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

[Adrien Nader]

[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

[Łukasz Zemczak (sil2100)]

I am not too knowledgable in the partitioning code, but this looks good to me. I also asked Nick and Brian for opinion and they said the same. Let's try this out.

review: Approve

[Brian Murray (brian-murray)]

One comment regarding package versioning.

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