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fix: read the two shapes a shell string hid from the rm guard entry - #927
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The enclosing shell expands an alternative to nothing when its variable is
unset, so `sh -c "rm -rf ${X:+x}/y"` hands the string `rm -rf /y`. The
string was written out with nothing there, so no reading led with a
variable. The lead reading now writes an empty text as `${_}`, a reference
that can be empty, and is asked even when no named reading leads, but only
for a string where such a site has a `/` after it, so the cost bar holds; a
new shape in TestSpellingSetsStayLinear pins that.
A default written as an escaped reference ends at its first `}`:
`sh -c "rm -rf ${X:-\${Y}}/y"` hands the string `rm -rf ${Y}/y`. The
reading's word marks the brace as the variable's, so its marked shape did
not fit the string's word and it was never paired. A word now pairs when
its written text fits, too.
Resolves: iss-2610110025455728
Assisted-by: Claude:claude-opus-5-5
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What
This closes the two gaps #909 left in the
rm-unguarded-variable-pathguard entry. In both, a double-quoted shell string hid the shape from the entry.${X:+x}to nothing, sosh -c "rm -rf ${X:+x}/y"hands the stringrm -rf /y. The string's re-read wrote that spot as nothing, so no reading opened with a variable. The lead reading now writes it as${_}, a reference that can be empty, so the command refuses. That reading only decides whether a word opens with a variable; the value compare never sees it.sh -c "rm -rf ${X:-\${Y}}/y"hands the stringrm -rf ${Y}/y, because the default ends at its first}. The re-read's word placed that brace inside the variable, so its shape did not fit the string's word and was never paired. A word now also pairs when its written text fits.Cost
The lead reading is written out a second time only for a string where a site that can print nothing has a
/after it. Dropping that gate put the guard at 29 units of work per byte against a bar of 24. A new shape inTestSpellingSetsStayLinearpins the gated cost.Review
A Fable review returned SHIP. It ran every verdict case in the guard's tests (1,030 commands) plus 88 crafted shapes against both main and this branch. 36 verdicts changed, all from allow or warn to block, and none the other way. Among them are correct new home-directory refusals:
sh -c "rm -rf ${X:-\${HOME}}/"now refuses asrm-rf-root-or-home.Its two test notes and one older gap it found in
kill-by-searchfollow in a separate change.Resolves: iss-2610110025455728
Refs: iss-2610091942156774
Assisted-by: Claude:claude-opus-5-5