better fix for arch version detection (#15705)

* better fix for arch version detection

fixes  #15696

* be extra safe about tracebacks in facts.py

* add comments to explain the setup
* make allowempty more conservative, ignore file content
* wrap function call in try/except
  * should never happen, but if it happens the bug should be distribtion=N/A and not a traceback

lib/ansible/module_utils/facts.py

@@ -604,6 +604,10 @@ class Distribution(object):
     This is unit tested. Please extend the tests to cover all distributions if you have them available.
     """
 
+    # every distribution name mentioned here, must have one of
+    #  - allowempty == True
+    #  - be listed in SEARCH_STRING
+    #  - have a function get_distribution_DISTNAME implemented
     OSDIST_LIST = (
         {'path': '/etc/oracle-release', 'name': 'OracleLinux'},
         {'path': '/etc/slackware-version', 'name': 'Slackware'},
@@ -687,11 +691,11 @@ class Distribution(object):
 
                 if not os.path.exists(path):
                     continue
-                if os.path.getsize(path) == 0:
+                # if allowempty is set, we only check for file existance but not content
                 if 'allowempty' in ddict and ddict['allowempty']:
                     self.facts['distribution'] = name
                     break
-                    else:
+                if os.path.getsize(path) == 0:
                     continue
 
                 data = get_file_content(path)
@@ -707,6 +711,7 @@ class Distribution(object):
                     break
                 else:
                     # call a dedicated function for parsing the file content
+                    try:
                         distfunc = getattr(self, 'get_distribution_' + name)
                         parsed = distfunc(name, data, path)
                         if parsed is None or parsed:
@@ -714,6 +719,11 @@ class Distribution(object):
                             # break only if parsing was succesful
                             # otherwise continue with other distributions
                             break
+                    except AttributeError:
+                        # this should never happen, but if it does fail quitely and not with a traceback
+                        pass
+
+
 
                     # to debug multiple matching release files, one can use:
                     # self.facts['distribution_debug'].append({path + ' ' + name:
@@ -780,10 +790,6 @@ class Distribution(object):
         if release:
             self.facts['distribution_release'] = release.groups()[0]
 
-    def get_distribution_Archlinux(self, name, data, path):
-        self.facts['distribution'] = 'Archlinux'
-        self.facts['distribution_version'] = data
-
     def get_distribution_Alpine(self, name, data, path):
         self.facts['distribution'] = 'Alpine'
         self.facts['distribution_version'] = data