fix CPU spin on stdin EOF in InteractiveBackend (#42)

CMakeLists.txt

@@ -92,4 +92,9 @@ if (COMMANDLINE_TESTS)
     endif ()
     enable_testing()
     add_test(NAME commandline_tests COMMAND commandline_tests)
+    if (UNIX)
+        add_test(NAME regression_issue42
+            COMMAND bash "${CMAKE_SOURCE_DIR}/tests/regression_issue42.sh" "${CMAKE_BINARY_DIR}"
+            WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}")
+    endif ()
 endif ()

src/backends/InteractiveBackend.cpp

@@ -1,5 +1,7 @@
 #include "InteractiveBackend.h"
 
+#include <cstdio>
+
 #include "impls.h"
 
 lk::InteractiveBackend::InteractiveBackend(const std::string& prompt)
@@ -235,6 +237,10 @@ void lk::InteractiveBackend::input_thread_main() {
             if (m_key_debug) {
                 fprintf(stderr, "c: 0x%.2x\n", c);
             }
+            if (c == EOF) {
+                m_shutdown.store(true);
+                break;
+            }
             std::unique_lock<std::mutex> guard(m_current_buffer_mutex);
             if (c != '\t') {
             }

tests/regression_issue42.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+# Regression test for #42: GNU screen -dmS causes commandline to break and CPU spin.
+#
+# Forced interactive mode + /dev/null stdin simulates the detached-screen scenario:
+# stdin reaches EOF immediately.  The input thread must detect EOF and shut down
+# instead of spinning.
+set -euo pipefail
+
+if [ ! -e /dev/null ] || ! command -v /usr/bin/time >/dev/null 2>&1; then
+    echo "SKIP: /usr/bin/time and /dev/null required (Unix-only regression test)"
+    exit 0
+fi
+
+BINARY="${1:-build}/commandline_test"
+
+TIME_OUT=$(mktemp)
+trap 'rm -f "$TIME_OUT"' EXIT
+
+COMMANDLINE_FORCE_INTERACTIVE=1 \
+    /usr/bin/time -o "$TIME_OUT" -v timeout 3 "$BINARY" \
+        < /dev/null >/dev/null 2>/dev/null || true
+
+total_cpu=$(awk '
+    /User time \(seconds\)/  { u = $NF }
+    /System time \(seconds\)/ { s = $NF }
+    END {
+        if (u == "" || s == "") exit 1
+        print u + s
+    }
+' "$TIME_OUT") || { echo "FAIL: could not parse CPU time from /usr/bin/time output"; exit 1; }
+
+if awk "BEGIN {exit !($total_cpu >= 0.3)}"; then
+    echo "FAIL: CPU spin detected: ${total_cpu}s CPU in 3s"
+    exit 1
+fi
+
+echo "PASS: no CPU spin (${total_cpu}s CPU in 3s)"