Never truncate the conf file when a saved value breaks the rewrite sed

A value holding a backslash-digit sequence (a backreference to sed) or an
embedded line break made _setopt's replace command fail after the shell
had already truncated the conf file, wiping the whole domain conf; the
next renewal then fails with an empty Le_API and no validation method.
Same class as #2426, which escaped only '&' and '|'.

Escape the backslash too, write the sed output back only when sed
succeeds, reject values holding a line break, and rewrite the file with
printf instead of echo in the append path and in _clear_conf: dash's
builtin echo interprets backslash escapes and corrupted such values on
every rewrite.

https://github.com/acmesh-official/acme.sh/issues/7213

acme.sh

@@ -2499,6 +2499,15 @@ _send_signed_request() {
 
 }
 
+#Reads a value from stdin, prints it escaped for use as the replacement text
+#of a sed s command delimited by '|'. The backslash must go first: a bare one
+#starts an escape sequence and backslash-digit is a backreference, both make
+#sed error out. Then '&' (the whole-match reference) and the '|' delimiter.
+#https://github.com/acmesh-official/acme.sh/issues/7213
+_sed_escape_rhs() {
+  sed -e 's/\\/\\\\/g' -e 's/&/\\&/g' -e 's/|/\\|/g'
+}
+
 #setopt "file"  "opt"  "="  "value" [";"]
 _setopt() {
   __conf="$1"
@@ -2514,34 +2523,50 @@ _setopt() {
     touch "$__conf"
     chmod 600 "$__conf"
   fi
+  __nl="
+"
+  case "$__val" in
+  *"$__nl"*)
+    #the conf format is line based and the file is sourced by the shell, so a
+    #value holding a line break cannot be represented in it (it would also
+    #make the replace sed below fail with an unterminated 's' command)
+    _err "The value of '$__opt' contains a line break, it cannot be saved to $__conf."
+    return 1
+    ;;
+  esac
   if [ -n "$(_tail_c 1 <"$__conf")" ]; then
     echo >>"$__conf"
   fi
 
   if grep -n "^$__opt$__sep" "$__conf" >/dev/null; then
     _debug3 OK
-    if _contains "$__val" "&"; then
-      __val="$(echo "$__val" | sed 's/&/\\&/g')"
-    fi
-    if _contains "$__val" "|"; then
-      __val="$(echo "$__val" | sed 's/|/\\|/g')"
-    fi
+    __val="$(printf -- "%s\n" "$__val" | _sed_escape_rhs)"
     text="$(cat "$__conf")"
-    printf -- "%s\n" "$text" | sed "s|^$__opt$__sep.*$|$__opt$__sep$__val$__end|" >"$__conf"
+    #capture first, write only on success: redirecting sed straight into the
+    #conf file truncates it before sed runs, so a failing sed (e.g. on an
+    #unescaped special character in the value) wiped the whole conf (#2426)
+    if __text="$(printf -- "%s\n" "$text" | sed "s|^$__opt$__sep.*$|$__opt$__sep$__val$__end|")"; then
+      printf -- "%s\n" "$__text" >"$__conf"
+    else
+      _err "Cannot save '$__opt' to $__conf."
+      return 1
+    fi
 
   elif grep -n "^#$__opt$__sep" "$__conf" >/dev/null; then
-    if _contains "$__val" "&"; then
-      __val="$(echo "$__val" | sed 's/&/\\&/g')"
-    fi
-    if _contains "$__val" "|"; then
-      __val="$(echo "$__val" | sed 's/|/\\|/g')"
-    fi
+    __val="$(printf -- "%s\n" "$__val" | _sed_escape_rhs)"
     text="$(cat "$__conf")"
-    printf -- "%s\n" "$text" | sed "s|^#$__opt$__sep.*$|$__opt$__sep$__val$__end|" >"$__conf"
+    if __text="$(printf -- "%s\n" "$text" | sed "s|^#$__opt$__sep.*$|$__opt$__sep$__val$__end|")"; then
+      printf -- "%s\n" "$__text" >"$__conf"
+    else
+      _err "Cannot save '$__opt' to $__conf."
+      return 1
+    fi
 
   else
     _debug3 APP
-    echo "$__opt$__sep$__val$__end" >>"$__conf"
+    #printf, not echo: dash's builtin echo interprets backslash escapes in
+    #the value and would corrupt it
+    printf -- "%s\n" "$__opt$__sep$__val$__end" >>"$__conf"
   fi
   _debug3 "$(grep -n "^$__opt$__sep" "$__conf")"
 }
@@ -2569,7 +2594,9 @@ _clear_conf() {
   _sdkey="$2"
   if [ "$_c_c_f" ]; then
     _conf_data="$(cat "$_c_c_f")"
-    echo "$_conf_data" | sed "/^$_sdkey *=.*$/d" >"$_c_c_f"
+    #printf, not echo: dash's builtin echo interprets backslash escapes and
+    #would corrupt saved values that contain them on every rewrite
+    printf -- "%s\n" "$_conf_data" | sed "/^$_sdkey *=.*$/d" >"$_c_c_f"
   else
     _err "Config file is empty, cannot clear"
   fi