postgresql/src/timezone/README

src/timezone/README

This is a PostgreSQL adapted version of the IANA timezone library from

	https://www.iana.org/time-zones

The latest version of the timezone data and library source code is
available right from that page.  It's best to get the merged file
tzdb-NNNNX.tar.lz, since the other archive formats omit tzdata.zi.
Historical versions, as well as release announcements, can be found
elsewhere on the site.

Since time zone rules change frequently in some parts of the world,
we should endeavor to update the data files before each PostgreSQL
release.  The code need not be updated as often, but we must track
changes that might affect interpretation of the data files.


Time Zone data
==============

We distribute the time zone source data as-is under src/timezone/data/.
Currently, we distribute just the abbreviated single-file format
"tzdata.zi", to reduce the size of our tarballs as well as churn
in our git repo.  Feeding that file to zic produces the same compiled
output as feeding the bulkier individual data files would do.

While data/tzdata.zi can just be duplicated when updating, manual effort
is needed to update the time zone abbreviation lists under tznames/.
These need to be changed whenever new abbreviations are invented or the
UTC offset associated with an existing abbreviation changes.  To detect
if this has happened, after installing new files under data/ do
	make abbrevs.txt
which will produce a file showing all abbreviations that are in current
use according to the data/ files.  Compare this to known_abbrevs.txt,
which is the list that existed last time the tznames/ files were updated.
Update tznames/ as seems appropriate, then replace known_abbrevs.txt
in the same commit.  Usually, if a known abbreviation has changed meaning,
the appropriate fix is to make it refer to a long-form zone name instead
of a fixed GMT offset.

The core regression test suite does some simple validation of the zone
data and abbreviations data (notably by checking that the pg_timezone_names
and pg_timezone_abbrevs views don't throw errors).  It's worth running it
as a cross-check on proposed updates.

When there has been a new release of Windows (probably including Service
Packs), findtimezone.c's mapping from Windows zones to IANA zones may
need to be updated.  We have two approaches to doing this:
1. Consult the CLDR project's windowsZones.xml file, and add any zones
   listed there that we don't have.  Use their "territory=001" mapping
   if there's more than one IANA zone listed.
2. Run the script in src/tools/win32tzlist.pl on a Windows machine
   running the new release, and add any new timezones that it detects.
   (This is not a full substitute for #1, though, as win32tzlist.pl
   can't tell you which IANA zone to map to.)
In either case, never remove any zone names that have disappeared from
Windows, since we still need to match properly on older versions.


Time Zone code
==============

The code in this directory is currently synced with tzcode release 2020d.
There are many cosmetic (and not so cosmetic) differences from the
original tzcode library, but diffs in the upstream version should usually
be propagated to our version.  Here are some notes about that.

For the most part we want to use the upstream code as-is, but there are
several considerations preventing an exact match:

* For readability/maintainability we reformat the code to match our own
conventions; this includes pgindent'ing it and getting rid of upstream's
overuse of "register" declarations.  (It used to include conversion of
old-style function declarations to C89 style, but thank goodness they
fixed that.)

* We need the code to follow Postgres' portability conventions; this
includes relying on configure's results rather than hand-hacked
#defines (see private.h in particular).

* Similarly, avoid relying on <stdint.h> features that may not exist on old
systems.  In particular this means using Postgres' definitions of the int32
and int64 typedefs, not int_fast32_t/int_fast64_t.  Likewise we use
PG_INT32_MIN/MAX not INT32_MIN/MAX.  (Once we desupport all PG versions
that don't require C99, it'd be practical to rely on <stdint.h> and remove
this set of diffs; but that day is not yet.)

* Since Postgres is typically built on a system that has its own copy
of the <time.h> functions, we must avoid conflicting with those.  This
mandates renaming typedef time_t to pg_time_t, and similarly for most
other exposed names.

* zic.c's typedef "lineno" is renamed to "lineno_t", because having
"lineno" in our typedefs list would cause unfortunate pgindent behavior
in some other files where we have variables named that.

* We have exposed the tzload() and tzparse() internal functions, and
slightly modified the API of the former, in part because it now relies
on our own pg_open_tzfile() rather than opening files for itself.

* tzparse() is adjusted to never try to load the TZDEFRULES zone.

* There's a fair amount of code we don't need and have removed,
including all the nonstandard optional APIs.  We have also added
a few functions of our own at the bottom of localtime.c.

* In zic.c, we have added support for a -P (print_abbrevs) switch, which
is used to create the "abbrevs.txt" summary of currently-in-use zone
abbreviations that was described above.


The most convenient way to compare a new tzcode release to our code is
to first run the tzcode source files through a sed filter like this:

    sed -r \
        -e 's/^([ \t]*)\*\*([ \t])/\1 *\2/' \
        -e 's/^([ \t]*)\*\*$/\1 */' \
        -e 's|^\*/| */|' \
        -e 's/\bregister[ \t]//g' \
        -e 's/\bATTRIBUTE_PURE[ \t]//g' \
        -e 's/int_fast32_t/int32/g' \
        -e 's/int_fast64_t/int64/g' \
        -e 's/intmax_t/int64/g' \
        -e 's/INT32_MIN/PG_INT32_MIN/g' \
        -e 's/INT32_MAX/PG_INT32_MAX/g' \
        -e 's/INTMAX_MIN/PG_INT64_MIN/g' \
        -e 's/INTMAX_MAX/PG_INT64_MAX/g' \
        -e 's/struct[ \t]+tm\b/struct pg_tm/g' \
        -e 's/\btime_t\b/pg_time_t/g' \
        -e 's/lineno/lineno_t/g' \

and then run them through pgindent.  (The first three sed patterns deal
with conversion of their block comment style to something pgindent
won't make a hash of; the remainder address other points noted above.)
After that, the files can be diff'd directly against our corresponding
files.  Also, it's typically helpful to diff against the previous tzcode
release (after processing that the same way), and then try to apply the
diff to our files.  This will take care of most of the changes
mechanically.