Support timezone abbreviations that sometimes change.

Up to now, PG has assumed that any given timezone abbreviation (such as
"EDT") represents a constant GMT offset in the usage of any particular
region; we had a way to configure what that offset was, but not for it
to be changeable over time.  But, as with most things horological, this
view of the world is too simplistic: there are numerous regions that have
at one time or another switched to a different GMT offset but kept using
the same timezone abbreviation.  Almost the entire Russian Federation did
that a few years ago, and later this month they're going to do it again.
And there are similar examples all over the world.

To cope with this, invent the notion of a "dynamic timezone abbreviation",
which is one that is referenced to a particular underlying timezone
(as defined in the IANA timezone database) and means whatever it currently
means in that zone.  For zones that use or have used daylight-savings time,
the standard and DST abbreviations continue to have the property that you
can specify standard or DST time and get that time offset whether or not
DST was theoretically in effect at the time.  However, the abbreviations
mean what they meant at the time in question (or most recently before that
time) rather than being absolutely fixed.

The standard abbreviation-list files have been changed to use this behavior
for abbreviations that have actually varied in meaning since 1970.  The
old simple-numeric definitions are kept for abbreviations that have not
changed, since they are a bit faster to resolve.

While this is clearly a new feature, it seems necessary to back-patch it
into all active branches, because otherwise use of Russian zone
abbreviations is going to become even more problematic than it already was.
This change supersedes the changes in commit 513d06ded et al to modify the
fixed meanings of the Russian abbreviations; since we've not shipped that
yet, this will avoid an undesirably incompatible (not to mention incorrect)
change in behavior for timestamps between 2011 and 2014.

This patch makes some cosmetic changes in ecpglib to keep its usage of
datetime lookup tables as similar as possible to the backend code, but
doesn't do anything about the increasingly obsolete set of timezone
abbreviation definitions that are hard-wired into ecpglib.  Whatever we
do about that will likely not be appropriate material for back-patching.
Also, a potential free() of a garbage pointer after an out-of-memory
failure in ecpglib has been fixed.

This patch also fixes pre-existing bugs in DetermineTimeZoneOffset() that
caused it to produce unexpected results near a timezone transition, if
both the "before" and "after" states are marked as standard time.  We'd
only ever thought about or tested transitions between standard and DST
time, but that's not what's happening when a zone simply redefines their
base GMT offset.

In passing, update the SGML documentation to refer to the Olson/zoneinfo/
zic timezone database as the "IANA" database, since it's now being
maintained under the auspices of IANA.

1 files changed, 14 insertions, 11 deletions

diff --git a/src/timezone/tznames/README b/src/timezone/tznames/README
index 6cb0ae88c9..c80caa3786 100644
--- a/src/timezone/tznames/README
+++ b/src/timezone/tznames/README
@@ -6,26 +6,29 @@ tznames
 This directory contains files with timezone sets for PostgreSQL.  The problem
 is that time zone abbreviations are not unique throughout the world and you
 might find out that a time zone abbreviation in the `Default' set collides
-with the one you wanted to use.  All other files except for `Default' are
-intended to override values from the `Default' set.  So you might already have
-a file here that serves your needs.  If not, you can create your own.
+with the one you wanted to use.  This can be fixed by selecting a timezone
+set that defines the abbreviation the way you want it.  There might already
+be a file here that serves your needs.  If not, you can create your own.
 
 In order to use one of these files, you need to set
 
    timezone_abbreviations = 'xyz'
 
 in any of the usual ways for setting a parameter, where xyz is the filename
-that contains the desired time zone names.
+that contains the desired time zone abbreviations.
 
-If you do not find an appropriate set of time zone names for your geographic
+If you do not find an appropriate set of abbreviations for your geographic
 location supplied here, please report this to <pgsql-hackers@postgresql.org>.
-Your set of time zone names can then be included in future releases.
+Your set of time zone abbreviations can then be included in future releases.
 For the time being you can always add your own set.
 
+Typically a custom abbreviation set is made by including the `Default' set
+and then adding or overriding abbreviations as necessary.  For examples,
+see the `Australia' and `India' files.
+
 The files named Africa.txt, etc, are not intended to be used directly as
 time zone abbreviation files. They contain reference definitions of time zone
-names that can be copied into a custom abbreviation file as needed.
-
-Note that these files (*.txt) are already a subset of the zic timezone
-database files: we tried to list only those time zones that (according to
-the zic timezone database) appear to be still in use.
+abbreviations that can be copied into a custom abbreviation file as needed.
+Note that these files (*.txt) are already a subset of the IANA timezone
+database files: we tried to list only those time zone abbreviations that
+(according to the IANA timezone database) appear to be still in use.