Selftest 340 flakes once in 4096 runs: an unreadable pg_config's stamp key keeps 3 hex characters #1478

The bug

pgc_source_stamp_path (test/lib.sh) keys an unreadable pg_config's build stamp on md5(path), cut to 3 hex characters:

_pgc_sp_id="nolib$(printf '%s' "$_pgc_sp_cfg" | md5sum | cut -c1-3)"

That is 4096 values. Selftest 340's check "two unreadable pg_configs do not alias onto one stamp" builds its two broken pg_configs in a random mktemp directory, so they collide about once in 4096 runs. #1476's suites (PG 18) job hit one (run 38855440878):

>> FAIL  two unreadable pg_configs do not alias onto one stamp: got [SAME] want [different]

The same head passed twice locally.

Reproduced deterministically with lib.sh's own function. In /tmp/pgc-collide-870, pg_config.broken and pg_config.broken2 both get /tree/.pgc_source_stamp.0.nolib235: SAME. At 8 characters they differ (23593d63 against 2356c898). The readable branch already keeps 8 characters.

Fix

Keep 8 characters in the nolib branch. Add a selftest arm that uses those two colliding paths, so the property is tested every run, not once in 4096.

🤖 Generated with Claude Code