Applying patch from Helmut Grohne <helmut@subdivi.de> to fix broken upgrade from bookworm due to /usr-move mitigations (Closes: #1104306).

Signed-off-by: Daniel Baumann <daniel@debian.org>

debian/control

@@ -18,11 +18,6 @@ Architecture: any
 Depends:
  ${misc:Depends},
  ${shlibs:Depends},
-Breaks:
-# We must ensure that gzip is upgraded before zutils.postinst runs. As it is
-# essential, Breaks is sufficient here and the janitor may propose dropping
-# this relation eventually.
- gzip (<< 1.12-1.1~),
 Suggests:
  bzip2,
  lzip,

debian/zutils.preinst

@@ -4,14 +4,45 @@ set -e
 
 # DEP17 M18: Duplicate diversion in aliased location /bin.
 
+die() {
+	printf '%s, cannot proceed\n' "$*" 1>&2
+	exit 1
+}
+
 case "${1}" in
 	install)
+		# Situations considered:
+		#  * bookworm or earlier gzip is fully installed.
+		#    -> No diversions, /bin/${FILE}
+		#  * trixie or later gzip is unpacked.
+		#    -> No diversions, /usr/bin/${FILE}
+		#  * trixie or later gzip has been unpacked while bookworm's
+		#    zutils was installed, but zutils has since been removed.
+		#    -> /usr/bin/FILE -> /usr/bin/FILE.usr-is-merged
+		#  * trixie or later gzip is fully installed.
+		#    -> No diversions, /usr/bin/${FILE}
+		#
+		# We cannot run between gzip.preinst and gzip unpack.
+		GZIP_VERSION=$(dpkg-query -f '${Version}' -W gzip)
+		GZIP_PREFIX=/usr
+		dpkg --compare-versions "$GZIP_VERSION" lt 1.12-1.1~ && GZIP_PREFIX=
 		for FILE in zcat zcmp zdiff zegrep zfgrep zgrep
 		do
-			# We may move $FILE to $FILE.gzip when we expected $FILE.gzip.usr-is-merged here.
+			TRUENAME=$(dpkg-divert --truename "/usr/bin/${FILE}")
-			# This is ok, because gzip will be upgraded and overwrite $FILE.gzip.
+			if [ "${TRUENAME}" = "/usr/bin/${FILE}.usr-is-merged" ]
-			dpkg-divert --package zutils --quiet --add --rename --divert "/usr/bin/${FILE}.gzip" "/usr/bin/${FILE}"
+			then
-			dpkg-divert --package zutils --quiet --add --rename --divert "/bin/${FILE}.gzip.usr-is-merged" "/bin/${FILE}"
+				dpkg-divert --package zutils --quiet --remove --no-rename --divert "${TRUENAME}" "/usr/bin/${FILE}"
+			elif [ "${TRUENAME}" != "/usr/bin/${FILE}" ]
+			then
+				die "unexpected diversion of /usr/bin/${FILE} to ${TRUENAME}"
+			fi
+			# We cannot --rename, because it'll rename any existing
+			# file without checking whether the file is owned or
+			# not. Correctly compute the required rename depending
+			# on the gzip version.
+			dpkg-divert --package zutils --quiet --add --no-rename --divert "/usr/bin/${FILE}.gzip" "/usr/bin/${FILE}"
+			dpkg-divert --package zutils --quiet --add --no-rename --divert "/bin/${FILE}.gzip.usr-is-merged" "/bin/${FILE}"
+			mv "${DPKG_ROOT:-}${TRUENAME}" "${DPKG_ROOT:-}${GZIP_PREFIX}/bin/${FILE}.gzip${GZIP_PREFIX:+.usr-is-merged}"
 			dpkg-divert --package zutils --quiet --add --rename --divert /usr/share/man/man1/${FILE}.gzip.1.gz /usr/share/man/man1/${FILE}.1.gz
 		done
 		;;
@@ -23,12 +54,23 @@ case "${1}" in
 
 			if [ "${TRUENAME}" = "/usr/bin/${FILE}.usr-is-merged" ]
 			then
-				# gzip.preinst duplicated the diversion for us
+
+				# This branch should be dead code. The
+				# diversion indicates that trixie's gzip has
+				# been unpacked, but trixie's gzip also
+				# conflicts with zutils (<< trixie), so either
+				# we're not installing or we are upgrading from
+				# trixie.
+				if ! [ -e "${DPKG_ROOT}${TRUENAME}" ]
+				then
+					die "diverted file ${TRUENAME} does not exist"
+				fi
 				dpkg-divert --package zutils --quiet --remove --no-rename --divert "/usr/bin/${FILE}.usr-is-merged" "/usr/bin/${FILE}"
 				dpkg-divert --package zutils --quiet --remove --no-rename "/bin/${FILE}"
 				dpkg-divert --package zutils --quiet --add --no-rename --divert "/usr/bin/${FILE}.gzip" "/usr/bin/${FILE}"
 				dpkg-divert --package zutils --quiet --add --no-rename --divert "/bin/${FILE}.gzip.usr-is-merged" "/bin/${FILE}"
-			elif [ "${TRUENAME}" != "/usr/bin/${FILE}.gzip" ]
+				mv "${DPKG_ROOT}${TRUENAME}" "${DPKG_ROOT}/usr/bin/${FILE}.gzip"
+			elif [ "${TRUENAME}" = "/usr/bin/${FILE}" ]
 			then
 				dpkg-divert --package zutils --quiet --add --no-rename --divert "/usr/bin/${FILE}.gzip" "/usr/bin/${FILE}"
 
@@ -43,6 +85,9 @@ case "${1}" in
 						mv "${DPKG_ROOT}${TRUENAME}" "${DPKG_ROOT}/bin/${FILE}.gzip.usr-is-merged"
 					fi
 				fi
+			elif [ "${TRUENAME}" != "/usr/bin/${FILE}.gzip" ]
+			then
+				die "unexpected diversion of /usr/bin/${FILE} to ${TRUENAME}"
 			fi
 		done
 		;;