feat(ssh-agent): allow using external helper to ask for passwords (#7631)

plugins/ssh-agent/README.md

@@ -55,6 +55,15 @@ ssh-add -K -c -a /run/user/1000/ssh-auth <identities>
 
 For valid `ssh-add` arguments run `ssh-add --help` or `man ssh-add`.
 
+----
+
+To set an **external helper** to ask for the passwords and possibly store
+them in the system keychain use the `helper` style. For example:
+
+```zsh
+zstyle :omz:plugins:ssh-agent helper ksshaskpass
+```
+
 ## Credits
 
 Based on code from Joseph M. Reagle: https://www.cygwin.com/ml/cygwin/2001-06/msg00537.html

plugins/ssh-agent/ssh-agent.plugin.zsh

@@ -27,7 +27,7 @@ function _add_identities() {
     # key list found on `ssh-add` man page's DESCRIPTION section
     for id in id_rsa id_dsa id_ecdsa id_ed25519 identity; do
       # check if file exists
-			[[ -f "$HOME/.ssh/$id" ]] && identities+=$id
+      [[ -f "$HOME/.ssh/$id" ]] && identities+=($id)
     done
   fi
 
@@ -44,13 +44,33 @@ function _add_identities() {
     # check for filename match, otherwise try for signature match
     if [[ ${loaded_ids[(I)$HOME/.ssh/$id]} -le 0 ]]; then
       sig="$(ssh-keygen -lf "$HOME/.ssh/$id" | awk '{print $2}')"
-			[[ ${loaded_sigs[(I)$sig]} -le 0 ]] && not_loaded+="$HOME/.ssh/$id"
+      [[ ${loaded_sigs[(I)$sig]} -le 0 ]] && not_loaded+=("$HOME/.ssh/$id")
     fi
   done
 
+  # abort if no identities need to be loaded
+  if [[ ${#not_loaded} -eq 0 ]]; then
+    return
+  fi
+
+  # pass extra arguments to ssh-add
   local args
   zstyle -a :omz:plugins:ssh-agent ssh-add-args args
-	[[ -n "$not_loaded" ]] && ssh-add "${args[@]}" ${^not_loaded}
+
+  # use user specified helper to ask for password (ksshaskpass, etc)
+  local helper
+  zstyle -s :omz:plugins:ssh-agent helper helper
+
+  if [[ -n "$helper" ]]; then
+    if [[ -z "${commands[$helper]}" ]]; then
+      echo "ssh-agent: the helper '$helper' has not been found."
+    else
+      SSH_ASKPASS="$helper" ssh-add "${args[@]}" ${^not_loaded} < /dev/null
+      return $?
+    fi
+  fi
+
+  ssh-add "${args[@]}" ${^not_loaded}
 }
 
 # Get the filename to store/lookup the environment from