dpkg-maintscript-helper: Add new dir_to_symlink command

Closes: #583585

debian/changelog

@@ -81,6 +81,7 @@ dpkg (1.17.2) UNRELEASED; urgency=low
     we might get us to read garbage from memory or segfault.
   * Add new symlink_to_dir command to dpkg-maintscript-helper. Closes: #720712
     Based on a patch by Bastien ROUCARIÈS <roucaries.bastien@gmail.com>.
+  * Add new dir_to_symlink command to dpkg-maintscript-helper. Closes: #583585
 
   [ Updated programs translations ]
   * German (Sven Joachim).

man/dpkg-maintscript-helper.1

@@ -30,6 +30,8 @@ dpkg\-maintscript\-helper \- works around known dpkg limitations in maintainer s
 \fBmv_conffile\fP \fIold-conffile\fP \fInew-conffile\fP [\fIprior-version\fP [\fIpackage\fP]]
 .P
 \fBsymlink_to_dir\fP \fIpathname\fP \fIold-target\fP [\fIprior-version\fP [\fIpackage\fP]]
+.P
+\fBdir_to_symlink\fP \fIpathname\fP \fInew-target\fP [\fIprior-version\fP [\fIpackage\fP]]
 .
 .SH DESCRIPTION
 .P
@@ -172,13 +174,43 @@ the \fBpostinst\fP removes \fIpathname\fP\fB.dpkg\-backup\fP if
 abort\-upgrade/abort\-install, the \fBpostrm\fP renames
 \fIpathname\fP\fB.dpkg\-backup\fP back to \fIpathname\fP if required.
 .
+.SS Switching a directory to symlink
+.
+If a real directory is converted to a symlink, you need to make sure
+before unpacking that the directory is removed. This may seem a simple
+change to the \fBpreinst\fP script at first, however that will result
+in some problems in case the directory contains conffiles, pathnames
+owned by other packages, locally created pathnames, or when downgrading
+the package.
+.P
+Graceful switching can be implemented by putting the following shell
+snippet in the \fBpreinst\fP, \fBpostinst\fP and \fBpostrm\fP maintainer
+scripts:
+.P
+    dpkg\-maintscript\-helper dir_to_symlink \\
+        \fIpathname\fP \fInew-target\fP \fIprior-version\fP \fIpackage\fP \-\- "$@"
+.P
+\fIpathname\fP is the name of the of the old directory (the path will be a
+symlink at the end of the installation) and \fInew-target\fP is the target
+of the new symlink at \fIpathname\fP.
+.P
+Current implementation: the \fBpreinst\fP checks if the directory
+exists, does not contain conffiles, pathnames owned by other packages,
+or locally created pathnames, if not then it's left in place, otherwise
+it's renamed to \fIpathname\fP\fB.dpkg\-backup\fP. On configuration,
+the \fBpostinst\fP removes \fIpathname\fP\fB.dpkg\-backup\fP if
+\fIpathname\fP\fB.dpkg\-backup\fP is still a directory. On
+abort\-upgrade/abort\-install, the \fBpostrm\fP renames
+\fIpathname\fP\fB.dpkg\-backup\fP back to \fIpathname\fP if required.
+.
 .SH INTEGRATION IN PACKAGES
 .P
 Given that \fBdpkg\-maintscript\-helper\fP is used in the \fBpreinst\fP,
 using it unconditionally requires a pre-dependency to ensure that the
 required version of \fBdpkg\fP has been unpacked before. The required version
 depends on the command used, for \fBrm_conffile\fP and \fBmv_conffile\fP
-it is 1.15.7.2, for \fBsymlink_to_dir\fP it is 1.17.2:
+it is 1.15.7.2, for \fBsymlink_to_dir\fP and \fBdir_to_symlink\fP
+it is 1.17.2:
 .P
     \fBPre\-Depends:\fP dpkg (>= 1.17.2)
 .P

