test: an unreadable pg_config's stamp key keeps 8 hex characters, not 3 (#1478)

test/check_ledger.tsv

@@ -1355,6 +1355,7 @@ harness_selftest	340-the-binary-must-be-built-from	the stamp writer reports fail
 harness_selftest	340-the-binary-must-be-built-from	the writer writes the file the reader looks for	15;16;17;18;19	never	-
 harness_selftest	340-the-binary-must-be-built-from	two installations of one major get different stamp paths	15;16;17;18;19	never	-
 harness_selftest	340-the-binary-must-be-built-from	two unreadable pg_configs do not alias onto one stamp	15;16;17;18;19	never	-
+harness_selftest	340-the-binary-must-be-built-from	two unreadable pg_configs whose short hashes collide still get different stamps	15;16;17;18;19	never	-
 harness_selftest	350-the-pytest-corpus-must-be	and derives the pins from requirements-test.txt	15;16;17;18;19	never	-
 harness_selftest	350-the-pytest-corpus-must-be	and it derives the file list rather than repeating it	15;16;17;18;19	never	-
 harness_selftest	350-the-pytest-corpus-must-be	and the job asserts the driver is absent rather than assuming it	15;16;17;18;19	never	-

test/check_ledger_budget.txt

@@ -3277,4 +3277,12 @@ suites_not_covered 201
 #     never         2923 + 18 = 2941
 #     dated         1125 + 35 = 1160
 #     2941 + 1160 == 4101
-checks_never_observed_red 2941
+# #1478, selftest 340 gains an arm for two unreadable pg_configs whose short hashes
+# collide (jdatcmd): 2941 -> 2942. The arm is new and has not been seen red on a
+# merged tree, so it enters as never observed red. No row is renamed or removed.
+#
+#     ledger rows   4101 + 1 = 4102
+#     never         2941 + 1 = 2942
+#     dated         1160 + 0 = 1160
+#     2942 + 1160 == 4102
+checks_never_observed_red 2942

test/lib.sh

@@ -1221,11 +1221,13 @@ pgc_source_stamp_path() {	# pgc_source_stamp_path DIR PG_CONFIG
 	_pgc_sp_lib="$("$_pgc_sp_cfg" --pkglibdir 2>/dev/null)"
 	# An unreadable pg_config gets a key that matches nothing rather than one
 	# every broken config shares: `unknown` would alias them together, which is
-	# the defect being fixed, one level down.
+	# the defect being fixed, one level down. Eight hex characters, as the
+	# readable branch keeps (#1478): three aliased two broken configs once in
+	# about 4096 runs, and selftest 340 failed CI on it.
 	if [ -n "$_pgc_sp_lib" ]; then
 		_pgc_sp_id="$(printf '%s' "$_pgc_sp_lib" | md5sum | cut -c1-8)"
 	else
-		_pgc_sp_id="nolib$(printf '%s' "$_pgc_sp_cfg" | md5sum | cut -c1-3)"
+		_pgc_sp_id="nolib$(printf '%s' "$_pgc_sp_cfg" | md5sum | cut -c1-8)"
 	fi
 	printf '%s/.pgc_source_stamp.%s.%s\n' \
 		"$_pgc_sp_dir" "${_pgc_sp_major:-0}" "$_pgc_sp_id"

test/selftest/340-the-binary-must-be-built-from.sh

@@ -410,6 +410,16 @@ check "and two pg_configs for one prefix share a stamp, keyed on pkglibdir" \
 # the aliasing defect one level down.
 printf '#!/bin/sh\nexit 1\n' > "$_fs_cfg/pg_config.broken"; chmod 755 "$_fs_cfg/pg_config.broken"
 printf '#!/bin/sh\nexit 1\n' > "$_fs_cfg/pg_config.broken2"; chmod 755 "$_fs_cfg/pg_config.broken2"
+# AND NOT ONE TIME IN 4096 (#1478). The check above uses a random directory, so a
+# key cut too short aliases the two only now and then: three hex characters failed
+# it once in about 4096 runs. These two paths collide on their first three md5 hex
+# characters (235) and differ at eight, so a three-character key aliases them on
+# every run. They need not exist: the key hashes the path string, and a pg_config
+# that cannot run is exactly the unreadable case.
+check "two unreadable pg_configs whose short hashes collide still get different stamps" \
+	"$([ "$(pgc_source_stamp_path /tree /tmp/pgc-collide-870/pg_config.broken)" \
+	    != "$(pgc_source_stamp_path /tree /tmp/pgc-collide-870/pg_config.broken2)" ] \
+	    && echo different || echo SAME)" "different"
 check "two unreadable pg_configs do not alias onto one stamp" \
 	"$([ "$(pgc_source_stamp_path /tree "$_fs_cfg/pg_config.broken")" \
 	    != "$(pgc_source_stamp_path /tree "$_fs_cfg/pg_config.broken2")" ] \
@@ -430,7 +440,7 @@ unset _fs_bad _fs_ok _fs_rp _fs_before _fs_cfg _fs_n _fs_pa _fs_pb
 # What the mismatch costs, measured by driving the real function both ways:
 #
 #     correct (PG_CONFIG):     /t/.pgc_source_stamp.18.603d6145
-#     a major passed instead:  /t/.pgc_source_stamp.0.nolib6f4
+#     a major passed instead:  /t/.pgc_source_stamp.0.nolib6f4922f4
 #
 # `pgc_major_of 18` finds no version in the string "18", so the major becomes 0
 # and the id becomes a hash of the literal "18". The writer and the reader then