Probe closed stdin with an fd duplication in _autolint_read_files0

lib/checkrun/autolint.sh

@@ -106,7 +106,15 @@ _autolint_read_files0() {
     # field for both EOF and descriptor errors. Reject a closed descriptor and
     # directory source before reading so an invalid stream cannot become a
     # silent no-op. Pipes, regular files, sockets, and terminals remain valid.
-    if { [ ! -r /dev/stdin ] && [ ! -r /dev/fd/0 ]; } ||
+    # Test descriptor openness by duplicating fd 0 into a subshell rather
+    # than by probing /dev/stdin readability: on the ubuntu-26.04 runner
+    # a closed stdin arrives as /dev/null (the runner's bash reopens it
+    # at process startup, verified with a fresh `bash -c` probe), so the
+    # /dev probe passes and the read loop treats it as EOF. Duplicating
+    # a closed descriptor fails; duplicating an open one succeeds
+    # regardless of what it points at. (Duplicating fd 0 onto itself is
+    # a no-op even when closed, so the target must differ.)
+    if ! ( exec 3<&0 ) 2>/dev/null ||
       [ -d /dev/stdin ] || [ -d /dev/fd/0 ]; then
       echo "autolint: --files0-from - requires a readable stream on standard input" >&2
       return 2

test/suites/autolint-test

@@ -3056,7 +3056,13 @@ _assert_exit "files0 stdin: empty input is a no-op" 0 "$rc"
 _assert_eq "files0 stdin: empty input is quiet" "" "$output"
 
 set +e
-output=$("$AUTOLINT" --files0-from - <&- 2>&1)
+# Invoke in-process, not via bin/autolint: the ubuntu-26.04 runner's bash
+# reopens /dev/null for a closed stdin at process startup (verified: a
+# fresh `bash -c` probe with closed stdin arrives with fd 0 -> /dev/null),
+# so a subprocess invocation can never observe the closed descriptor the
+# guard is meant to reject. A command-substitution subshell forks without
+# re-running bash startup, so the closed fd survives to _autolint_main.
+output=$(. "$CHECKRUN_TEST_ROOT/lib/checkrun/autolint.sh"; _autolint_main --files0-from - <&- 2>&1)
 rc=$?
 set -e
 _assert_exit "files0 stdin: closed descriptor exits 2" 2 "$rc"