build-windows-base.sh: show a window by default, fix macOS bash 3.2 and fork crashes

Replace --window with --headless (VNC); the window is the default. Fill the
QEMU config placeholders without nested quotes (bash 3.2 kept them literally),
and run QEMU as a background job instead of -daemonize when it shows a window
(Cocoa crashes in fork()).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -130,7 +130,7 @@ It needs `qemu`, `qemu-img`, an ISO tool (`xorriso`, `genisoimage` or `mkisofs`)
 and `socat` or `nc`; there is no SSH to the guest.
 
 ```bash
-./build-windows-base.sh [--force] [--compress] [--from-img] [--window] [OUTPUT.qcow2]
+./build-windows-base.sh [--force] [--compress] [--from-img] [--headless] [OUTPUT.qcow2]
 ```
 
 It refuses to replace an existing base image without `--force`, because the

build-windows-base.sh

@@ -24,7 +24,7 @@
 # install-iso/ (see README.md).
 #
 # Usage: build-windows-base.sh [--force] [--keep-work] [--compress] [--from-img]
-#                              [--window] [OUTPUT.qcow2]
+#                              [--headless] [OUTPUT.qcow2]
 # OUTPUT defaults to the path that "download-images.sh --print-path windows11"
 # prints.
 set -euo pipefail
@@ -38,7 +38,7 @@ win_dir="${images_dir}/windows11"
 usage() {
   cat <<EOF
 Usage: $(basename "$0") [--force] [--keep-work] [--compress] [--from-img]
-                         [--window] [OUTPUT.qcow2]
+                         [--headless] [OUTPUT.qcow2]
 
 Install Windows 11 from the ISO, generalize it with sysprep and store the disk
 as the read-only, generic base image that VM disks are cloned from.
@@ -49,8 +49,9 @@ as the read-only, generic base image that VM disks are cloned from.
   --from-img      Install from the disk image made by build-windows-install-disk.sh
                   (the ISO path ending in .img), which reads much faster than
                   the emulated CD-ROM
-  --window        Show the guest in a QEMU window (needs a desktop session)
+  --headless      Serve the guest screen over VNC on 127.0.0.1:5990 instead of
-                  instead of the VNC display on 127.0.0.1:5990
+                  showing a QEMU window (the default; without a desktop
+                  session on Linux, VNC is used as well)
   -h, --help      Show this help
 
 Environment variables:
@@ -60,7 +61,7 @@ Environment variables:
   WIN11_VM_DISK_SIZE    Size of the system disk (default: 64G)
   WIN11_VM_UEFI_CODE    OVMF read-only firmware (auto-detected if unset)
   WIN11_VM_UEFI_VARS    OVMF variable-store template (auto-detected if unset)
-  WIN11_VM_DISPLAY      QEMU display with --window (default: cocoa on macOS,
+  WIN11_VM_DISPLAY      QEMU window display (default: cocoa on macOS,
                         gtk elsewhere)
   BASE_BUILD_TMPDIR     Parent of the scratch state directory, which needs about
                         25 GB (default: /var/tmp)
@@ -77,7 +78,7 @@ force=0
 keep_work=0
 compress=0
 from_img=0
-window=0
+window=1
 output=""
 while (($#)); do
   case "$1" in
@@ -86,7 +87,7 @@ while (($#)); do
     --keep-work) keep_work=1 ;;
     --compress) compress=1 ;;
     --from-img) from_img=1 ;;
-    --window) window=1 ;;
+    --headless) window=0 ;;
     -*) usage >&2; exit 2 ;;
     *) [[ -z "$output" ]] || { usage >&2; exit 2; }; output="$1" ;;
   esac
@@ -240,13 +241,15 @@ config_text="$(<"$config_template")"
 for placeholder in uefi_code uefi_vars disk_image monitor_socket; do
   value="${!placeholder}"
   [[ "$value" != *\"* ]] || die "${placeholder} must not contain double quotes: ${value}"
-  config_text="${config_text//"\${${placeholder}}"/"$value"}"
+  # No outer double quotes: bash 3.2 (macOS) keeps inner quotes literally.
+  config_text=${config_text//"\${${placeholder}}"/"$value"}
 done
 printf '%s\n' "$config_text" >"$qemu_config"
 
+if ((window)) && [[ -z "${DISPLAY:-}${WAYLAND_DISPLAY:-}" && "$host_os" != darwin ]]; then
+  window=0
+fi
 if ((window)); then
-  [[ -n "${DISPLAY:-}${WAYLAND_DISPLAY:-}" || "$host_os" == darwin ]] \
-    || die "--window needs a desktop session (DISPLAY or WAYLAND_DISPLAY is not set)."
   if [[ -n "${WIN11_VM_DISPLAY:-}" ]]; then display="$WIN11_VM_DISPLAY"
   elif [[ "$host_os" == darwin ]]; then display=cocoa
   else display=gtk; fi
@@ -263,8 +266,11 @@ qemu_args=(
   -netdev user,id=net0
   -nic none
   -display "$display"
-  -serial "file:${work}/console.log" -daemonize -pidfile "$pid_file"
+  -serial "file:${work}/console.log" -pidfile "$pid_file"
 )
+# The Cocoa/GTK window needs the original process (macOS crashes in fork()), so
+# only the VNC display is daemonized; the window runs as a background job.
+((window)) || qemu_args+=(-daemonize)
 if [[ "$host_os" == darwin && "$arch" == arm64 ]] && ((!from_img)); then
   # With the HVF in-kernel GICv3, QEMU's usb-storage races on the status packet
   # and Windows Setup stalls 10 s after every burst of reads (utmapp/UTM#7970),
@@ -308,7 +314,16 @@ press_key() {
   fi
 }
 
+if ((window)); then
+  "$qemu_binary" "${qemu_args[@]}" </dev/null &
+  qemu_job=$!
+  until [[ -s "$pid_file" ]]; do
+    kill -0 "$qemu_job" 2>/dev/null || die "QEMU exited before it started the guest."
+    sleep 0.2
+  done
+else
   "$qemu_binary" "${qemu_args[@]}"
+fi
 if ((!from_img)); then
   (
     deadline=$((SECONDS + 30))