diff --git a/countess/core/cmd.py b/countess/core/cmd.py
index e934a20..a8d0364 100644
--- a/countess/core/cmd.py
+++ b/countess/core/cmd.py
@@ -26,6 +26,7 @@ def main() -> None:
     run(sys.argv[1:])
 
     logging_handler.stop()
+    logging_queue.close()
 
 
 if __name__ == "__main__":
diff --git a/countess/utils/parallel.py b/countess/utils/parallel.py
index 0ccfed8..e0d120d 100644
--- a/countess/utils/parallel.py
+++ b/countess/utils/parallel.py
@@ -1,10 +1,9 @@
 import gc
 import logging
-import threading
-import time
 from multiprocessing import Process, Queue, Value
 from os import cpu_count, getpid
-from queue import Empty
+from queue import Empty, Full
+import time
 from typing import Callable, Iterable
 
 try:
@@ -22,6 +21,37 @@
 logger = logging.getLogger(__name__)
 
 
+class IterableMultiprocessQueue:
+    """This connects a multiprocessing.Queue with a multiprocessing.Value
+    and gives us a queue that multiple reader processes can iterate over and
+    they'll each get a StopIteration when the Queue is both finished *and*
+    empty."""
+
+    def __init__(self, maxsize=3):
+        self.queue = Queue(maxsize=maxsize)
+        self.finished = Value("b", False)
+
+    def put(self, value, timeout=None):
+        if self.finished.value:
+            raise ValueError("IterableMultiprocessQueue Stopped")
+        self.queue.put(value, timeout=timeout)
+
+    def close(self):
+        self.finished.value = True
+        self.queue.close()
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        while True:
+            try:
+                return self.queue.get(timeout=0.1)
+            except Empty as exc:
+                if self.finished.value:
+                    raise StopIteration from exc
+
+
 def multiprocess_map(
     function: Callable[Concatenate[V, P], Iterable[D]], values: Iterable[V], *args: P.args, **kwargs: P.kwargs
 ) -> Iterable[D]:
@@ -37,58 +67,60 @@ def multiprocess_map(function, values, *args, **kwargs):
 
     # Start up several workers.
     nproc = ((cpu_count() or 1) + 1) // 2
-    input_queue: Queue = Queue()
+    input_queue = IterableMultiprocessQueue(maxsize=nproc)
     output_queue: Queue = Queue(maxsize=3)
 
-    # XXX is it actually necessary to have this in a separate thread or
-    # would it be sufficient to add items to input_queue alternately with
-    # removing items from output_queue?
-
-    enqueue_running = Value("b", True)
-
-    def __enqueue():
-        for v in values:
-            input_queue.put(v)
-        enqueue_running.value = False
+    def __process():
+        for data_in in input_queue:
+            for data_out in function(data_in, *args, **(kwargs or {})):
+                output_queue.put(data_out)
+
+                # Make sure large data is disposed of before we
+                # go around for the next loop
+                del data_out
+            del data_in
+            gc.collect()
 
-    thread = threading.Thread(target=__enqueue)
-    thread.start()
+            # Prevent processes from using up all available memory while waiting
+            # XXX this is probably a bad idea
+            while psutil.virtual_memory().percent > 90:
+                logger.warning("PID %d LOW MEMORY %f%%", getpid(), psutil.virtual_memory().percent)
+                time.sleep(1)
 
-    # XXX is this necessary?
-    time.sleep(1)
+    processes = [Process(target=__process, name=f"worker {n}") for n in range(0, nproc)]
+    for p in processes:
+        p.start()
 
-    def __process():
+    # push each of the input values onto the input_queue, if it gets full
+    # then also try to drain the output_queue.
+    for v in values:
         while True:
             try:
-                while True:
-                    # Prevent processes from using up all
-                    # available memory while waiting
-                    # XXX this is probably a bad idea
-                    while psutil.virtual_memory().percent > 90:
-                        logger.warning("PID %d LOW MEMORY alert %f%%", getpid(), psutil.virtual_memory().percent)
-                        time.sleep(1)
-
-                    data_in = input_queue.get(timeout=1)
-                    for data_out in function(data_in, *args, **(kwargs or {})):
-                        output_queue.put(data_out)
-
-                        # Make sure large data is disposed of before we
-                        # go around for the next loop
-                        del data_out
-                    del data_in
+                input_queue.put(v, timeout=0.1)
+                break
+            except Full:
+                try:
+                    yield output_queue.get(timeout=0.1)
+                except Empty:
+                    # Waiting for the next output, might as well tidy up
                     gc.collect()
 
-            except Empty:
-                if not enqueue_running.value:
-                    break
+    # we're finished with input values, so close the input_queue to
+    # signal to all the processes that there will be no new entries
+    # and once the queue is empty they can finish.
+    input_queue.close()
 
