test: coverage.sh takes no args, always full serial suite

coverage.sh

@@ -1,8 +1,10 @@
 #!/bin/sh
 # Line coverage of kick.c unioned across the test suite (gcc/gcov route).
 #
-#   ./coverage.sh [pytest args...]   # e.g. ./coverage.sh tests/test_smoke.py::test_cycle_tabs
-#                                    # with no args: full default suite
+#   ./coverage.sh   # always the full default suite, no args: partial runs
+#                   # give a misleading union, and -n/pytest-xdist shares
+#                   # .gcda files across workers (read-merge-write race),
+#                   # so parallel coverage is not supported.
 #
 # Builds build-cov/ instrumented (--coverage + KICK_COVERAGE flush hook for
 # the SIGTERM/_exit teardown path), zeroes counters, runs pytest against
@@ -11,6 +13,11 @@
 set -e
 cd "$(dirname "$0")"
 
+if [ $# -ne 0 ]; then
+    echo "usage: $0 (no arguments; always the full suite)" >&2
+    exit 2
+fi
+
 if [ ! -f build-cov/build.ninja ]; then
     meson setup build-cov -Db_coverage=true "-Dc_args=-DKICK_COVERAGE"
 else
@@ -19,7 +26,7 @@ fi
 meson compile -C build-cov kick wltest
 find build-cov -name '*.gcda' -delete
 
-KICK_BIN="$PWD/build-cov/kick" pytest "$@"
+KICK_BIN="$PWD/build-cov/kick" pytest
 
 echo "--- kick.c line coverage (union of all runs above) ---"
 gcovr -r . --filter 'kick\.c$' --txt