Add tests for multiprocessing-driven hmmsearch and fix some exit …

…conditions

src/pyhmmer/hmmer/_base.py

@@ -294,9 +294,11 @@ def run(self) -> None:
     def is_killed(self) -> bool:
         try:
             return self.kill_switch.is_set()
-        except BrokenPipeError:
+        except BrokenPipeError:  # the connection was closed already
             return True
-        except ConnectionResetError:
+        except ConnectionResetError:  # the connection was closed already
+            return True
+        except FileNotFoundError:  # the Event manager has been closed already
             return True
 
     def kill(self) -> None:

src/pyhmmer/hmmer/_jackhmmer.py

@@ -157,6 +157,8 @@ def _new_worker(
         query_count: "multiprocessing.Value[int]",  # type: ignore
         kill_switch: threading.Event,
     ) -> _JACKHMMERWorker[_I]:
+        if self.backend != "threading":
+            raise ValueError(f"Invalid backend for `jackhmmer`: {self.backend!r}")
         return _JACKHMMERWorker(
             self.targets,
             query_queue,

src/pyhmmer/tests/test_hmmer.py

@@ -277,7 +277,19 @@ def get_hits(self, hmm, seqs):
     def test_no_queries(self):
         with self.seqs_file("938293.PRJEB85.HG003687", digital=True) as seqs_file:
             seqs = list(seqs_file)
-        hits = pyhmmer.hmmsearch([], seqs)
+        hits = pyhmmer.hmmsearch([], seqs, cpus=1)
+        self.assertIs(None, next(hits, None))
+
+
+class TestHmmsearchProcess(TestHmmsearch, unittest.TestCase):
+    def get_hits(self, hmm, seqs):
+        return list(pyhmmer.hmmsearch(hmm, seqs, cpus=2, backend="multiprocessing"))[0]
+
+    @unittest.skipUnless(resource_files, "importlib.resources not available")
+    def test_no_queries(self):
+        with self.seqs_file("938293.PRJEB85.HG003687", digital=True) as seqs_file:
+            seqs = list(seqs_file)
+        hits = pyhmmer.hmmsearch([], seqs, cpus=2, backend="multiprocessing")
         self.assertIs(None, next(hits, None))