Merge ~adrien/ubiquity:lp-1991083-add-margin-to-partition-minsize into ubiquity:main

[Adrien Nader]

513cfb4... by Adrien Nader on 2022-10-12

Add changelog entry for minimal partition size margin.

[Sergio Costas (rastersoft-gmail)]

     minsize=$size
+ minsize="$(add_margin_to_minsize "$minsize" "$cursize")"

What about changing this to:

- minsize=$size
+ minsize="$(add_margin_to_minsize "$size" "$cursize")"

And the same in the other change?

review: Needs Fixing

[Adrien Nader (adrien)]

> minsize=$size
> + minsize="$(add_margin_to_minsize "$minsize"
> "$cursize")"
>
> What about changing this to:
>
> - minsize=$size
> + minsize="$(add_margin_to_minsize "$size"
> "$cursize")"
>
> And the same in the other change?

Yeah, absolutely. I've pushed an updated change.

review: Approve

Unmerged commits

513cfb4... by Adrien Nader on 2022-10-12

Add changelog entry for minimal partition size margin.

3e4225d... by Adrien Nader on 2022-10-12

partman/resize: add a margin to the minimal size for partition shrinking

The current estimation of the minimal size of a filesystem is used as-is
without any margin. However these are only estimations and filesystems
shrunk to their minimum are not usable anymore. This is especially the
case for ext4 since the estimation does not take into account the
numerous filesystem features.

For ext4 we could improve the estimation too but this is a difficult
task with shell scripting and probably not worth considering the issue
has probably existed for a long time and no user seems to have
complained so far (maybe they worked around the issue by not trying to
shrink as much as possible).

Therefore, add the same margin as in subiquity: max(minsize*0.25, 2GiB).

LP: #1991083

diff --git a/d-i/source/partman-partitioning/lib/resize.sh b/d-i/source/partman-partitioning/lib/resize.sh
index fb7419d..b93b401 100644
--- a/d-i/source/partman-partitioning/lib/resize.sh
+++ b/d-i/source/partman-partitioning/lib/resize.sh
@@ -45,6 +45,50 @@ do_ntfsresize () {
 	return $RET
 }
 
+# add_margin_to_minsize: add a max(minsize*0.25, 2GiB) margin to the minimal
+# size of a filesystem but also cap the result to its current size.
+#
+# When minsize is first computed, it is only an estimation and it doesn't leave
+# any margin to continue actually using the filesystem that is about to be
+# shrunk. Below is an excerpt from ntfsresize(1) which is applicable to every
+# filesystem and operating system:
+#   Practically  the  smallest  shrunken size generally is at around
+#   "used space" + (20-200 MB). Please also take into  account  that
+#   Windows  might  need  about  50-100  MB  free space left to boot
+#   safely.
+#
+# The margin is max(minsize*0.25, 2GiB). This is the same as in subiquity.
+# Expect the same accuracy as you would expect from any arithmetic done in
+# shell scripts.
+#
+# I /think/ there is no issue with filesystem alignment because we don't change
+# the beginning of the filesystem and tools are supposed to properly align the
+# beginning of partitions/filesystems no matter their input.
+add_margin_to_minsize()
+{
+	local minsize cursize plus_25_percents plus_2G
+	minsize="$1"
+	cursize="$2"
+
+	plus_25_percents="$(expr 125 \* "$minsize" / 100)"
+	plus_2G="$(expr "$minsize" + 2 \* 1024 \* 1024 \* 1024)"
+
+	if longint_le "$plus_25_percents" "$plus_2G"; then
+		plus_25_percents_or_2G="$plus_2G"
+	else
+		plus_25_percents_or_2G="$plus_25_percents"
+	fi
+
+	if longint_le "$cursize" "$plus_25_percents_or_2G"; then
+		logger -t partman "Filesystem is already as small as it can reasonably be"
+		minsize="$cursize"
+	else
+		minsize="$plus_25_percents_or_2G"
+	fi
+
+	echo $minsize
+}
+
 get_ntfs_resize_range () {
 	local bdev size
 	open_dialog GET_VIRTUAL_RESIZE_RANGE $oldid
@@ -71,7 +115,7 @@ get_ntfs_resize_range () {
 				unset minsize cursize maxsize prefsize
 				return 1
 			else
-				minsize=$size
+				minsize="$(add_margin_to_minsize "$size" "$cursize")"
 			fi
 		fi
 	fi
@@ -108,7 +152,7 @@ get_ext2_resize_range () {
 				unset minsize cursize maxsize prefsize
 				return 1
 			else
-				minsize="$real_minsize"
+				minsize="$(add_margin_to_minsize "$real_minsize" "$cursize")"
 			fi
 		fi
 	fi
diff --git a/debian/changelog b/debian/changelog
index a9ed647..f1ab086 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,10 @@
+ubiquity (22.10.8ubuntu1) UNRELEASED; urgency=medium
+
+  * partman/resize: add a margin to the minimal size for partition shrinking
+    (LP: #1991083)
+
+ -- Adrien Nader <[e-mail]>  Wed, 12 Oct 2022 10:14:27 +0000
+
 ubiquity (22.10.8) kinetic; urgency=medium
 
   *  Increase the window size in the QT/KDE version of the installer dailog so