protect new cons cell in named StretchyList::push_back() - #1490
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kevinushey merged 1 commit intoAug 4, 2026
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The named variant of push_back() allocated the new cons cell before constructing the Symbol for its tag. If the tag name was not yet interned, Rf_install() could allocate and trigger a garbage collection that reclaimed the still-unreachable cell. Construct the Symbol first, matching push_front(). Fixes #1489.
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August 4, 2026 23:57
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Fixes #1489.
The named variant of StretchyList::push_back() allocated the new cons cell before constructing the Symbol for its tag. If the tag name was not yet interned, Rf_install() could allocate and trigger a garbage collection that reclaimed the still-unreachable cell; SET_TAG() then wrote into a reclaimed node, which was subsequently linked into the list. See #1489 for a reproducible example via Rcpp::sourceCpp() and gctorture().
The fix constructs the Symbol before calling Rf_cons(), matching the ordering already used by the named variant of push_front().
Also adds a regression test exercising named push_back() with a freshly generated (never-interned) tag name under gctorture(); the test fails without the header change and passes with it.
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