scripts/dpkg-maintscript-helper.sh

@@ -295,6 +295,115 @@ symlink_to_dir() {
 	esac
 }
 
+##
+## Functions to replace a directory with a symlink
+##
+dir_to_symlink() {
+	local PATHNAME="$1"
+	local SYMLINK_TARGET="$2"
+	local LASTVERSION="$3"
+	local PACKAGE="$4"
+
+	if [ "$LASTVERSION" = "--" ]; then
+		LASTVERSION=""
+		PACKAGE="$DPKG_MAINTSCRIPT_PACKAGE${DPKG_MAINTSCRIPT_ARCH:+:$DPKG_MAINTSCRIPT_ARCH}"
+	fi
+	if [ "$PACKAGE" = "--" -o -z "$PACKAGE" ]; then
+		PACKAGE="$DPKG_MAINTSCRIPT_PACKAGE${DPKG_MAINTSCRIPT_ARCH:+:$DPKG_MAINTSCRIPT_ARCH}"
+	fi
+
+	# Skip remaining parameters up to --
+	while [ "$1" != "--" -a $# -gt 0 ]; do shift; done
+	[ $# -gt 0 ] || badusage "missing arguments after --"
+	shift
+
+	[ -n "$DPKG_MAINTSCRIPT_NAME" ] || \
+		error "environment variable DPKG_MAINTSCRIPT_NAME is required"
+	[ -n "$PACKAGE" ] || error "cannot identify the package"
+	[ -n "$PATHNAME" ] || error "directory parameter is missing"
+	[ -n "$SYMLINK_TARGET" ] || error "new symlink target is missing"
+	[ -n "$LASTVERSION" ] || error "last version is missing"
+	[ -n "$1" ] || error "maintainer script parameters are missing"
+
+	debug "Executing $0 dir_to_symlink in $DPKG_MAINTSCRIPT_NAME" \
+	      "of $DPKG_MAINTSCRIPT_PACKAGE"
+	debug "PATHNAME=$PATHNAME SYMLINK_TARGET=$SYMLINK_TARGET" \
+	      "PACKAGE=$PACKAGE LASTVERSION=$LASTVERSION ACTION=$1 PARAM=$2"
+
+	case "$DPKG_MAINTSCRIPT_NAME" in
+	preinst)
+		if [ "$1" = "install" -o "$1" = "upgrade" ] &&
+		   [ -n "$2" ] && [ -d "$PATHNAME" ] &&
+		   dpkg --compare-versions "$2" le-nl "$LASTVERSION"; then
+			prepare_dir_to_symlink "$PACKAGE" "$PATHNAME"
+		fi
+		;;
+	postinst)
+		if [ "$1" = "configure" ] &&
+		   [ -d "${PATHNAME}.dpkg-backup" ] && [ -h "$PATHNAME" ] &&
+		   [ "$(readlink -f $PATHNAME)" = "$SYMLINK_TARGET" ] &&
+		    dpkg --compare-versions "$2" le-nl "$LASTVERSION"; then
+			# By now, dpkg will have updated the symlink to point
+			# to the new location, but we are left behind the old
+			# files owned by this package in the backup directory,
+			# just remove it.
+			rm -rf "${PATHNAME}.dpkg-backup"
+		fi
+		;;
+	postrm)
+		if [ "$1" = "purge" ] && [ -d "${PATHNAME}.dpkg-backup" ]; then
+		    rm -rf "${PATHNAME}.dpkg-backup"
+		fi
+		if [ "$1" = "abort-install" -o "$1" = "abort-upgrade" ] &&
+		   [ -n "$2" ] &&
+		   [ -d "${PATHNAME}.dpkg-backup" ] && [ -h "$PATHNAME" ] &&
+		   [ "$(readlink -f $PATHNAME)" = "$SYMLINK_TARGET" ] &&
+		   dpkg --compare-versions "$2" le-nl "$LASTVERSION"; then
+			echo "Restoring backup of $PATHNAME ..."
+			rm -f "$PATHNAME"
+			mv "${PATHNAME}.dpkg-backup" "$PATHNAME"
+		fi
+		;;
+	*)
+		debug "$0 dir_to_symlink not required in $DPKG_MAINTSCRIPT_NAME"
+		;;
+	esac
+}
+
+prepare_dir_to_symlink()
+{
+	local PACKAGE="$1"
+	local PATHNAME="$2"
+
+	# If there are conffiles we should not perform the switch.
+	if dpkg-query -W -f='${Conffiles}' "$PACKAGE" | \
+	   grep -q "$PATHNAME/."; then
+		error "directory '$PATHNAME' contains conffiles," \
+		      "cannot switch to symlink"
+	fi
+
+	# If there are locally created files or files owned by another package
+	# we should not perform the switch.
+	find "$PATHNAME" -print0 | xargs -0 -n1 sh -c '
+		package="$1"
+		file="$2"
+		if ! dpkg-query -L "$package" | grep -q -x "$file"; then
+			return 1
+		fi
+		return 0
+	' subcommand "$PACKAGE" || \
+		error "directory '$PATHNAME' contains files not owned by" \
+		      "package $PACKAGE, cannot switch to symlink"
+
+	# Move the directory aside and make a temporary symlink to reduce the
+	# time the contents are not available. dpkg will not be able to remove
+	# the old files from the backup directory after unpack, because it
+	# will have updated the symlink to point to the new location already,
+	# we'll remove them ourselves later on.
+	mv -f "$PATHNAME" "${PATHNAME}.dpkg-backup"
+	ln -s "${PATHNAME}.dpkg-backup" "$PATHNAME"
+}
+
 # Common functions
 ensure_package_owns_file() {
 	local PACKAGE="$1"
@@ -338,6 +447,9 @@ Commands:
   symlink_to_dir <pathname> <old-symlink-target> [<last-version> [<package>]]
 	Replace a symlink with a directory. Must be called in preinst,
 	postinst and postrm.
+  dir_to_symlink <pathname> <new-symlink-target> [<last-version> [<package>]]
+	Replace a directory with a symlink. Must be called in preinst,
+	postinst and postrm.
   help
 	Display this usage information.
 END
@@ -362,7 +474,7 @@ shift
 case "$command" in
 supports)
 	case "$1" in
-	rm_conffile|mv_conffile|symlink_to_dir)
+	rm_conffile|mv_conffile|symlink_to_dir|dir_to_symlink)
 		code=0
 		;;
 	*)
@@ -388,6 +500,9 @@ mv_conffile)
 symlink_to_dir)
 	symlink_to_dir "$@"
 	;;
+dir_to_symlink)
+	dir_to_symlink "$@"
+	;;
 --help|help|-?)
 	usage
 	;;