[PATCH v2] scripts/make-gpt.sh: allow setting partition size

[Alyssa Ross <hi@alyssa.is>]

We want non-minimally-sized partitions to leave space for updates.
All such partitions will also be labelled, so we can just add another
optional field at the end.

Since we don't parse partition specifications in sh, we can't keep a
running total any more, so instead we just go through the table at the
end and add up all the sizes, taking advantage of our knowledge that
the size will always be the last thing in each line in our tables.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Message-ID: <20251127174054.2056835-2-hi@alyssa.is>
---
v2: fix conflicts after applying Demi's optimization, and avoid awk
    to sum partition sizes, taking inspiration from the same.
v1: https://spectrum-os.org/lists/archives/spectrum-devel/20251127174054.2056835-2-hi@alyssa.is/

 scripts/make-gpt.sh      | 30 ++++++++++++++++++------------
 scripts/sfdisk-field.awk | 22 +++++++++++++++++-----
 2 files changed, 35 insertions(+), 17 deletions(-)

diff --git a/scripts/make-gpt.sh b/scripts/make-gpt.sh
index 3cae441..cb19868 100755
--- a/scripts/make-gpt.sh
+++ b/scripts/make-gpt.sh
@@ -1,10 +1,10 @@
 #!/bin/sh -eu
 #
-# SPDX-FileCopyrightText: 2021-2023 Alyssa Ross <hi@alyssa.is>
+# SPDX-FileCopyrightText: 2021-2023, 2025 Alyssa Ross <hi@alyssa.is>
 # SPDX-FileCopyrightText: 2022 Unikie
 # SPDX-License-Identifier: EUPL-1.2+
 #
-# usage: make-gpt.sh GPT_PATH PATH:PARTTYPE[:PARTUUID[:PARTLABEL]]...
+# usage: make-gpt.sh GPT_PATH PATH:PARTTYPE[:PARTUUID[:PARTLABEL[:PARTMiB]]]...
 
 ONE_MiB=1048576
 
@@ -33,21 +33,27 @@ scriptsDir="$(dirname "$0")"
 out="$1"
 shift
 
-nl='
-'
-table="label: gpt"
+table=$(for partition; do
+	awk -f "$scriptsDir/sfdisk-field.awk" \
+		-v partition="$partition" \
+		-v size="$(sizeMiB "${partition%%:*}")"
+done)
 
 # Keep 1MiB free at the start, and 1MiB free at the end.
-gptBytes=$((ONE_MiB * 2))
-for partition; do
-	sizeMiB="$(sizeMiB "${partition%%:*}")"
-	table="$table${nl}size=${sizeMiB}MiB,$(awk -f "$scriptsDir/sfdisk-field.awk" -v partition="$partition")"
-	gptBytes="$((gptBytes + sizeMiB * ONE_MiB))"
-done
+gptMiB=2
+while read -r partition; do
+	# Here we rely on sfdisk-field.awk always putting size last.
+	: $((gptMiB += ${partition##*=}))
+done <<EOF
+$table
+EOF
 
 rm -f "$out"
-truncate -s "$gptBytes" "$out"
+truncate -s "${gptMiB}M" "$out"
+
 sfdisk --no-reread --no-tell-kernel "$out" <<EOF
+label: gpt
+sector-size: $ONE_MiB
 $table
 EOF
 
diff --git a/scripts/sfdisk-field.awk b/scripts/sfdisk-field.awk
index e13c86d..78b438e 100644
--- a/scripts/sfdisk-field.awk
+++ b/scripts/sfdisk-field.awk
@@ -1,7 +1,7 @@
 #!/usr/bin/awk -f
 #
 # SPDX-License-Identifier: EUPL-1.2+
-# SPDX-FileCopyrightText: 2022, 2024 Alyssa Ross <hi@alyssa.is>
+# SPDX-FileCopyrightText: 2022, 2024-2025 Alyssa Ross <hi@alyssa.is>
 
 BEGIN {
 	types["root.aarch64"] = "b921b045-1df0-41c3-af44-4c6f280d3fae"
@@ -9,12 +9,11 @@ BEGIN {
 	types["verity.aarch64"] = "df3300ce-d69f-4c92-978c-9bfb0f38d820"
 	types["verity.x86_64"] = "2c7357ed-ebd2-46d9-aec1-23d437ec2bf5"
 
-	# Field #1 is the partition path, which make-gpt.sh will turn into
-	# the size field.  Since it's handled elsewhere, we skip that
-	# first field.
+	# Field #1 is the partition path, which is read by make-gpt.sh
+	# but not relevant for running sfdisk, so skip it.
 	skip=1
 
-	split("type uuid name", keys)
+	split("type uuid name size", keys)
 	split(partition, fields, ":")
 
 	arch = ENVIRON["ARCH"]
@@ -31,8 +30,21 @@ BEGIN {
 		if (keys[n - skip] == "type") {
 			if (uuid = types[fields[n] "." arch])
 				fields[n] = uuid
+		} else if (keys[n - skip] == "size") {
+			if (fields[n] < size) {
+				printf "%s MiB partition content is too big for %s MiB partition\n",
+					size, fields[n] > "/dev/stderr"
+				exit 1
+			}
+
+			size = fields[n]
+			continue # Handled at the end.
 		}
 
 		printf "%s=%s,", keys[n - skip], fields[n]
 	}
+
+	# Always output a size field, either supplied in input or
+	# default value of the size variable.
+	printf "size=%s\n", size
 }

base-commit: 1970f432152730ec19bf97a8db4613a591ee00be
-- 
2.51.0