plugin.py: Extend Plugin.call() API

Plugin.Call() now takes fail_temporarily as an optional parameter,
when supplied it will cause subsequent failures to trigger temporary
errors as opposed to permanent errors.

buildstream/_exceptions.py

@@ -136,8 +136,8 @@ class BstError(Exception):
 # or by the base :class:`.Plugin` element itself.
 #
 class PluginError(BstError):
-    def __init__(self, message, reason=None):
-        super().__init__(message, domain=ErrorDomain.PLUGIN, reason=reason)
+    def __init__(self, message, reason=None, temporary=False):
+        super().__init__(message, domain=ErrorDomain.PLUGIN, reason=reason, temporary=False)
 
 
 # LoadErrorReason

buildstream/plugin.py

@@ -478,13 +478,15 @@ class Plugin():
                                            silent_nested=silent_nested):
             yield
 
-    def call(self, *popenargs, fail=None, **kwargs):
+    def call(self, *popenargs, fail=None, fail_temporarily=False, **kwargs):
         """A wrapper for subprocess.call()
 
         Args:
            popenargs (list): Popen() arguments
            fail (str): A message to display if the process returns
                        a non zero exit code
+            fail_temporarily (bool): Whether any exceptions should
+                       be raised as temporary. (*Added in 1.4*)
            rest_of_args (kwargs): Remaining arguments to subprocess.call()
 
         Returns:
@@ -507,7 +509,7 @@ class Plugin():
               "Failed to download ponies from {}".format(
                   self.mirror_directory))
         """
-        exit_code, _ = self.__call(*popenargs, fail=fail, **kwargs)
+        exit_code, _ = self.__call(*popenargs, fail=fail, fail_temporarily=fail_temporarily, **kwargs)
         return exit_code
 
     def check_output(self, *popenargs, fail=None, **kwargs):
@@ -619,7 +621,7 @@ class Plugin():
 
     # Internal subprocess implementation for the call() and check_output() APIs
     #
-    def __call(self, *popenargs, collect_stdout=False, fail=None, **kwargs):
+    def __call(self, *popenargs, collect_stdout=False, fail=None, fail_temporarily=False, **kwargs):
 
         with self._output_file() as output_file:
             if 'stdout' not in kwargs:
@@ -634,7 +636,8 @@ class Plugin():
             exit_code, output = utils._call(*popenargs, **kwargs)
 
             if fail and exit_code:
-                raise PluginError("{plugin}: {message}".format(plugin=self, message=fail))
+                raise PluginError("{plugin}: {message}".format(plugin=self, message=fail),
+                                  temporary=fail_temporarily)
 
         return (exit_code, output)