bpo-38021: Modify AIX platform_tag so it covers PEP 425 needs (GH-17303)

Provides a richer platform tag for AIX that we expect to be sufficient for PEP 425
binary distribution identification. Any backports to earlier Python versions will be
handled via setuptools.

Patch by Michael Felt.

Lib/_aix_support.py

@@ -0,0 +1,99 @@
+"""Shared AIX support functions."""
+
+import sys
+from sysconfig import get_config_var
+
+# subprocess is not necessarily available early in the build process
+# if not available, the config_vars are also definitely not available
+# supply substitutes to bootstrap the build
+try:
+    import subprocess
+    _have_subprocess = True
+    _tmp_bd = get_config_var("AIX_BUILDDATE")
+    _bgt = get_config_var("BUILD_GNU_TYPE")
+except ImportError:  # pragma: no cover
+    _have_subprocess = False
+    _tmp_bd = None
+    _bgt = "powerpc-ibm-aix6.1.7.0"
+
+# if get_config_var("AIX_BUILDDATE") was unknown, provide a substitute,
+# impossible builddate to specify 'unknown'
+_MISSING_BD = 9898
+try:
+    _bd = int(_tmp_bd)
+except TypeError:
+    _bd = _MISSING_BD
+
+# Infer the ABI bitwidth from maxsize (assuming 64 bit as the default)
+_sz = 32 if sys.maxsize == (2**31-1) else 64
+
+
+def _aix_tag(vrtl, bd):
+    # type: (List[int], int) -> str
+    # vrtl[version, release, technology_level]
+    return "aix-{:1x}{:1d}{:02d}-{:04d}-{}".format(vrtl[0], vrtl[1], vrtl[2], bd, _sz)
+
+
+# extract version, release and technology level from a VRMF string
+def _aix_vrtl(vrmf):
+    # type: (str) -> List[int]
+    v, r, tl = vrmf.split(".")[:3]
+    return [int(v[-1]), int(r), int(tl)]
+
+
+def _aix_bosmp64():
+    # type: () -> Tuple[str, int]
+    """
+    Return a Tuple[str, int] e.g., ['7.1.4.34', 1806]
+    The fileset bos.mp64 is the AIX kernel. It's VRMF and builddate
+    reflect the current ABI levels of the runtime environment.
+    """
+    if _have_subprocess:
+        # We expect all AIX systems to have lslpp installed in this location
+        out = subprocess.check_output(["/usr/bin/lslpp", "-Lqc", "bos.mp64"])
+        out = out.decode("utf-8").strip().split(":")  # type: ignore
+        # Use str() and int() to help mypy see types
+        return str(out[2]), int(out[-1])
+    else:
+        from os import uname
+
+        osname, host, release, version, machine = uname()
+        return "{}.{}.0.0".format(version, release), _MISSING_BD
+
+
+def aix_platform():
+    # type: () -> str
+    """
+    AIX filesets are identified by four decimal values: V.R.M.F.
+    V (version) and R (release) can be retreived using ``uname``
+    Since 2007, starting with AIX 5.3 TL7, the M value has been
+    included with the fileset bos.mp64 and represents the Technology
+    Level (TL) of AIX. The F (Fix) value also increases, but is not
+    relevant for comparing releases and binary compatibility.
+    For binary compatibility the so-called builddate is needed.
+    Again, the builddate of an AIX release is associated with bos.mp64.
+    AIX ABI compatibility is described  as guaranteed at: https://www.ibm.com/\
+    support/knowledgecenter/en/ssw_aix_72/install/binary_compatability.html
+
+    For pep425 purposes the AIX platform tag becomes:
+    "aix-{:1x}{:1d}{:02d}-{:04d}-{}".format(v, r, tl, builddate, bitsize)
+    e.g., "aix-6107-1415-32" for AIX 6.1 TL7 bd 1415, 32-bit
+    and, "aix-6107-1415-64" for AIX 6.1 TL7 bd 1415, 64-bit
+    """
+    vrmf, bd = _aix_bosmp64()
+    return _aix_tag(_aix_vrtl(vrmf), bd)
+
+
+# extract vrtl from the BUILD_GNU_TYPE as an int
+def _aix_bgt():
+    # type: () -> List[int]
+    assert _bgt
+    return _aix_vrtl(vrmf=_bgt)
+
+
+def aix_buildtag():
+    # type: () -> str
+    """
+    Return the platform_tag of the system Python was built on.
+    """
+    return _aix_tag(_aix_bgt(), _bd)