postgresql_user: Add support for PUBLIC

This change enables users to add and remove permissions to all roles,
using the PUBLIC user. This is equivalent to using the PUBLIC keyword
in GRANT and DENY postgres SQL commands.

For example, see: <http://www.postgresql.org/docs/current/interactive/sql-grant.html>

Fixes #1833

postgresql_user

@@ -129,6 +129,9 @@ else:
 
 
 def user_exists(cursor, user):
+    # The PUBLIC user is a special case that is always there
+    if user == 'PUBLIC':
+        return True
     query = "SELECT rolname FROM pg_roles WHERE rolname=%(user)s"
     cursor.execute(query, {'user': user})
     return cursor.rowcount > 0
@@ -144,6 +147,14 @@ def user_alter(cursor, user, password, role_attr_flags):
     """Change user password"""
     changed = False
 
+    if user == 'PUBLIC':
+        if password is not None:
+            module.fail_json(msg="cannot change the password for PUBLIC user")
+        elif role_attr_flags != '':
+            module.fail_json(msg="cannot change the role_attr_flags for PUBLIC user")
+        else:
+            return False
+
     # Handle passwords.
     if password is not None or role_attr_flags is not None:
         # Select password and all flag-like columns in order to verify changes.
@@ -241,14 +252,20 @@ def has_database_privilege(cursor, user, db, priv):
 
 def grant_database_privilege(cursor, user, db, priv):
     prev_priv = get_database_privileges(cursor, user, db)
-    query = 'GRANT %s ON DATABASE \"%s\" TO \"%s\"' % (priv, db, user)
+    if user == "PUBLIC":
+        query = 'GRANT %s ON DATABASE \"%s\" TO PUBLIC' % (priv, db)
+    else:
+        query = 'GRANT %s ON DATABASE \"%s\" TO \"%s\"' % (priv, db, user)
     cursor.execute(query)
     curr_priv = get_database_privileges(cursor, user, db)
     return len(curr_priv) > len(prev_priv)
 
 def revoke_database_privilege(cursor, user, db, priv):
     prev_priv = get_database_privileges(cursor, user, db)
-    query = 'REVOKE %s ON DATABASE \"%s\" FROM \"%s\"' % (priv, db, user)
+    if user == "PUBLIC":
+        query = 'REVOKE %s ON DATABASE \"%s\" FROM PUBLIC' % (priv, db)
+    else:
+        query = 'REVOKE %s ON DATABASE \"%s\" FROM \"%s\"' % (priv, db, user)
     cursor.execute(query)
     curr_priv = get_database_privileges(cursor, user, db)
     return len(curr_priv) < len(prev_priv)