feat(system): allow a newer Bash to skip a fork the 3.0 floor requires - #1362
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bashunit floors at Bash 3.0, so it forks where a newer shell has a
builtin, and there was no way to ship a faster body for the newer shell:
the compatibility rules reject a too-new construct anywhere in src/,
including inside a branch 3.0 never takes. Deliberately blunt, because
the Bash 3.0 job only catches a too-new construct when a test happens to
execute the line.
This is the mechanism, not a speed run. The floor does not move.
src/system/bash.sh declares one flag per version boundary in use, named
for its tier, and a leaf module picks between two bodies with a column-0
if/else. Measured over 100k calls, best of 5, against a single ungated
definition: picking the definition once costs between -0.07 and +0.09
microseconds per call, while a predicate function per call costs +17.9
on 3.0 and +4.3 on 5.3. Selection is free; the pattern already in use is
the slow one. The five existing runtime predicates are correct and stay.
Each compatibility rule now declares its construct's minimum version as
a tier, and a construct is allowed only inside a column-0 gate whose
tier is at least that. Tier 0 means no gate is ever enough: the
parse-time constructs, which kill the file from inside a branch that
shell never takes, and the rules whose boundary is not a version at all
-- `[[ =~ ]]` changed semantics between 3.0 and 3.2, and the
temporary-locale prefix is a 5.3.9 segfault.
Rejected, and why, since both are tempting: a trailing pragma is the
author's claim rather than a structural fact, so honouring it is a
smuggling path by construction; a per-file allow-list lets an ungated
construct elsewhere in the same file ship.
The first gated helper is lpad, chosen because it removes a fork rather
than because a builtin exists: `printf -v` writes into a variable, the
3.0 body has to capture. Both bodies hand the same format and the same
value to the same printf, so the output is identical by construction --
the textbook `${v,,}` example would not be, since it disagrees with `tr`
on non-ASCII (#1351), which is the whole reason a gated helper needs an
equivalence test.
Verified end to end: the build emits both blocks intact, the artifact
parses on 3.0, 3.2, 4.4, 5.2 and 5, and the helper's tests pass on each
with the branch its tier calls for -- `tr`-free `printf -v` above 3.1,
the capture on real 3.00.0.
Two guards the mechanism needs and did not have: every flag a gate names
must be one the flags file declares, or a typo leaves the fast body
unreachable and the fallback in use everywhere, green and slower; and
build_test's duplicate check anchors at column 0, so it cannot see
either body of a gated helper, which is defined twice by design.
Closes #1352
Claude-Session: https://claude.ai/code/session_01EXYWTGLjf7qM8Ru3GakDRm
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🤔 Background
Related #1352
bashunit floors at Bash 3.0, so it forks where a newer shell has a builtin — and there was no way to ship a faster body for the newer shell, because the compatibility rules reject a too-new construct anywhere in
src/, including inside a branch 3.0 never takes.This is the mechanism, not a speed run. The floor does not move.
💡 Changes
src/system/bash.shdeclares one flag per version boundary, named for its tier; a leaf module picks between two bodies with a column-0if/else. Selection at load time costs between -0.07 and +0.09 µs per call, against +17.9 on Bash 3.0 for the runtime-predicate pattern.[[ =~ ]], the 5.3.9 locale segfault). A pragma and a per-file allow-list were both rejected; the commit says why.printf -vwrites into a variable where the 3.0 body must capture. Both bodies hand the same format and value to the sameprintf, so equivalence holds by construction — unlike the textbook${v,,}example, which disagrees withtron non-ASCII (assert_contains_ignore_case: the non-ASCII folding comment is wrong for GNUtr#1351).adrs/adr-013, a parse test over everysrc/file, and two guards the mechanism needed: every gate must name a declared flag, andbuild_test's column-0 duplicate check cannot see either body of a gated helper.https://claude.ai/code/session_01EXYWTGLjf7qM8Ru3GakDRm