bpo-40503: PEP 615: Tests and implementation for zoneinfo (GH-19909)

This is the initial implementation of PEP 615, the zoneinfo module,
ported from the standalone reference implementation (see
https://www.python.org/dev/peps/pep-0615/#reference-implementation for a
link, which has a more detailed commit history).

This includes (hopefully) all functional elements described in the PEP,
but documentation is found in a separate PR. This includes:

1. A pure python implementation of the ZoneInfo class
2. A C accelerated implementation of the ZoneInfo class
3. Tests with 100% branch coverage for the Python code (though C code
   coverage is less than 100%).
4. A compile-time configuration option on Linux (though not on Windows)

Differences from the reference implementation:

- The module is arranged slightly differently: the accelerated module is
  `_zoneinfo` rather than `zoneinfo._czoneinfo`, which also necessitates
  some changes in the test support function. (Suggested by Victor
  Stinner and Steve Dower.)
- The tests are arranged slightly differently and do not include the
  property tests. The tests live at test/test_zoneinfo/test_zoneinfo.py
  rather than test/test_zoneinfo.py or test/test_zoneinfo/__init__.py
  because we may do some refactoring in the future that would likely
  require this separation anyway; we may:
        - include the property tests
        - automatically run all the tests against both pure Python and C,
          rather than manually constructing C and Python test classes (similar
          to the way this works with test_datetime.py, which generates C
          and Python test cases from datetimetester.py).
- This includes a compile-time configuration option on Linux (though not
  on Windows); added with much help from Thomas Wouters.
- Integration into the CPython build system is obviously different from
  building a standalone zoneinfo module wheel.
- This includes configuration to install the tzdata package as part of
  CI, though only on the coverage jobs. Introducing a PyPI dependency as
  part of the CI build was controversial, and this is seen as less of a
  major change, since the coverage jobs already depend on pip and PyPI.

Additional changes that were introduced as part of this PR, most / all of
which were backported to the reference implementation:

- Fixed reference and memory leaks

    With much debugging help from Pablo Galindo

- Added smoke tests ensuring that the C and Python modules are built

    The import machinery can be somewhat fragile, and the "seamlessly falls
    back to pure Python" nature of this module makes it so that a problem
    building the C extension or a failure to import the pure Python version
    might easily go unnoticed.

- Adjustments to zoneinfo.__dir__

    Suggested by Petr Viktorin.

- Slight refactorings as suggested by Steve Dower.

- Removed unnecessary if check on std_abbr

    Discovered this because of a missing line in branch coverage.

1 files changed, 36 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 21c47b56358..497d7c191d5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2946,6 +2946,42 @@ esac
 ],
 [AC_MSG_RESULT(default)])
 
+validate_tzpath() {
+    # Checks that each element of hte path is an absolute path
+    if test -z "$1"; then
+        # Empty string is allowed: it indicates no system TZPATH
+        return 0
+    fi
+
+    # Bad paths are those that don't start with /
+    dnl quadrigraphs "@<:@" and "@:>@" produce "[" and "]" in the output
+    if ( echo $1 | grep -qE '(^|:)(@<:@^/@:>@|$)' ); then
+        AC_MSG_ERROR([--with-tzpath must contain only absolute paths, not $1])
+        return 1;
+    fi
+}
+
+TZPATH="/usr/share/zoneinfo:/usr/lib/zoneinfo:/usr/share/lib/zoneinfo:/etc/zoneinfo"
+AC_MSG_CHECKING(for --with-tzpath)
+AC_ARG_WITH(tzpath,
+            AS_HELP_STRING([--with-tzpath=<list of absolute paths separated by pathsep>]
+                           [Select the default time zone search path for zoneinfo.TZPATH]),
+[
+case "$withval" in
+    yes)
+        AC_MSG_ERROR([--with-tzpath requires a value])
+        ;;
+    *)
+        validate_tzpath "$withval"
+        TZPATH="$withval"
+        AC_MSG_RESULT("$withval")
+        ;;
+esac
+],
+[validate_tzpath "$TZPATH"
+ AC_MSG_RESULT("$TZPATH")])
+AC_SUBST(TZPATH)
+
 AC_MSG_CHECKING(for --with-address-sanitizer)
 AC_ARG_WITH(address_sanitizer,
             AS_HELP_STRING([--with-address-sanitizer],