Weird Maya bug that involves referencing shared unknown nodes and threading

README.md

Maya version: 2018 update 6 OS: Windows 10

1. Save those two .ma file in a folder.
2. Open work.ma, it should have upstream.ma referenced.
3. Execute the script crash.py.
4. Save the scene and Maya will crash.

crash.py

import contextlib
import threading
from maya import cmds, utils


@contextlib.contextmanager
def maintained_selection():
    try:
        cmds.ls(selection=True)
        yield
    finally:
        cmds.select(clear=True)


@contextlib.contextmanager
def attribute_locks(attrs, lock):
    original = [(attr, cmds.getAttr(attr, lock=True)) for attr in attrs]
    try:
        for attr in attrs:
            cmds.setAttr(attr, lock=lock)
        yield
    finally:
        for attr, value in original:
            cmds.setAttr(attr, lock=value)


@contextlib.contextmanager
def attribute_values(attr_values):
    original = [(attr, cmds.getAttr(attr)) for attr in attr_values]
    try:
        for attr, value in attr_values.items():
            cmds.setAttr(attr, value)
        yield
    finally:
        for attr, value in original:
            cmds.setAttr(attr, value)


# 1. This was actually an unknown node that was generated via Redshift
#    from the artist who has the plugin loaded, but the next artist who
#    takes over does not have that plugin so became referencing unknown
#    node.
#    And the unknow node was required to be deleted for publish.
cmds.delete("sharedUnknownNode")

# 2. We need at least 12 attributes to cause the crash
node_attrs = {}
for i in range(12):
    node = cmds.group(world=True, empty=True)
    node_attrs[node + ".tx"] = 0


def run():
    # 3. This step was actually processing file texture nodes, changing
    #    file path, colorspace.. and other attributes.
    #    And ensure the attribute not locked before setting them.
    with contextlib.nested(
        maintained_selection(),
        attribute_locks(node_attrs.keys(), lock=False),
        attribute_values(node_attrs),
    ):
        cmds.select(clear=True)


# Only cause the crash in other thread, the above script will not crash if
# executed in main thread.
thd = threading.Thread(target=lambda: utils.executeInMainThreadWithResult(run))
thd.start()


# Once the above thread completed, save the file will crash Maya.
# cmds.file(rename="/any/path/output.ma")
# cmds.file(save=True, force=True)  # <------CRASH

upstream.ma

//Maya ASCII 2018ff09 scene
//Name: upstream.ma
//Last modified: Fri, Feb 07, 2020 05:57:55 PM
//Codeset: 950
createNode unknownA -s -n "sharedUnknownNode";
createNode unknownB -n "upstreamUnknownNode";
connectAttr "upstreamUnknownNode.msg" ":sharedUnknownNode.inmessage";
// End of upstream.ma

work.ma

//Maya ASCII 2018ff09 scene
//Name: work.ma
//Last modified: Fri, Feb 07, 2020 05:59:41 PM
//Codeset: 950
file -rdi 1 -ns "upstream" -rfn "upstreamRN" -op "v=0;" -typ "mayaAscii"
		 "upstream.ma";
file -r -ns "upstream" -dr 1 -rfn "upstreamRN" -op "v=0;" -typ "mayaAscii"
		 "upstream.ma";

createNode unknownA -s -n "sharedUnknownNode";
createNode reference -n "upstreamRN";
	setAttr ".phl[1]" 0;
	setAttr ".ed" -type "dataReferenceEdits" 
		"upstreamRN"
		"upstreamRN" 0
		"upstreamRN" 14
		3 "upstreamRN" "upstream:upstreamUnknownNode.message" ":sharedUnknownNode.inmessage" 
		"";
	setAttr ".ptag" -type "string" "";
lockNode -l 1 ;
// End of work.ma