rebuild the image with final sizes

after shrinking the btrfs it will have dangling space at the end. we'll
split the image into its partitions with actual filesystem sizes (i.e.
the new btrfs size) and then re-assmble the image using the partitions
as copyblocks.
this gives us a tightly packed image

build.sh

@@ -130,6 +130,9 @@ fi
 cp -v "${OUTPUT}_live.efi" "${OUTPUT}/efi-template/EFI/Linux/$EFI"
 systemd-repart --no-pager --empty=allow --size=auto --dry-run=no --root="${OUTPUT}" --definitions=mkosi.repart --defer-partitions=root "$IMG"
 
+# Finally rebuild the actual image file with appropriate partition sizing. In particular with squeezed btrfs.
+./part-rebuild.py "$IMG"
+
 # TODO before accepting new uploads perform sanity checks on the artifacts (e.g. the tar being well formed)
 chmod go+r ./*.efi # efi images are 700, make them readable so the server can serve them
 ls -lah

mkosi.repart-rebuild/00-esp.conf

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2023 Harald Sitter <sitter@kde.org>
+
+[Partition]
+Type=esp
+SplitName=esp
+CopyBlocks=/esp.raw

mkosi.repart-rebuild/50-root.conf

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2023 Harald Sitter <sitter@kde.org>
+
+[Partition]
+Type=root
+Label=KDEOSLive
+SplitName=root
+CopyBlocks=/KDEOSLive.raw

mkosi.repart-rebuild/README.md

@@ -0,0 +1,9 @@
+<!--
+SPDX-License-Identifier: CC0-1.0
+SPDX-FileCopyrightText: 2024 Harald Sitter <sitter@kde.org>
+-->
+
+This isn't actually used by mkosi but named like it so it is next to the actual mkosi directory.
+The configs in here simply pick up split partitions from the original repart run into a rebuild run.
+
+Also see part-rebuild.py

mkosi.repart/50-root.conf

@@ -5,6 +5,7 @@
 Type=root
 Format=btrfs
 SizeMinBytes=1G
+# Do not use spaces. We need to extract this later as a file path.
 Label=KDEOSLive
 SplitName=root
 # NOTE: the FS isn't read only because we have mutable subvolumes. Individual subvolumes may be readonly though.

part-rebuild.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2024 Harald Sitter <sitter@kde.org>
+
+# Rebuilds the image using appropriate partition sizes.
+
+import json
+import math
+import subprocess
+from subprocess import check_output
+import sys
+import os
+
+RAW = sys.argv[1]
+
+# Load the partition table
+out = check_output(["sfdisk", "--json", RAW])
+data = json.loads(out)
+partitiontable = data["partitiontable"]
+sectorsize = partitiontable["sectorsize"]
+partitions = partitiontable["partitions"]
+
+# Find the root partition
+root_partition = None
+for index, partition in enumerate(partitions):
+    if partition["name"] == "KDEOSLive":
+        root_partition = partition
+
+# Update its size to what it actually is on the filesystem level
+btrfs = json.loads(open("btrfs.json").read())
+root_partition["size"] = math.ceil(btrfs['size'] / sectorsize)
+
+# Fish out the partitions with appropriate skip and count values (i.e. the actual filesystem sizes)
+for index, partition in enumerate(partitions):
+    subprocess.run(["dd", f"if={RAW}", f"of={partition["name"]}.raw", f"bs={sectorsize}", f"skip={partition["start"]}", f"count={partition["size"]}"], check=True)
+
+# Then reassemble them into a new image using those squeezed partition images.
+open(RAW, "w").close()
+subprocess.run(["systemd-repart", "--empty=allow", "--dry-run=no", "--definitions=mkosi.repart-rebuild", "--root", os.getcwd(), RAW], check=True)