Implement method for JobCalculation to create a restart builder

The `get_builder` method will return a "clean" builder for the given
`JobProcess`, without any inputs defined. Here we implement `get_builder_restart`
for the `JobCalculation` that will also return a `JobProcessBuilder` but with
the inputs and options pre-set with those that were used for the original node.
The builder can then immediately be resubmitted to run another instance of that
calculation, while of course giving the chance to change one or more inputs.

For example:

    calculation = load_node(<pk>)  # Some completed JobCalculation node
    builder = calculation.get_builder_restart()
    results = run(builder)

aiida/backends/tests/work/job_processes.py

@@ -19,6 +19,7 @@
 from aiida.orm.calculation.job.simpleplugins.templatereplacer import TemplatereplacerCalculation
 from aiida.work.persistence import ObjectLoader
 from aiida.work.job_processes import JobProcess
+from aiida.work.process_builder import JobProcessBuilder
 from aiida.work import Process
 
 from . import utils
@@ -72,9 +73,20 @@ def tearDown(self):
         self.runner = None
         work.set_runner(None)
 
-    def test_class_loader(self):
-        cl = ObjectLoader()
-        TemplatereplacerProcess = JobProcess.build(TemplatereplacerCalculation)
+    def test_job_calculation_process(self):
+        """Verify that JobCalculation.process returns a sub class of JobProcess with correct calculation class."""
+        process = TemplatereplacerCalculation.process()
+        self.assertTrue(issubclass(process, JobProcess))
+        self.assertEqual(process._calc_class, TemplatereplacerCalculation)
+
+    def test_job_calculation_get_builder(self):
+        """Verify that JobCalculation.get_builder() returns an instance of JobProcessBuilder."""
+        process = TemplatereplacerCalculation.process()
+        builder = TemplatereplacerCalculation.get_builder()
+        self.assertTrue(isinstance(builder, JobProcessBuilder))
+
+        # Class objects are actually different memory instances so can't use assertEqual on simply instances
+        self.assertEqual(builder.process_class.__name__, process.__name__)
 
     def test_job_process_set_label_and_description(self):
         """

aiida/backends/tests/work/test_process_builder.py

@@ -110,6 +110,23 @@ def test_dynamic_getters_doc_string(self):
         self.assertEquals(builder.__class__.c.__doc__, str(TestWorkChain.spec().inputs['c']))
         self.assertEquals(builder.c.__class__.d.__doc__, str(TestWorkChain.spec().inputs['c']['d']))
 
+    def test_job_calculation_get_builder_restart(self):
+        """
+        Test the get_builder_restart method of JobCalculation class
+        """
+        from aiida.orm.calculation.job import JobCalculation
+
+        original = JobCalculation()
+        original.set_option('resources', {'num_machines': 1, 'num_mpiprocs_per_machine': 1})
+        original.set_option('max_wallclock_seconds', 1800)
+        original.set_computer(self.computer)
+        original.label = 'original'
+        original.store()
+
+        builder = original.get_builder_restart()
+
+        self.assertDictEqual(builder.options, original.get_options())
+
     def test_code_get_builder(self):
         """
         Test that the get_builder method of Code returns a builder

aiida/orm/implementation/general/calculation/job/__init__.py

@@ -112,13 +112,63 @@ def get_hash(self, ignore_errors=True, ignored_folder_content=('raw_input',), **
 
     @classmethod
     def process(cls):
+        """
+        Return the JobProcess class constructed based on this JobCalculatin class
+
+        :return: JobProcess class
+        """
         from aiida.work.job_processes import JobProcess
         return JobProcess.build(cls)
 
     @classmethod
     def get_builder(cls):
+        """
+        Return a JobProcessBuilder instance, tailored for this calculation class
+
+        This builder is a mapping of the inputs of the JobCalculation class, supports tab-completion, automatic
+        validation when settings values as well as automated docstrings for each input
+
+        :return: JobProcessBuilder instance
+        """
         return cls.process().get_builder()
 
+    def get_builder_restart(self):
+        """
+        Return a JobProcessBuilder instance, tailored for this calculation instance
+
+        This builder is a mapping of the inputs of the JobCalculation class, supports tab-completion, automatic
+        validation when settings values as well as automated docstrings for each input.
+
+        The fields of the builder will be pre-populated with all the inputs recorded for this instance as well as
+        settings all the options that were explicitly set for this calculation instance.
+
+        This builder can then directly be launched again to effectively run a duplicate calculation. But more useful
+        is that it serves as a starting point to, after changing one or more inputs, launch a similar calculation by
+        using this already completed calculation as a starting point.
+
+        :return: JobProcessBuilder instance
+        """
+        from aiida.work.job_processes import JobProcess
+        from aiida.work.ports import PortNamespace
+
+        inputs = self.get_inputs_dict()
+        options = self.get_options()
+        builder = self.get_builder()
+
+        for port_name, port in self.process().spec().inputs.items():
+            if port_name == JobProcess.OPTIONS_INPUT_LABEL:
+                setattr(builder, port_name, options)
+            elif isinstance(port, PortNamespace):
+                namespace = port_name + '_'
+                sub = {link[len(namespace):]: node for link, node in inputs.items() if link.startswith(namespace)}
+                if sub:
+                    setattr(builder, port_name, sub)
+            else:
+                if port_name in inputs:
+                    setattr(builder, port_name, inputs[port_name])
+
+        return builder
+
     def _init_internal_params(self):
         """
         Define here internal parameters that should be defined right after the __init__