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perf(coverage): the trap caches die with every test subshell and get rebuilt #1104
Description
Problem
record_line caches two decisions per file — whether it is tracked, and its
normalized path — in shell variables. The test body runs in a subshell, so
those variables are a copy that dies with it, and the next test rebuilds them
for every file it touches.
Instrumenting one run of tests/unit/assert/basic_test.sh against
--coverage-paths src: 5718 record_line calls, 1427 of them cache misses
over only 13 distinct files. Thirteen files, eleven hundred redundant
misses.
A miss is the expensive path (Bash 3.2 arm64, per 500 calls):
should_track |
1130 ms |
normalize_path |
1843 ms (424 ms after #1102) |
lookup_put x2 |
62 ms |
Proposal
Warm the caches in the parent, before the first test subshell, from the
tracked list seed_tracked_files already builds (#1053). A subshell inherits
the parent's variables, so every worker starts with the decisions already
made and the miss path runs about thirteen times per run instead of 1427.
The spelling matters: the trap keys on whatever BASH_SOURCE holds, which is
how the file was sourced — often ./src/x.sh rather than the absolute path
the seed list carries. Warming both spellings covers the common cases, and a
miss still falls through to the current path, so nothing breaks when a file
arrives spelled some third way.
Related
#1102 makes each miss ~4x cheaper; this removes most of them entirely. The two
compose.
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Chemaclass commentedon Aug 13, 2026
Found the concrete mechanism, and it is cheaper to fix than warming.
should_trackalready has a disk cache, and it reads it like this:That is a command substitution plus
grepplushead— three processes perlookup — and it runs on every in-memory miss, which is 1427 times per run of
one test file because the in-memory cache dies with each test subshell.
Measured on Bash 3.2 arm64, a 13-entry cache, 500 lookups:
grep | headper lookup2 ms versus 0.064 ms. At 1427 lookups that is ~2.85s per run — essentially all
of the recording time left after #1102.
Same anti-pattern #1076 removed from the stats cache: a cache that costs more
than the computation it replaces.
Plan
Read
cache.datonce per process into parallel arrays on first use, then matchin memory; append to the file as new decisions are made, as today. That keeps
the cross-process sharing that the file exists for (parallel workers), removes
the per-lookup forks, and needs no knowledge of the tracked list up front —
which matters because seeding happens at report time (#1053), so the parent has
no file list to warm from when the trap starts.
Chemaclass commentedon Aug 13, 2026
Done in #1109 (merged).