--- a/mercurial/scmutil.py	Mon Jul 08 13:57:44 2019 -0400
+++ b/mercurial/scmutil.py	Mon Jul 08 14:01:01 2019 -0400
@@ -1541,7 +1541,12 @@
                 pass # we ignore data for nodes that don't exist locally
     finally:
         if proc:
-            proc.communicate()
+            try:
+                proc.communicate()
+            except ValueError:
+                # This happens if we started iterating src and then
+                # get a parse error on a line. It should be safe to ignore.
+                pass
         if src:
             src.close()
     if proc and proc.returncode != 0:

--- a/tests/test-extdata.t	Mon Jul 08 13:57:44 2019 -0400
+++ b/tests/test-extdata.t	Mon Jul 08 14:01:01 2019 -0400
@@ -66,9 +66,14 @@
   > 9de260b1e88e
   > EOF
 
-BUG: this should print the revset parse error
-  $ hg log -qr "extdata(badparse)" 2>&1 | grep ValueError
-  ValueError: Mixing iteration and read methods would lose data
+It might be nice if this error message mentioned where the bad string
+came from (eg line X of extdata source S), but the important thing is
+that we don't crash before we can print the parse error.
+  $ hg log -qr "extdata(badparse)"
+  hg: parse error at 0: not a prefix: +
+  (+---------------------------------------+
+   ^ here)
+  [255]
 
 test template support: