test(main): pin set_shard_or_exit's --shard spec validation

tests/unit/main_test.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# First unit tests for src/main.sh: pin set_shard_or_exit's validation, which
+# guards --shard's slice arithmetic against off-by-one specs. It exits the
+# shell on invalid input, so every call runs inside a subshell.
+
+function _shard_status() {
+  (
+    bashunit::main::set_shard_or_exit "$1" >/dev/null 2>&1
+    echo "ok $BASHUNIT_SHARD_INDEX/$BASHUNIT_SHARD_TOTAL"
+  ) 2>/dev/null || echo "rejected"
+}
+
+function test_shard_spec_accepts_the_full_range_boundaries() {
+  assert_same "ok 1/4" "$(_shard_status 1/4)"
+  assert_same "ok 4/4" "$(_shard_status 4/4)"
+  assert_same "ok 1/1" "$(_shard_status 1/1)"
+}
+
+function test_shard_spec_rejects_index_zero() {
+  assert_same "rejected" "$(_shard_status 0/4)"
+}
+
+function test_shard_spec_rejects_total_zero() {
+  assert_same "rejected" "$(_shard_status 1/0)"
+}
+
+function test_shard_spec_rejects_missing_slash() {
+  assert_same "rejected" "$(_shard_status 3)"
+}
+
+function test_shard_spec_rejects_empty_and_partial_specs() {
+  assert_same "rejected" "$(_shard_status "")"
+  assert_same "rejected" "$(_shard_status "/4")"
+  assert_same "rejected" "$(_shard_status "2/")"
+}
+
+function test_shard_spec_rejects_negative_and_decorated_numbers() {
+  assert_same "rejected" "$(_shard_status "-1/4")"
+  assert_same "rejected" "$(_shard_status "1/+4")"
+  assert_same "rejected" "$(_shard_status "1.5/4")"
+}