arch-chroot: move resolve_link and chroot_add_resolv_link to common

chroot_add_resolve_link is cleaned up to be simpler.
(We shouldn't need the complex symlink handling logic for target)

arch-chroot.in

@@ -30,53 +30,6 @@ itself to make it one, i.e. 'mount --bind chroot_dir chroot_dir'.
 EOF
 }
 
-resolve_link() {
-  local target=$1
-  local root=$2
-
-  # If a root was given, make sure it ends in a slash.
-  [[ -n $root && $root != */ ]] && root=$root/
-
-  while [[ -L $target ]]; do
-    target=$(readlink -m "$target")
-    # If a root was given, make sure the target is under it.
-    # Make sure to strip any leading slash from target first.
-    [[ -n $root && $target != $root* ]] && target=$root${target#/}
-  done
-
-  printf %s "$target"
-}
-
-chroot_add_resolv_conf() {
-  local chrootdir=$1
-  local src=$(resolve_link /etc/resolv.conf)
-  local dest=$(resolve_link "$chrootdir/etc/resolv.conf" "$chrootdir")
-
-  # If we don't have a source resolv.conf file, there's nothing useful we can do.
-  [[ -e $src ]] || return 0
-
-  if [[ ! -e $dest ]]; then
-    # There are two reasons the destination might not exist:
-    #
-    #   1. There may be no resolv.conf in the chroot.  In this case, $dest won't exist,
-    #      and it will be equal to $1/etc/resolv.conf.  In this case, we'll just exit.
-    #      The chroot environment must not be concerned with DNS resolution.
-    #
-    #   2. $1/etc/resolv.conf is (or resolves to) a broken link.  The environment
-    #      clearly intends to handle DNS resolution, but something's wrong.  Maybe it
-    #      normally creates the target at boot time.  We'll (try to) take care of it by
-    #      creating a dummy file at the target, so that we have something to bind to.
-
-    # Case 1.
-    [[ $dest = $chrootdir/etc/resolv.conf ]] && return 0
-
-    # Case 2.
-    install -Dm644 /dev/null "$dest" || return 1
-  fi
-
-  chroot_add_mount "$src" "$dest" --bind
-}
-
 if [[ -z $1 || $1 = @(-h|--help) ]]; then
   usage
   exit $(( $# ? 0 : 1 ))

common

@@ -103,6 +103,35 @@ rev_array() {
   mapfile -t _arr < <(printf '%s\n' "${_arr[@]}" | tac)
 }
 
+resolve_link() {
+  local link="$1" root="$2" target
+
+  if [[ ! $root ]]; then
+    target="$(realpath -eq "$link")"
+  else
+    # This is tricky to do. We read from $link in a loop
+    # and prepend it with $root if it's not under it.
+    # This can't handle e.g. $root is /mnt and $link
+    # is /mnt/1 pointing to /mnt/2, which should actually
+    # be /mnt/mnt/2. Luckily these edge cases shouldn't
+    # bother most of the time.
+
+    root="$(realpath -e "$root")"
+    target="$link"
+    while [[ -L "$target" ]]; do
+      target="$(readlink -m "$target")"
+      if [[ "$target" != "$root"/* ]]; then
+        target="$root/${target#/}"
+      fi
+    done
+
+    # Normalize the path and make sure the resolved target exists
+    target="$(realpath -eq "$target")"
+  fi
+
+  printf '%s' "$target"
+}
+
 chroot_add_mount() {
   mount "$@" && CHROOT_ACTIVE_MOUNTS+=("$2")
 }
@@ -128,6 +157,27 @@ chroot_add_link() {
   ln -sf "$1" "$2" && CHROOT_ACTIVE_FILES+=("$2")
 }
 
+chroot_add_resolv_conf() {
+  local src="$(resolve_link /etc/resolv.conf)" dest="$1/etc/resolv.conf"
+
+  # If we don't have a source resolv.conf file, there's nothing useful we can do.
+  if [[ ! -f "$src" ]]; then
+    warning 'Cannot find a usable resolv.conf. DNS requests might fail in chroot'
+    return 0
+  fi
+
+  # If resolv.conf in the chroot is a symlink, we make a backup of it
+  # and create a plain file so we can bind mount to it.
+  # The backup is restore during chroot_teardown().
+
+  if [[ -L "$dest" ]]; then
+    mv "$dest" "$dest.orig"
+  fi
+  touch -a "$dest"
+
+  chroot_add_mount "$src" "$dest" --bind
+}
+
 chroot_setup() {
   CHROOT_ACTIVE_MOUNTS=()
   [[ $(trap -p EXIT) ]] && die '(BUG): attempting to overwrite existing EXIT trap'
@@ -145,11 +195,17 @@ chroot_setup() {
 }
 
 chroot_teardown() {
+  local root="$1"
+
   if (( ${#CHROOT_ACTIVE_MOUNTS[@]} )); then
     rev_array CHROOT_ACTIVE_MOUNTS
     umount "${CHROOT_ACTIVE_MOUNTS[@]}"
   fi
   unset CHROOT_ACTIVE_MOUNTS
+
+  if [[ -L "$root"/etc/resolv.conf.orig ]]; then
+    mv "$root"/etc/resolv.conf.orig "$root"/etc/resolv.conf
+  fi
 }
 
 unshare_setup() {
@@ -175,7 +231,7 @@ unshare_setup() {
 }
 
 unshare_teardown() {
-  chroot_teardown
+  chroot_teardown "$1"
 
   if (( ${#CHROOT_ACTIVE_LAZY[@]} )); then
     rev_array CHROOT_ACTIVE_LAZY