From 7c4409714028c15c81cb0e3284faf588526d2ebb Mon Sep 17 00:00:00 2001
From: Bilal Godil <bg2002@gmail.com>
Date: Mon, 13 Apr 2026 10:19:40 -0700
Subject: [PATCH] emulator: replace docker save/nuke/reload with in-place prune

After flattening, reclaim intermediate layers with `docker rmi` +
`docker image prune -af` rather than round-tripping the final image
through a tar and wiping /var/lib/docker. The round-trip cost ~15 min
under same-arch TCG on the arm64 runner because every byte of the
image is read, written to tar, then read and written back. Relies on
the drive's `discard=on,detect-zeroes=unmap` + fstrim to return freed
clusters to the qcow2, which also lets the zero-fill `dd` go.
---
 .../qemu/cloud-init/emulator/user-data        | 24 +++++--------------
 1 file changed, 6 insertions(+), 18 deletions(-)

diff --git a/docker/local-emulator/qemu/cloud-init/emulator/user-data b/docker/local-emulator/qemu/cloud-init/emulator/user-data
index c1d0d0f9b..ad70bdc8d 100644
--- a/docker/local-emulator/qemu/cloud-init/emulator/user-data
+++ b/docker/local-emulator/qemu/cloud-init/emulator/user-data
@@ -399,28 +399,16 @@ write_files:
         - stack-local-emulator:final
       log "Flatten done."
 
-      log "Saving final image to /var/tmp..."
+      log "Pruning intermediate images in place..."
       docker rm flatten
-      docker save stack-local-emulator:final -o /var/tmp/final-image.tar
-      mv /var/lib/docker/volumes /var/tmp/volumes-backup
-      log "Nuking Docker storage and reloading..."
-      systemctl stop docker containerd
-      rm -rf /var/lib/docker /var/lib/containerd
-      systemctl start docker containerd
-      until docker info >/dev/null 2>&1; do sleep 1; done
-      docker load -i /var/tmp/final-image.tar
+      docker rmi stack-local-emulator stack-local-emulator-slim || true
       docker tag stack-local-emulator:final stack-local-emulator
       docker rmi stack-local-emulator:final || true
-      rm -f /var/tmp/final-image.tar
-      systemctl stop docker
-      rm -rf /var/lib/docker/volumes
-      mv /var/tmp/volumes-backup /var/lib/docker/volumes
-      systemctl start docker
-      log "Docker storage rebuilt."
+      docker builder prune -af || true
+      docker image prune -af || true
+      log "Intermediate images pruned."
 
-      log "Zeroing free space for qcow2 compression..."
-      dd if=/dev/zero of=/zero.fill bs=1M 2>/dev/null || true
-      rm -f /zero.fill
+      log "Releasing free space for qcow2 compression (fstrim)..."
       sync
       fstrim -av 2>/dev/null || true
       log "slim-docker-image done."