Fix segfault from <attach> mjCwrap#3157
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mar-yan24 wants to merge 1 commit intogoogle-deepmind:mainfrom
Open
Fix segfault from <attach> mjCwrap#3157mar-yan24 wants to merge 1 commit intogoogle-deepmind:mainfrom
mar-yan24 wants to merge 1 commit intogoogle-deepmind:mainfrom
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mar-yan24
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quagla
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Mar 7, 2026
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Summary
Fixes #3152: Accessing MjsTendonPath.MjsWrap Python bindings causes a segfault with a body/attach model
It honestly took me sorta a long time to trace/figure this out but the bug originates in
mjCTendon::CopyFromSpec()insrc/user/user_objects.cc.When the model is loaded using the block, MuJoCo runs CopyList() for each of the tendons:
candidate->model = this;candidate->CopyFromSpec();candidate->ResolveReferences(this);This shows you how the model gets updated, but
CopyFromSpec()never propagated that to themjCWrapobjects inpath[]. This means that each wrap had like a null or old pointer from the child.This solution also wasn't exactly new since the
mjCTendon::SetMode()function already does this; I'm not sure whyCopyFromSpecwasn't doing the same thing.I lowkey decided on omitting the test-cases for this since it is such a small fix if that's ok.