Enable snapshot <> snapshot diffing

This change adds in the ability to diff a snapshot against another
snapshot, instead of always against the current state of the filesystem.

The tab key is used to select one or two snapshots that can be passed
into the diff browser.

If two snapshots are selected and a non-diff action is chosen, the
second snapshot (as determined by fzf) is discarded from the return
value.

Since there can potentially be confusion as to which snapshot was
selected, the clone/duplicate screens now show the snapshot to be
cloned/duplicated at the top of the screen.

The diff browser screen no longer shows the snapshot name as the input
prompt - a generic '> ' is instead used.

Lastly, zfsbootmenu-preview now accepts an optional third argument to be
shown on a 3rd preview line. This can be used to provide additional /
relevant help on the screen - such as noting the tab key can be used to
select two snapshots for diffing.

Author: zdykstra

90zfsbootmenu/zfsbootmenu-lib.sh

@@ -540,7 +540,7 @@ draw_kernel() {
   expects="--expect=alt-d,alt-u"
 
   if ! selected="$( HELP_SECTION=kernel-management ${FUZZYSEL} \
-     --prompt "${benv} > " --tac --with-nth=2 --header="${header}" \
+      --prompt "${benv} > " --tac --with-nth=2 --header="${header}" \
       ${expects} ${expects//alt-/ctrl-} ${expects//alt-/ctrl-alt-} \
       --preview="/libexec/zfsbootmenu-preview ${benv} ${BOOTFS}"  \
       --preview-window="up:${PREVIEW_HEIGHT}" < "${_kernels}" )"; then
@@ -556,7 +556,7 @@ draw_kernel() {
 }
 
 # arg1: ZFS filesystem name
-# prints: selected snapshot
+# prints: selected snapshot name, optionally a second snapshot
 # returns: 130 on error, 0 otherwise
 
 draw_snapshots() {
@@ -575,18 +575,19 @@ draw_snapshots() {
     "[CTRL+X] clone and promote" "[CTRL+C] clone only" "" \
     "[CTRL+J] jump into chroot" "[CTRL+D] show diff" "" \
     "[CTRL+L] view logs" "[CTRL+H] help" )"
+  context="Note: for diff viewer, use tab to select/deselect up to two items"
 
   expects="--expect=alt-x,alt-c,alt-j,alt-o"
 
   if ! selected="$( zfs list -t snapshot -H -o name "${benv}" -S "${sort_key}" |
       HELP_SECTION=snapshot-management ${FUZZYSEL} \
-        --prompt "Snapshot > " --header="${header}" --tac \
+        --prompt "Snapshot > " --header="${header}" --tac --multi 2 \
         ${expects} ${expects//alt-/ctrl-} ${expects//alt-/ctrl-alt-} \
-        --bind='alt-d:execute[ /libexec/zfunc draw_diff {} ]' \
-        --bind='ctrl-d:execute[ /libexec/zfunc draw_diff {} ]' \
-        --bind='ctrl-alt-d:execute[ /libexec/zfunc draw_diff {} ]' \
-        --preview="/libexec/zfsbootmenu-preview ${benv} ${BOOTFS}" \
-        --preview-window="up:${PREVIEW_HEIGHT}" )"; then
+        --bind='alt-d:execute[ /libexec/zfunc draw_diff {+} ]' \
+        --bind='ctrl-d:execute[ /libexec/zfunc draw_diff {+} ]' \
+        --bind='ctrl-alt-d:execute[ /libexec/zfunc draw_diff {+} ]' \
+        --preview="/libexec/zfsbootmenu-preview ${benv} ${BOOTFS} '${context}'" \
+        --preview-window="up:$(( PREVIEW_HEIGHT + 1 ))" )"; then
     return 1
   fi
 
