Do not silently swallow errors in the write_image deploy step

Calling join() does not raise, we need to explicitly check the result.

Change-Id: I81d3d727af220c2b50358edab8139f07874611f0
Story: #2008240
Task: #41083

ironic_python_agent/extensions/base.py

@@ -79,6 +79,17 @@ class BaseCommandResult(encoding.SerializableComparable):
         """:returns: result of completed command."""
         return self
 
+    def wait(self):
+        """Join the result and extract its value.
+
+        Raises if the command failed.
+        """
+        self.join()
+        if self.command_error is not None:
+            raise self.command_error
+        else:
+            return self.command_result
+
 
 class SyncCommandResult(BaseCommandResult):
     """A result from a command that executes synchronously."""

ironic_python_agent/hardware.py

@@ -2139,7 +2139,7 @@ class GenericHardwareManager(HardwareManager):
         ext = ext_base.get_extension('standby')
         cmd = ext.prepare_image(image_info=image_info, configdrive=configdrive)
         # The result is asynchronous, wait here.
-        cmd.join()
+        return cmd.wait()
 
     def generate_tls_certificate(self, ip_address):
         """Generate a TLS certificate for the IP address."""

ironic_python_agent/tests/unit/extensions/test_base.py

@@ -149,6 +149,12 @@ class TestExtensionDecorators(test_base.IronicAgentTest):
                          result.command_result)
         self.agent.force_heartbeat.assert_called_once_with()
 
+    def test_wait_async_command_success(self):
+        result = self.extension.execute('fake_async_command', param='v1')
+        self.assertIsInstance(result, base.AsyncCommandResult)
+        result = result.wait()
+        self.assertEqual({'result': 'fake_async_command: v1'}, result)
+
     def test_async_command_success_without_agent(self):
         extension = FakeExtension(agent=None)
         result = extension.execute('fake_async_command', param='v1')
@@ -182,6 +188,11 @@ class TestExtensionDecorators(test_base.IronicAgentTest):
         self.assertIsNone(result.command_result)
         self.agent.force_heartbeat.assert_called_once_with()
 
+    def test_wait_async_command_execution_failure(self):
+        result = self.extension.execute('fake_async_command', param='v2')
+        self.assertIsInstance(result, base.AsyncCommandResult)
+        self.assertRaises(ExecutionError, result.wait)
+
     def test_async_command_name(self):
         self.assertEqual(
             'other_async_name',

releasenotes/notes/prepare-image-49744276cef719d5.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixes the ``write_image`` deploy step to actually check and return any
+    errors during its execution.