fix: [rsync-backup-target] disallow using same key for different ident in any admin account

As key will map authorization to a specified directory, we can't allow
having 2 keys pointing to 2 different directories. This must be enforced.

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

rsync-backup-target/build/src/usr/local/sbin/ssh-key

@@ -65,19 +65,45 @@ ssh-key-add() {
     ## ident are unique by construction (they are struct keys)
     ## but keys need to be also unique
     declare -A keys
-    mkdir -p "${RSYNC_KEY_PATH}/backup/$label"
     content="$type $key $email"
     ident="${email##*@}"
     target="${RSYNC_KEY_PATH}/backup/$label/$ident.pub"
-    if [ -e "$target" ]; then
-        old_content=$(cat "$target")
-        if [ "$content" == "$old_content" ]; then
-            echo "Provided key already present for '$ident'." >&2
-            return 0
+
+    ## is key used already ? As key give access to a specified subdir,
+    ## we need to make sure it is unique.
+
+    for key_file in "${RSYNC_KEY_PATH}/backup/"*/*.pub; do
+        [ -e "$key_file" ] || continue
+        key_content=$(cat "$key_file")
+        if [ "$type $key" == "${key_content% *}" ]; then
+            if [ "$key_file" == "$target" ]; then
+                echo "Provided key already present for '$ident'." >&2
+                return 0
+            elif [[ "$key_file" == "${RSYNC_KEY_PATH}/"*"/$label/"*.pub ]]; then
+                type=${key_file#"${RSYNC_KEY_PATH}/"}
+                type=${type%"/$label/"*.pub}
+                key_ident=${key_file##*/}
+                key_ident=${key_ident%.pub}
+                echo "Provided key already used as $type key for '$key_ident'." >&2
+                return 1
+            else
+                olabel=${key_file#"${RSYNC_KEY_PATH}/"*/}
+                olabel=${olabel%/*.pub}
+                echo "Specified key is already used by '$olabel' account, please pick another one." >&2
+                return 1
+            fi
         fi
+    done
+
+    mkdir -p "${target%/*}"
+    if [ -e "$target" ]; then
         echo "Replacing key for '$ident'." >&2
-    elif [ -e "${RSYNC_KEY_PATH}/backup/"*"/$ident.pub" ]; then
-        echo "ident '$ident' is already reserved, please pick another one." >&2
+    elif [ -e "${RSYNC_KEY_PATH}/"*"/"*"/$ident.pub" ]; then
+        olabel=("${RSYNC_KEY_PATH}/"*"/"*"/$ident.pub")
+        olabel="${olabel[0]}"
+        olabel=${olabel#"${RSYNC_KEY_PATH}/"*/}
+        olabel=${olabel%/*.pub}
+        echo "ident '$ident' is already reserved by '$olabel', please pick another one." >&2
         return 1
     fi
     echo "$content" > "$target"