postgresql_ext: fix module's failing when available ext versions contain a pure string (#1099)

* postgresql_ext: fix module's failing when available ext versions contain a pure string

* Add unit tests

* Add changelog fragment

* fix

plugins/modules/database/postgresql/postgresql_ext.py

@@ -106,6 +106,7 @@ notes:
   then PostgreSQL must also be installed on the remote host.
 - For Ubuntu-based systems, install the C(postgresql), C(libpq-dev),
   and C(python-psycopg2) packages on the remote host before using this module.
+- Incomparable versions, for example PostGIS ``unpackaged``, cannot be installed.
 requirements: [ psycopg2 ]
 author:
 - Daniel Schep (@dschep)
@@ -286,16 +287,40 @@ def ext_get_versions(cursor, ext):
     cursor.execute(query, {'ext': ext})
     res = cursor.fetchall()
 
-    available_versions = [
-        line['version']
-        for line in res
-        if LooseVersion(line['version']) > LooseVersion(current_version)
-    ]
+    available_versions = parse_ext_versions(current_version, res)
 
     if current_version == '0':
         current_version = False
 
-    return (current_version, sorted(available_versions, key=LooseVersion))
+    return (current_version, available_versions)
+
+
+def parse_ext_versions(current_version, ext_ver_list):
+    """Parse ext versions.
+
+    Args:
+      current_version (str) -- version to compare elements of ext_ver_list with
+      ext_ver_list (list) -- list containing dicts with versions
+
+    Return a sorted list with versions that are higher than current_version.
+
+    Note: Incomparable versions (e.g., postgis version "unpackaged") are skipped.
+    """
+    available_versions = []
+
+    for line in ext_ver_list:
+        if line['version'] == 'unpackaged':
+            continue
+
+        try:
+            if LooseVersion(line['version']) > LooseVersion(current_version):
+                available_versions.append(line['version'])
+        except Exception:
+            # When a version cannot be compared, skip it
+            # (there's a note in the documentation)
+            continue
+
+    return sorted(available_versions, key=LooseVersion)
 
 # ===========================================
 # Module execution.