@@ -604,15 +605,34 @@ draw_snapshots() {
 # returns: nothing
 
 draw_diff() {
-  local snapshot pool diff_target mnt
+  local snapshot diff_target pool base_fs mnt
   local zfs_diff zfs_diff_PID
+  local line_one line_two left_pad
 
   snapshot="${1}"
   if [ -z "${snapshot}" ]; then
     zerror "snapshot is undefined"
     return 130
   fi
+
+  # if a second parameter was passed in and it's a snapshot, compare
+  # creation dates and make sure diff_target is newer than snapshot
+  if [ -n "${2}" ] ; then
+    local sd td
+    sd="$( zfs get -H -p -o value creation "${snapshot}" )"
+    td="$( zfs get -H -p -o value creation "${2}" )"
+    if [ "${sd}" -lt "${td}" ] ; then
+      diff_target="${2}"
+    else
+      diff_target="${snapshot}"
+      snapshot="${2}"
+    fi
+  else
+    diff_target="${snapshot%%@*}"
+  fi
+
   zdebug "snapshot: ${snapshot}"
+  zdebug "diff target: ${diff_target}"
 
   pool="${snapshot%%/*}"
   zdebug "pool: ${pool}"
@@ -622,24 +642,30 @@ draw_diff() {
     return
   fi
 
-  diff_target="${snapshot%%@*}"
-  zdebug "base filesystem: ${diff_target}"
+  base_fs="${snapshot%%@*}"
+  zdebug "base filesystem: ${base_fs}"
 
-  CLEAR_SCREEN=1 load_key "${diff_target}"
+  CLEAR_SCREEN=1 load_key "${base_fs}"
 
-  if ! mnt="$( mount_zfs "${diff_target}" )" ; then
-    zerror "unable to mount ${diff_target}"
+  if ! mnt="$( mount_zfs "${base_fs}" )" ; then
+    zerror "unable to mount ${base_fs}"
     return
   fi
 
+  zdebug "executing: zfs diff -F -H ${snapshot} ${diff_target}"
   coproc zfs_diff ( zfs diff -F -H "${snapshot}" "${diff_target}" )
 
   # Bash won't use an FD referenced in a variable on the left side of a pipe
   exec 3>&"${zfs_diff[0]}"
 
   # shellcheck disable=SC2154
-  sed "s,${mnt},," <&3 | HELP_SECTION=diff-viewer ${FUZZYSEL} --prompt "${snapshot} > " \
-    --preview="/libexec/zfsbootmenu-preview ${diff_target} ${BOOTFS}" \
+  line_one="$( center_string "---${snapshot}" )"
+  left_pad="${line_one//---${snapshot}/}"
+  line_one="$( colorize red "${line_one}" )"
+  line_two="${left_pad}$( colorize green "+++${diff_target}" )"
+
+  sed "s,${mnt},," <&3 | HELP_SECTION=diff-viewer ${FUZZYSEL} --prompt "> " \
+    --preview="echo -e '${line_one}\n${line_two}'" --no-sort \
     --preview-window="up:${PREVIEW_HEIGHT}"
 
   [ -n "${zfs_diff_PID}" ] && kill "${zfs_diff_PID}"

90zfsbootmenu/zfsbootmenu-preview.sh

@@ -43,3 +43,8 @@ fi
 
 colorize "${_COLOR}" "${selected_env_str}\n"
 echo "${selected_arguments}"
+
+if [ -n "${3}" ]; then
+  context="$( center_string "${3}" )"
+  colorize "orange" "${context}"
+fi

90zfsbootmenu/zfsbootmenu.sh

@@ -220,6 +220,9 @@ while true; do
 
       # shellcheck disable=SC2162
       IFS=, read subkey selected_snap <<< "${selection}"
+
+      # two snapshots were potentially returned - discard the second
+      selected_snap="${selected_snap%,*}"
       zdebug "selected snapshot: ${selected_snap}"
 
       # Parent of the selected dataset, must be nonempty
@@ -261,6 +264,9 @@ while true; do
       # Strip snapshot name and append NEW
       pre_populated="${pre_populated%%@*}_NEW"
 
+      header="$( center_string "${selected_snap}" )"
+      colorize green "${header}"
+
       while true; do
         echo -e "\nNew boot environment name (leave blank to abort)"
         new_be="$( /libexec/zfsbootmenu-input "${pre_populated}" )"

pod/online/snapshot-management.pod

@@ -39,7 +39,7 @@ The operation will fail gracefully if the pool can not be set I<read/write>.
 
 =item I<[MOD+D]> B<diff>
 
-Compare the differences between the selected snapshot and the current state of the boot environment.
+Compare the differences between snapshots and filesystems. A single snapshot can be selected and a diff will be generated between that and the current state of the filesystem. Two snapshots can be selected (and deselected) with the tab key and a diff will be generated between them.
 
 The operation will fail gracefully if the pool can not be set I<read/write>.