FIX: Type error in blocking task list, detailed exception message

pydra/engine/submitter.py

@@ -169,13 +169,22 @@ async def expand_workflow(self, wf, rerun=False):
                     await asyncio.sleep(1)
                     if ii > 60:
                         blocked = _list_blocked_tasks(graph_copy)
-                        get_runnable_tasks(graph_copy)
+                        # get_runnable_tasks(graph_copy)  # Uncomment to debug `get_runnable_tasks`
                         raise Exception(
                             "graph is not empty, but not able to get more tasks "
                             "- something may have gone wrong when retrieving the results "
-                            "of predecessor tasks caused by a file-system error or a bug "
-                            "in the internal workflow logic.\n\nBlocked tasks\n-------------\n"
-                            + "\n".join(blocked)
+                            "of predecessor tasks. This could be caused by a file-system "
+                            "error or a bug in the internal workflow logic, but is likely "
+                            "to be caused by the hash of an upstream node being unstable."
+                            " \n\nHash instability can be caused by an input of the node being "
+                            "modified in place, or by psuedo-random ordering of `set` or "
+                            "`frozenset` inputs (or nested attributes of inputs) in the hash "
+                            "calculation. To ensure that sets are hashed consistently you can "
+                            "you can try set the environment variable PYTHONHASHSEED=0 for "
+                            "all processes, but it is best to try to identify where the set "
+                            "objects are occurring and manually hash their sorted elements. "
+                            "(or use list objects instead)"
+                            "\n\nBlocked tasks\n-------------\n" + "\n".join(blocked)
                         )
             for task in tasks:
                 # grab inputs if needed
@@ -307,7 +316,7 @@ def _list_blocked_tasks(graph):
                                 matching_name.append(
                                     f"{saved_tsk.name} ({tsk_work_dir.name})"
                                 )
-                blocking.append(pred, ", ".join(matching_name))
+                blocking.append((pred, ", ".join(matching_name)))
         if blocking:
             blocked.append(
                 f"\n{tsk.name} ({tsk.checksum}) is blocked by "

pydra/engine/tests/test_submitter.py

@@ -1,6 +1,5 @@
 from dateutil import parser
 import re
-import shutil
 import subprocess as sp
 import time
 
@@ -14,10 +13,11 @@
     gen_basic_wf_with_threadcount_concurrent,
 )
 from ..core import Workflow
-from ..submitter import Submitter
+from ..submitter import Submitter, get_runnable_tasks, _list_blocked_tasks
 from ... import mark
 from pathlib import Path
 from datetime import datetime
+from pydra.engine.specs import LazyField
 
 
 @mark.task
@@ -575,3 +575,43 @@ def test_sge_no_limit_maxthreads(tmpdir):
         out_job2_dict["start_time"][0], f"%a %b %d %H:%M:%S %Y"
     )
     assert job_1_endtime > job_2_starttime
+
+
+def test_wf_with_blocked_tasks(tmpdir):
+    wf = Workflow(name="wf_with_blocked_tasks", input_spec=["x"])
+    wf.add(identity(name="taska", x=wf.lzin.x))
+    wf.add(alter_input(name="taskb", x=wf.taska.lzout.out))
+    wf.add(to_tuple(name="taskc", x=wf.taska.lzout.out, y=wf.taskb.lzout.out))
+    wf.set_output([("out", wf.taskb.lzout.out)])
+
+    wf.inputs.x = A(1)
+
+    wf.cache_dir = tmpdir
+
+    with pytest.raises(Exception, match="graph is not empty,"):
+        with Submitter("serial") as sub:
+            sub(wf)
+
+
+class A:
+    def __init__(self, a):
+        self.a = a
+
+    def __hash__(self):
+        return hash(self.a)
+
+
+@mark.task
+def identity(x):
+    return x
+
+
+@mark.task
+def alter_input(x):
+    x.a = 2
+    return x
+
+
+@mark.task
+def to_tuple(x, y):
+    return (x, y)