vmspawn: apply --grow-image= to the ephemeral copy

Combining --ephemeral with --grow-image= truncated the file passed to
--image= to the requested size before the VM was started, so a mode whose
entire purpose is to leave the original image untouched modified it on
disk.

Skip growing the original image when running ephemeral, and instead create
the qcow2 overlay with the requested virtual size, so the guest sees the
larger disk while the base image stays read-only. As only the overlay is
sized in that case, the base image format no longer matters, so allow
--grow-image= together with qcow2 images when running ephemeral. The round
up to a multiple of 4096 moves to option parsing so both paths apply the
same value.

The vmspawn drive test grows an ephemeral VM and checks that the image
passed to --image= keeps its size, and that the overlay QEMU runs off is
created at the requested one.

test/units/TEST-87-AUX-UTILS-VM.vmspawn-drives.sh

@@ -53,7 +53,7 @@ WORKDIR="$(mktemp -d)"
 
 at_exit() {
     set +e
-    for m in "${MACHINE_MULTI:-}" "${MACHINE_EPHEMERAL:-}"; do
+    for m in "${MACHINE_MULTI:-}" "${MACHINE_EPHEMERAL:-}" "${MACHINE_GROW:-}"; do
         [[ -n "$m" ]] || continue
         if machinectl status "$m" &>/dev/null; then
             machinectl terminate "$m" 2>/dev/null
@@ -62,6 +62,7 @@ at_exit() {
     done
     [[ -n "${VMSPAWN_MULTI_PID:-}" ]] && kill "$VMSPAWN_MULTI_PID" 2>/dev/null && wait "$VMSPAWN_MULTI_PID" 2>/dev/null
     [[ -n "${VMSPAWN_EPHEMERAL_PID:-}" ]] && kill "$VMSPAWN_EPHEMERAL_PID" 2>/dev/null && wait "$VMSPAWN_EPHEMERAL_PID" 2>/dev/null
+    [[ -n "${VMSPAWN_GROW_PID:-}" ]] && kill "$VMSPAWN_GROW_PID" 2>/dev/null && wait "$VMSPAWN_GROW_PID" 2>/dev/null
     rm -rf "$WORKDIR"
 }
 trap at_exit EXIT
@@ -185,4 +186,59 @@ timeout 10 bash -c "while machinectl status '$MACHINE_EPHEMERAL' &>/dev/null; do
 timeout 10 bash -c "while kill -0 '$VMSPAWN_EPHEMERAL_PID' 2>/dev/null; do sleep .5; done"
 echo "Ephemeral VM terminated cleanly"
 
+# --- Test 3: Ephemeral overlay grown with --grow-image= ---
+# In ephemeral mode the requested size has to land on the qcow2 overlay. The
+# image passed to --image= is opened read-only and has to come out of the run
+# at its original size.
+
+MACHINE_GROW="test-vmspawn-grow-$$"
+GROW_SIZE=$((512 * 1024 * 1024))
+IMAGE_SIZE="$(stat -c %s "$WORKDIR/root.raw")"
+
+systemd-vmspawn \
+    --machine="$MACHINE_GROW" \
+    --ram=256M \
+    --image="$WORKDIR/root.raw" \
+    --ephemeral \
+    --grow-image="$GROW_SIZE" \
+    --linux="$KERNEL" \
+    --tpm=no \
+    --console=headless \
+    root=/dev/vda rw \
+    &>"$WORKDIR/vmspawn-grow.log" &
+VMSPAWN_GROW_PID=$!
+
+wait_for_machine "$MACHINE_GROW" "$VMSPAWN_GROW_PID" "$WORKDIR/vmspawn-grow.log"
+echo "Grown ephemeral machine '$MACHINE_GROW' registered with machined"
+
+assert_eq "$(stat -c %s "$WORKDIR/root.raw")" "$IMAGE_SIZE"
+echo "Image passed to --image= was left at its original size"
+
+# The overlay is unlinked, so QEMU's fd is the only way to reach it. vmspawn
+# registers QEMU itself as the machine leader, so machined knows its PID.
+QEMU_PID="$(varlinkctl call /run/systemd/machine/io.systemd.Machine \
+    io.systemd.Machine.List "{\"name\":\"$MACHINE_GROW\"}" | jq -r '.leader.pid')"
+
+OVERLAY=""
+for fd in /proc/"$QEMU_PID"/fd/*; do
+    target="$(readlink "$fd" 2>/dev/null)" || continue
+    # O_TMPFILE in the runtime directory, or the memfd fallback.
+    [[ "$target" == */vmspawn/"$MACHINE_GROW"/#* || "$target" == /memfd:vmspawn-overlay* ]] || continue
+    OVERLAY="$fd"
+    break
+done
+assert_neq "$OVERLAY" ""
+echo "Ephemeral overlay is QEMU fd ${OVERLAY##*/}"
+
+# qcow2 header: the magic, followed by the virtual size as a big endian u64 at
+# offset 24. Read it out of the header directly, qemu-img may not be installed.
+assert_eq "$(od -An -tx1 -N4 "$OVERLAY" | tr -d ' ')" "514649fb"
+assert_eq "$(od -An -tu8 -j24 -N8 --endian=big "$OVERLAY" | tr -d ' ')" "$GROW_SIZE"
+echo "Ephemeral overlay was created at the requested size"
+
+machinectl terminate "$MACHINE_GROW"
+timeout 10 bash -c "while machinectl status '$MACHINE_GROW' &>/dev/null; do sleep .5; done"
+timeout 10 bash -c "while kill -0 '$VMSPAWN_GROW_PID' 2>/dev/null; do sleep .5; done"
+echo "Grown ephemeral VM terminated cleanly"
+
 echo "All vmspawn drive setup tests passed"