-    processes = [Process(target=__process, name=f"worker {n}") for n in range(0, nproc)]
-    for p in processes:
-        p.start()
-
-    while thread.is_alive() or any(p.is_alive() for p in processes):
+    # wait for all processes to finish and yield any data which appears
+    # on the output_queue
+    while any(p.is_alive() for p in processes):
         try:
-            yield output_queue.get(timeout=0.1)
+            while True:
+                yield output_queue.get(timeout=0.1)
         except Empty:
-            # Waiting for the next input, might as well tidy up
+            # Waiting for the next output, might as well tidy up
             gc.collect()
+
+    # once all processes have finished, we can clean up the queue.
+    for p in processes:
+        p.join()
+    output_queue.close()
diff --git a/tests/gui/test_main.py b/tests/gui/test_main.py
index 11731dc..d0ce589 100644
--- a/tests/gui/test_main.py
+++ b/tests/gui/test_main.py
@@ -44,6 +44,7 @@ def test_main():
 
     root.update()
 
+    wrap.destroy()
     root.destroy()
 
 
diff --git a/tests/plugins/test_csv.py b/tests/plugins/test_csv.py
index 3a29209..549851a 100644
--- a/tests/plugins/test_csv.py
+++ b/tests/plugins/test_csv.py
@@ -142,7 +142,7 @@ def test_save_csv_bz2():
 df2 = pd.DataFrame([[10, 11, 12]], columns=["a", "b", "d"])
 
 
-def test_save_csv_multi():
+def test_save_csv_multi(caplog):
     plugin = SaveCsvPlugin()
     plugin.set_parameter("header", True)
     plugin.set_parameter("filename", "tests/output2.csv")
@@ -154,3 +154,4 @@ def test_save_csv_multi():
     with open("tests/output2.csv", "r", encoding="utf-8") as fh:
         text = fh.read()
         assert text == "a,b,c\n1,2,3\n4,5,6\n7,8,9\n10,11,,12\n"
+    assert "Added CSV Column" in caplog.text
diff --git a/tests/test_plugins.py b/tests/test_plugins.py
index 0bbb3fb..a658f1c 100644
--- a/tests/test_plugins.py
+++ b/tests/test_plugins.py
@@ -1,8 +1,11 @@
 import importlib.metadata
 from unittest.mock import patch
 
+import pandas as pd
+import pytest
+
 import countess
-from countess.core.plugins import BasePlugin, get_plugin_classes
+from countess.core.plugins import BasePlugin, FileInputPlugin, PandasProductPlugin, get_plugin_classes
 
 empty_entry_points_dict = {"countess_plugins": []}
 
@@ -31,3 +34,94 @@ def test_get_plugin_classes_wrongparent(caplog):
         with patch("importlib.metadata.EntryPoint.load", lambda x: NoParentPlugin):
             get_plugin_classes()
             assert "not a valid CountESS plugin" in caplog.text
+
+
+class PPP(PandasProductPlugin):
+    version = "0"
+
+    def process_dataframes(self, dataframe1, dataframe2):
+        return dataframe1 + dataframe2
+
+
+def test_product_plugin():
+    ppp = PPP()
+
+    df1 = pd.DataFrame([{"a": 1}])
+    df2 = pd.DataFrame([{"a": 2}])
+    df3 = pd.DataFrame([{"a": 4}])
+    df4 = pd.DataFrame([{"a": 8}])
+    df5 = pd.DataFrame([{"a": 16}])
+
+    ppp.prepare(["source1", "source2"])
+
+    dfs = list(ppp.process(df1, "source1"))
+    assert len(dfs) == 0
+
+    dfs = list(ppp.process(df2, "source1"))
+    assert len(dfs) == 0
+
+    dfs = list(ppp.process(df3, "source2"))
+    assert len(dfs) == 2
+    assert dfs[0]["a"][0] == 5
+    assert dfs[1]["a"][0] == 6
+
+    dfs = list(ppp.process(df4, "source1"))
+    assert len(dfs) == 1
+    assert dfs[0]["a"][0] == 12
+
+    dfs = list(ppp.finished("source1"))
+    assert len(dfs) == 0
+
+    dfs = list(ppp.process(df5, "source2"))
+    assert len(dfs) == 3
+    assert dfs[0]["a"][0] == 17
+    assert dfs[1]["a"][0] == 18
+    assert dfs[2]["a"][0] == 24
+
+    dfs = list(ppp.finished("source2"))
+    assert len(dfs) == 0
+
+
+def test_product_plugin_sources():
+    with pytest.raises(ValueError):
+        ppp = PPP()
+        ppp.prepare(["source1"])
+
+    with pytest.raises(ValueError):
+        ppp = PPP()
+        ppp.prepare(["source1", "source2", "source3"])
+
+    with pytest.raises(ValueError):
+        ppp = PPP()
+        ppp.prepare(["source1", "source2"])
+        list(ppp.process(pd.DataFrame(), "source3"))
+
+    with pytest.raises(ValueError):
+        ppp = PPP()
+        ppp.prepare(["source1", "source2"])
+        list(ppp.finished("source3"))
+
+
+class FIP(FileInputPlugin):
+    version = "0"
+
+    def num_files(self):
+        return 3
+
+    def load_file(self, file_number, row_limit):
+        if row_limit is None:
+            row_limit = 1000000
+        return [f"hello{file_number}"] * row_limit
+
+
+def test_fileinput(caplog):
+    caplog.set_level("INFO")
+    fip = FIP("fip")
+
+    fip.prepare([], 1000)
+    data = list(fip.finalize())
+
+    assert len(data) >= 999
+    assert sorted(set(data)) == ["hello0", "hello1", "hello2"]
+
+    assert "100%" in caplog.text