tests: Add tests for interrupting a fetch operation

This test is to help avoid a regression in this area, fetching should
terminate and not retry.

tests/frontend/interruptions.py

+import pytest
+import os
+import signal
+import re
+import time
+import multiprocessing as mp
+from multiprocessing import Process
+from multiprocessing.queues import Queue
+
+from buildstream import _yaml
+from tests.testutils import cli
+
+
+DATA_DIR = os.path.join(
+    os.path.dirname(os.path.realpath(__file__)),
+    "interruptions",
+)
+
+
+def cli_run_in_process(cli, path, args):
+    def do_run(cli, path, args, queue):
+        result = cli.run(project=path, args=args)
+        queue.put(result.output)
+        queue.put(result.stderr)
+
+    queue = mp.Queue()
+    p = mp.Process(target=do_run, args=[cli, path, args, queue])
+    p.start()
+    return queue, p
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_interrupt_fetch(cli, datafiles):
+    path = os.path.join(datafiles.dirname, datafiles.basename)
+
+    queue, proc = cli_run_in_process(cli, path, args=['--on-error', 'terminate',
+                                                      'fetch', 'delaymock.bst'])
+    # Wait a few seconds, fetch should then be in progress. With DelayedMockSource
+    # fetch should take 20s unless terminated
+    time.sleep(3)
+    os.kill(proc.pid, signal.SIGINT)
+
+    # 10 second timeout
+    try:
+        output = queue.get(timeout=10)
+        stderr = queue.get(timeout=10)
+    except mp.queues.Empty:
+        assert False, 'Fetch failed to terminate'
+
+    matches = re.findall(r'FAILURE\s*Fetch', stderr)
+    assert len(matches), "Unexpected success"
+
+    matches = re.findall(r'STATUS\s*Fetch terminating', stderr)
+    assert len(matches) != 0, "Fetch failed to terminate"
+    assert len(matches) == 1, "Fetch attempted to terminate more than once"

tests/frontend/interruptions/elements/delaymock.bst

+kind: import
+
+sources:
+  - kind: delayed
\ No newline at end of file

tests/frontend/interruptions/plugins/delayed.py

+import time
+from buildstream import Source, SourceError, Consistency
+
+
+class DelayedMockSource(Source):
+
+    def configure(self, node):
+        pass
+
+    def preflight(self):
+        pass
+
+    def get_unique_key(self):
+        return {}
+
+    def get_consistency(self):
+        return Consistency.RESOLVED
+
+    def get_ref(self):
+        return None
+
+    def set_ref(self, ref, node):
+        pass
+
+    def fetch(self):
+        LOOP_FOR = 40
+        def prog_percent(prog):
+            return "{}%".format(float(i)/float(LOOP_FOR) * 100)
+        for i in range(LOOP_FOR):
+            time.sleep(0.5)
+            self.status("Mock Source progress {}".format(prog_percent(i)))
+
+    def stage(self, directory):
+        pass
+
+
+def setup():
+    return DelayedMockSource

tests/frontend/interruptions/project.conf

+name: interruptions
+element-path: elements
+plugins:
+- origin: local
+  path: plugins
+  sources:
+    delayed: 0
\ No newline at end of file