diff --git a/newsfragments/4078.bugfix b/newsfragments/4078.bugfix
new file mode 100644
index 0000000000..12ca66bcfb
--- /dev/null
+++ b/newsfragments/4078.bugfix
@@ -0,0 +1 @@
+Fix a race condition with SegmentFetcher
\ No newline at end of file
diff --git a/src/allmydata/immutable/downloader/fetcher.py b/src/allmydata/immutable/downloader/fetcher.py
index 4e8b7f9266..130bb93dc2 100644
--- a/src/allmydata/immutable/downloader/fetcher.py
+++ b/src/allmydata/immutable/downloader/fetcher.py
@@ -63,13 +63,14 @@ def __init__(self, node, segnum, k, logparent):
         self._running = True
 
     def stop(self):
-        log.msg("SegmentFetcher(%r).stop" % self._node._si_prefix,
-                level=log.NOISY, parent=self._lp, umid="LWyqpg")
-        self._cancel_all_requests()
-        self._running = False
-        # help GC ??? XXX
-        del self._shares, self._shares_from_server, self._active_share_map
-        del self._share_observers
+        if self._running:
+            log.msg("SegmentFetcher(%r).stop" % self._node._si_prefix,
+                    level=log.NOISY, parent=self._lp, umid="LWyqpg")
+            self._cancel_all_requests()
+            self._running = False
+            # help GC ???
+            del self._shares, self._shares_from_server, self._active_share_map
+            del self._share_observers
 
 
     # called by our parent _Node
diff --git a/src/allmydata/immutable/downloader/node.py b/src/allmydata/immutable/downloader/node.py
index a1ef4b4853..102d29a0a2 100644
--- a/src/allmydata/immutable/downloader/node.py
+++ b/src/allmydata/immutable/downloader/node.py
@@ -132,8 +132,8 @@ def __repr__(self):
     def stop(self):
         # called by the Terminator at shutdown, mostly for tests
         if self._active_segment:
-            self._active_segment.stop()
-            self._active_segment = None
+            seg, self._active_segment = self._active_segment, None
+            seg.stop()
         self._sharefinder.stop()
 
     # things called by outside callers, via CiphertextFileNode. get_segment()
@@ -410,11 +410,11 @@ def want_more_shares(self):
 
     def fetch_failed(self, sf, f):
         assert sf is self._active_segment
+        self._active_segment = None
         # deliver error upwards
         for (d,c,seg_ev) in self._extract_requests(sf.segnum):
             seg_ev.error(now())
             eventually(self._deliver, d, c, f)
-        self._active_segment = None
         self._start_new_segment()
 
     def process_blocks(self, segnum, blocks):
@@ -434,6 +434,7 @@ def _deliver(result):
                     eventually(self._deliver, d, c, result)
             else:
                 (offset, segment, decodetime) = result
+                self._active_segment = None
                 for (d,c,seg_ev) in self._extract_requests(segnum):
                     # when we have two requests for the same segment, the
                     # second one will not be "activated" before the data is
@@ -446,7 +447,6 @@ def _deliver(result):
                     seg_ev.deliver(when, offset, len(segment), decodetime)
                     eventually(self._deliver, d, c, result)
             self._download_status.add_misc_event("process_block", start, now())
-            self._active_segment = None
             self._start_new_segment()
         d.addBoth(_deliver)
         d.addErrback(log.err, "unhandled error during process_blocks",
@@ -533,11 +533,12 @@ def _cancel_request(self, cancel):
         self._segment_requests = [t for t in self._segment_requests
                                   if t[2] != cancel]
         segnums = [segnum for (segnum,d,c,seg_ev,lp) in self._segment_requests]
+
         # self._active_segment might be None in rare circumstances, so make
         # sure we tolerate it
         if self._active_segment and self._active_segment.segnum not in segnums:
-            self._active_segment.stop()
-            self._active_segment = None
+            seg, self._active_segment = self._active_segment, None
+            seg.stop()
             self._start_new_segment()
 
     # called by ShareFinder to choose hashtree sizes in CommonShares, and by
diff --git a/src/allmydata/test/cli/test_run.py b/src/allmydata/test/cli/test_run.py
index ae0f92131c..c7ebcd8c0d 100644
--- a/src/allmydata/test/cli/test_run.py
+++ b/src/allmydata/test/cli/test_run.py
@@ -264,7 +264,7 @@ def test_non_numeric_pid(self):
         self.assertThat(runs, Equals([]))
         self.assertThat(result_code, Equals(1))
 
-    good_file_content_re = re.compile(r"\s[0-9]*\s[0-9]*\s", re.M)
+    good_file_content_re = re.compile(r"\s*[0-9]*\s[0-9]*\s*", re.M)
 
     @given(text())
     def test_pidfile_contents(self, content):