feat(updater): add `background-alpha` update mode (preview) (#11928)

NOTE: this feature is in alpha / preview mode, it is not guaranteed to work 100% of the
time in all cases. If you experience any issues, open an issue or search for an open
one describing your same situation.

To use this, use the zstyle update mode settings [1] with the value `background-alpha`:

  zstyle ':omz:update' mode background-alpha

[1] https://github.com/ohmyzsh/ohmyzsh#getting-updates

tools/check_for_upgrade.sh

@@ -9,6 +9,7 @@ fi
 # - prompt (default): the user is asked before updating when it's time to update
 # - auto: the update is performed automatically when it's time
 # - reminder: a reminder is shown to the user when it's time to update
+# - background-alpha: an experimental update-on-the-background option
 # - disabled: automatic update is turned off
 zstyle -s ':omz:update' mode update_mode || {
   update_mode=prompt
@@ -91,13 +92,37 @@ function is_update_available() {
 }
 
 function update_last_updated_file() {
+  local exit_status="$1" error="$2"
+
+  if [[ -z "${1}${2}" ]]; then
     echo "LAST_EPOCH=$(current_epoch)" >! "${ZSH_CACHE_DIR}/.zsh-update"
+    return
+  fi
+
+  cat >! "${ZSH_CACHE_DIR}/.zsh-update" <<EOD
+LAST_EPOCH=$(current_epoch)
+EXIT_STATUS=${exit_status}
+ERROR='${error//\'/’}'
+EOD
 }
 
 function update_ohmyzsh() {
+  local verbose_mode
   zstyle -s ':omz:update' verbose verbose_mode || verbose_mode=default
-  if ZSH="$ZSH" zsh -f "$ZSH/tools/upgrade.sh" -i -v $verbose_mode; then
+
+  if [[ "$update_mode" != background-alpha ]] \
+    && LANG= ZSH="$ZSH" zsh -f "$ZSH/tools/upgrade.sh" -i -v $verbose_mode; then
     update_last_updated_file
+    return $?
+  fi
+
+  local exit_status error
+  if error=$(LANG= ZSH="$ZSH" zsh -f "$ZSH/tools/upgrade.sh" -i -v silent 2>&1); then
+    update_last_updated_file 0 "Update successful"
+  else
+    exit_status=$?
+    update_last_updated_file $exit_status "$error"
+    return $exit_status
   fi
 }
 
@@ -126,6 +151,7 @@ function has_typed_input() {
   }
 }
 
+function handle_update() {
   () {
     emulate -L zsh
 
@@ -154,7 +180,7 @@ function has_typed_input() {
     trap "
       ret=\$?
       unset update_mode
-    unset -f current_epoch is_update_available update_last_updated_file update_ohmyzsh 2>/dev/null
+      unset -f current_epoch is_update_available update_last_updated_file update_ohmyzsh handle_update 2>/dev/null
       command rm -rf '$ZSH/log/update.lock'
       return \$ret
     " EXIT INT QUIT
@@ -185,14 +211,14 @@ function has_typed_input() {
     fi
 
     # If in reminder mode or user has typed input, show reminder and exit
-  if [[ "$update_mode" = reminder ]] || has_typed_input; then
+    if [[ "$update_mode" = reminder ]] || { [[ "$update_mode" != background-alpha ]] && has_typed_input }; then
       printf '\r\e[0K' # move cursor to first column and clear whole line
       echo "[oh-my-zsh] It's time to update! You can do that by running \`omz update\`"
       return 0
     fi
 
     # Don't ask for confirmation before updating if in auto mode
-  if [[ "$update_mode" = auto ]]; then
+    if [[ "$update_mode" = (auto|background-alpha) ]]; then
       update_ohmyzsh
       return $?
     fi
@@ -210,4 +236,60 @@ function has_typed_input() {
   }
 
   unset update_mode
-unset -f current_epoch is_update_available update_last_updated_file update_ohmyzsh
+  unset -f current_epoch is_update_available update_last_updated_file update_ohmyzsh handle_update
+}
+
+case "$update_mode" in
+  background-alpha)
+    autoload -Uz add-zsh-hook
+
+    _omz_bg_update() {
+      # do the update in a subshell
+      (handle_update) &|
+
+      # register update results function
+      add-zsh-hook precmd _omz_bg_update_status
+
+      # deregister background function
+      add-zsh-hook -d precmd _omz_bg_update
+      unset -f _omz_bg_update
+    }
+
+    _omz_bg_update_status() {
+      {
+        local LAST_EPOCH EXIT_STATUS ERROR
+        if [[ ! -f "$ZSH_CACHE_DIR"/.zsh-update ]]; then
+          return 1
+        fi
+
+        # check update results until timeout is reached
+        . "$ZSH_CACHE_DIR/.zsh-update"
+        if [[ -z "$EXIT_STATUS" || -z "$ERROR" ]]; then
+          return 1
+        fi
+
+        if [[ "$EXIT_STATUS" -eq 0 ]]; then
+          print -P "\n%F{green}[oh-my-zsh] Update successful.%f"
+          return 0
+        elif [[ "$EXIT_STATUS" -ne 0 ]]; then
+          print -P "\n%F{red}[oh-my-zsh] There was an error updating:%f"
+          printf "\n${fg[yellow]}%s${reset_color}" "$ERROR"
+          return 0
+        fi
+      } always {
+        if (( TRY_BLOCK_ERROR == 0 )); then
+          # if last update results have been handled, remove them from the status file
+          update_last_updated_file
+
+          # deregister background function
+          add-zsh-hook -d precmd _omz_bg_update_status
+          unset -f _omz_bg_update_status
+        fi
+      }
+    }
+
+    add-zsh-hook precmd _omz_bg_update
+    ;;
+  *)
+    handle_update ;;
+esac