable behavior of
* the C library ' s localtime ( ) function . The database zone that matches
* furthest into the past is the one to use . Often there will be several
* zones with identical rankings ( since the Olson database assigns multiple
* zones with identical rankings ( since the IANA database assigns multiple
* names to many zones ) . We break ties arbitrarily by preferring shorter ,
* then alphabetically earlier zone names .
*
* Many modern systems use the IANA database , so if we can determine the
* system ' s idea of which zone it is using and its behavior matches our zone
* of the same name , we can skip the rather - expensive search through all the
* zones in our database . This short - circuit path also ensures that we spell
* the zone name the same way the system setting does , even in the presence
* of multiple aliases for the same zone .
*
* Win32 ' s native knowledge about timezones appears to be too incomplete
* and too different from the Olson database for the above matching strategy
* and too different from the IANA database for the above matching strategy
* to be of any use . But there is just a limited number of timezones
* available , so we can rely on a handmade mapping table instead .
*/
@ -150,6 +158,8 @@ struct tztry
time_t test_times [ MAX_TEST_TIMES ] ;
} ;
static bool check_system_link_file ( const char * linkname , struct tztry * tt ,
char * bestzonename ) ;
static void scan_available_timezones ( char * tzdir , char * tzdirsub ,
struct tztry * tt ,
int * bestscore , char * bestzonename ) ;
@ -299,12 +309,19 @@ score_timezone(const char *tzname, struct tztry *tt)
return i ;
}
/*
* Test whether given zone name is a perfect match to localtime ( ) behavior
*/
static bool
perfect_timezone_match ( const char * tzname , struct tztry * tt )
{
return ( score_timezone ( tzname , tt ) = = tt - > n_test_times ) ;
}
/*
* Try to identify a timezone name ( in our terminology ) that best matches the
* observed behavior of the system timezone library . We cannot assume that
* the system TZ environment setting ( if indeed there is one ) matches our
* terminology , so we ignore it and just look at what localtime ( ) returns .
* observed behavior of the system localtime ( ) function .
*/
static const char *
identify_system_timezone ( void )
@ -339,7 +356,7 @@ identify_system_timezone(void)
* way of doing things , but experience has shown that system - supplied
* timezone definitions are likely to have DST behavior that is right for
* the recent past and not so accurate further back . Scoring in this way
* allows us to recognize zones that have some commonality with the Olson
* allows us to recognize zones that have some commonality with the IANA
* database , without insisting on exact match . ( Note : we probe Thursdays ,
* not Sundays , to avoid triggering DST - transition bugs in localtime
* itself . )
@ -374,7 +391,18 @@ identify_system_timezone(void)
tt . test_times [ tt . n_test_times + + ] = t ;
}
/* Search for the best-matching timezone file */
/*
* Try to avoid the brute - force search by seeing if we can recognize the
* system ' s timezone setting directly .
*
* Currently we just check / etc / localtime ; there are other conventions for
* this , but that seems to be the only one used on enough platforms to be
* worth troubling over .
*/
if ( check_system_link_file ( " /etc/localtime " , & tt , resultbuf ) )
return resultbuf ;
/* No luck, so search for the best-matching timezone file */
strlcpy ( tmptzdir , pg_TZDIR ( ) , sizeof ( tmptzdir ) ) ;
bestscore = - 1 ;
resultbuf [ 0 ] = ' \0 ' ;
@ -383,7 +411,7 @@ identify_system_timezone(void)
& bestscore , resultbuf ) ;
if ( bestscore > 0 )
{
/* Ignore Olson 's rather silly "Factory" zone; use GMT instead */
/* Ignore IANA 's rather silly "Factory" zone; use GMT instead */
if ( strcmp ( resultbuf , " Factory " ) = = 0 )
return NULL ;
return resultbuf ;
@ -472,7 +500,7 @@ identify_system_timezone(void)
/*
* Did not find the timezone . Fallback to use a GMT zone . Note that the
* Olson timezone database names the GMT - offset zones in POSIX style : plus
* IANA timezone database names the GMT - offset zones in POSIX style : plus
* is west of Greenwich . It ' s unfortunate that this is opposite of SQL
* conventions . Should we therefore change the names ? Probably not . . .
*/
@ -486,6 +514,94 @@ identify_system_timezone(void)
return resultbuf ;
}
/*
* Examine a system - provided symlink file to see if it tells us the timezone .
*
* Unfortunately , there is little standardization of how the system default
* timezone is determined in the absence of a TZ environment setting .
* But a common strategy is to create a symlink at a well - known place .
* If " linkname " identifies a readable symlink , and the tail of its contents
* matches a zone name we know , and the actual behavior of localtime ( ) agrees
* with what we think that zone means , then we may use that zone name .
*
* We insist on a perfect behavioral match , which might not happen if the
* system has a different IANA database version than we do ; but in that case
* it seems best to fall back to the brute - force search .
*
* linkname is the symlink file location to probe .
*
* tt tells about the system timezone behavior we need to match .
*
* If we successfully identify a zone name , store it in * bestzonename and
* return true ; else return false . bestzonename must be a buffer of length
* TZ_STRLEN_MAX + 1.
*/
static bool
check_system_link_file ( const char * linkname , struct tztry * tt ,
char * bestzonename )
{
# ifdef HAVE_READLINK
char link_target [ MAXPGPATH ] ;
int len ;
const char * cur_name ;
/*
* Try to read the symlink . If not there , not a symlink , etc etc , just
* quietly fail ; the precise reason needn ' t concern us .
*/
len = readlink ( linkname , link_target , sizeof ( link_target ) ) ;
if ( len < 0 | | len > = sizeof ( link_target ) )
return false ;
link_target [ len ] = ' \0 ' ;
# ifdef DEBUG_IDENTIFY_TIMEZONE
fprintf ( stderr , " symbolic link \" %s \" contains \" %s \" \n " ,
linkname , link_target ) ;
# endif
/*
* The symlink is probably of the form " /path/to/zones/zone/name " , or
* possibly it is a relative path . Nobody puts their zone DB directly in
* the root directory , so we can definitely skip the first component ; but
* after that it ' s trial - and - error to identify which path component begins
* the zone name .
*/
cur_name = link_target ;
while ( * cur_name )
{
/* Advance to next segment of path */
cur_name = strchr ( cur_name + 1 , ' / ' ) ;
if ( cur_name = = NULL )
break ;
/* If there are consecutive slashes, skip all, as the kernel would */
do
{
cur_name + + ;
} while ( * cur_name = = ' / ' ) ;
/*
* Test remainder of path to see if it is a matching zone name .
* Relative paths might contain " .. " ; we needn ' t bother testing if the
* first component is that . Also defend against overlength names .
*/
if ( * cur_name & & * cur_name ! = ' . ' & &
strlen ( cur_name ) < = TZ_STRLEN_MAX & &
perfect_timezone_match ( cur_name , tt ) )
{
/* Success! */
strcpy ( bestzonename , cur_name ) ;
return true ;
}
}
/* Couldn't extract a matching zone name */
return false ;
# else
/* No symlinks? Forget it */
return false ;
# endif
}
/*
* Recursively scan the timezone database looking for the best match to
* the system timezone behavior .
@ -586,7 +702,7 @@ static const struct
* HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ CurrentVersion \ Time
* Zones on Windows 10 and Windows 7.
*
* The zones have been matched to Olson timezones by looking at the cities
* The zones have been matched to IANA timezones by looking at the cities
* listed in the win32 display name ( in the comment here ) in most cases .
*/
{
Tom Lane
7 years ago