setup: when detecting platform, ask the Python Standard Library's platform.dist() before executing lsb_release, and cache the result in global (module) variables

This should make it sufficiently fast, while still giving a better answer on Ubuntu than platform.dist() currently does, and also falling back to lsb_release if platform.dist() says that it doesn't know.

src/allmydata/__init__.py

@@ -28,21 +28,21 @@ _release_cmdline_re = re.compile("(?:Release:)\s*(.*)", re.I)
 _distributor_id_file_re = re.compile("(?:DISTRIB_ID\s*=)\s*(.*)", re.I)
 _release_file_re = re.compile("(?:DISTRIB_RELEASE\s*=)\s*(.*)", re.I)
 
+global _distname,_version
+_distname = None
+_version = None
+
 def get_linux_distro():
     """ Tries to determine the name of the Linux OS distribution name.
 
     First, try to parse a file named "/etc/lsb-release".  If it exists, and
     contains the "DISTRIB_ID=" line and the "DISTRIB_RELEASE=" line, then return
-    the strings parsed from that file.  The reason we try this first is because
-    it is faster than the official method of invoking "lsb_release" (which takes
-    half a second on my high-performance Athlon64 Ubuntu workstation).  Also
-    because some distributions (at least Debian/Ubuntu) have /etc/lsb-release in
-    the "base-files" package (Priority: required) but /usr/bin/lsb_release in
-    the "lsb-release" package (Priority: important), so it is possible that
-    /etc/lsb-release is there even if /usr/bin/lsb_release isn't.
+    the strings parsed from that file.
 
-    If parsing /etc/lsb-release doesn't work, then try to execute "lsb_release",
-    as standardized in 2001:
+    If that doesn't work, then invoke platform.dist().
+
+    If that doesn't work, then try to execute "lsb_release", as standardized in
+    2001:
 
     http://refspecs.freestandards.org/LSB_1.0.0/gLSB/lsbrelease.html
 
@@ -50,14 +50,8 @@ def get_linux_distro():
 
     http://refspecs.freestandards.org/LSB_3.2.0/LSB-Core-generic/LSB-Core-generic/lsbrelease.html
 
-    If executing "lsb_release" raises no exception, and returns exit code 0, and
-    both the "distributor id" and "release" results are non-empty after being
-    stripped of whitespace, then return a two-tuple containing the information
     that lsb_release emitted, as strings.
 
-    If that doesn't work, then invoke platform.dist() and return the first two
-    elements of the tuple returned by that function.
-
     Returns a tuple (distname,version). Distname is what LSB calls a
     "distributor id", e.g. "Ubuntu".  Version is what LSB calls a "release",
     e.g. "8.04".
@@ -67,8 +61,9 @@ def get_linux_distro():
 
     http://bugs.python.org/issue3937
     """
-    _distname = None
-    _version = None
+    global _distname,_version
+    if _distname and _version:
+        return (_distname, _version)
 
     try:
         etclsbrel = open("/etc/lsb-release", "rU")
@@ -86,6 +81,10 @@ def get_linux_distro():
     except EnvironmentError:
             pass
 
+    (_distname, _version) = platform.dist()[:2]
+    if _distname and _version:
+        return (_distname, _version)
+
     try:
         p = subprocess.Popen(["lsb_release", "--all"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
         rc = p.wait()
@@ -105,7 +104,7 @@ def get_linux_distro():
     except EnvironmentError:
         pass
 
-    return platform.dist()[:2]
+    return (_distname,_version)
 
 def get_platform():
     # Our version of platform.platform(), telling us both less and more than the