Support $DPKG_ROOT in ucf and ucfr utilities

Thank you for merging and uploading the $DPKG_ROOT related fixes to the maintainer scripts. Now that ucf can be installed with --force-script-chrootless we can start making the ucf and ucfr utilities aware of $DPKG_ROOT. These changes are invasive. Please review carefully. I appreciate your attention to detail and sincere criticism.

• keep $DPKG_ROOT environment variable
• prefix statedir with $DPKG_ROOT
• use /etc directory from $DPKG_ROOT
• destination passed to ucf must not have the $DPKG_ROOT prefix
• path to configuration file passed to ucfr must not have prefix
• use dpkg-realpath instead of readlink as the former respects $DPKG_ROOT automatically and resolves even absolute symlinks relative to it
• other file arguments like --sum-file still require $DPKG_ROOT prefix if appropriate

This patch prefixes a lot of places with $DPKG_ROOT which makes the patch and the resulting code look quite messy. There is an alternative implementation which requires the caller of ucf (for example the openssh-server postinst maintainer script) to prefix the destination path with $DPKG_ROOT. With that done, ucf only needs to remove $DPKG_ROOT from a few places. The patch would look similar to this:

--- a/ucf
+++ b/ucf
@@ -217,24 +217,24 @@ replace_md5sum () {
 	    if [ "$VERBOSE" ]; then
 		echo >&2 "grep -v \"${dest_file_bre}\" \"$statedir/hashfile\""
 		grep -v "${dest_file_bre}" "$statedir/hashfile" >&2 || true
-		md5sum "$orig_new_file" | sed "s|$orig_new_file|$dest_file|" >&2
+		md5sum "$orig_new_file" | sed "s|$orig_new_file|${dest_file##"$DPKG_ROOT"}|" >&2
 	    fi
 	    grep -v "${dest_file_bre}" "$statedir/hashfile" > \
 		"$statedir/hashfile.tmp" || true
-	    md5sum "$orig_new_file" | sed "s|$orig_new_file|$dest_file|" >> \
+	    md5sum "$orig_new_file" | sed "s|$orig_new_file|${dest_file##"$DPKG_ROOT"}|" >> \
 		"$statedir/hashfile.tmp"
 	    mv -f "$statedir/hashfile.tmp"  "$statedir/hashfile"
 	else
 	    echo "(grep -v \"${dest_file_bre}\" \"$statedir/hashfile\""
-	    echo " md5sum \"$orig_new_file\" | sed \"s|$orig_new_file|$dest_file|\"; "
+	    echo " md5sum \"$orig_new_file\" | sed \"s|$orig_new_file|${dest_file##"$DPKG_ROOT"}|\"; "
 	    echo ") | sort > \"$statedir/hashfile\""
 	fi
     else
 	if [ "$docmd" = "YES" ]; then
-	    md5sum "$orig_new_file" | sed "s|$orig_new_file|$dest_file|"  > \
+	    md5sum "$orig_new_file" | sed "s|$orig_new_file|${dest_file##"$DPKG_ROOT"}|"  > \
 		"$statedir/hashfile"
 	else
-	    echo " md5sum \"$orig_new_file\" | sed \"s|$orig_new_file|$dest_file|\" >" \
+	    echo " md5sum \"$orig_new_file\" | sed \"s|$orig_new_file|${dest_file##"$DPKG_ROOT"}|\" >" \
 		"\"$statedir/hashfile\""
 	fi
     fi
@@ -402,7 +402,7 @@ if [ -n "$divert_line" ]; then
        dest_file=$(dpkg-divert --truename "$dest_file")
    fi
 fi
-dest_file_bre="[[:space:]]$(escape_bre "$dest_file")"'$'
+dest_file_bre="[[:space:]]$(escape_bre "${dest_file##"$DPKG_ROOT"}")"'$'
 
 ######################################################################
 ########                                                     #########
@@ -558,7 +558,10 @@ else
     old_mdsum_dir=""
 fi
 
-cached_file="$(echo "$dest_file" | tr / :)"
+# Remove $DPKG_ROOT prefix from $dest_file
+# The double quotes make sure that potential shell wildcards in $DPKG_ROOT are
+# not interpreted as such but as a fixed string.
+cached_file="$(echo "${dest_file##"$DPKG_ROOT"}" | tr / :)"
 ######################################################################
 ########                                                     #########
 ########                  Debugging dump                     #########

But this has disadvantages:

• I find the removal of the $DPKG_ROOT prefix untidy as well
• if maintainer scripts do not have to change how they call ucf, then that would make use of $DPKG_ROOT easier
• to also make the output of ucf cleaner, the $DPKG_ROOT prefix has to be removed from a lot more places than the patch above suggests

The code has been tested for correctness by the salsa CI pipeline associated to this MR: helmutg/dpkg-root-demo!8 (merged)

Since the $DPKG_ROOT environment variable is empty by default, most changes of this diff should not introduce new bugs. The biggest functional change is probably the introduction of dpkg-realpath.

The ucf utility is required by openssh-server which is a useful package to have early in the bootstrapping phase to set up a buildd server. This makes chrootless support interesting to have in ucf.

What do you think?