Michelson: fix inconsistency when reading and printing strings

docs/whitedoc/michelson.rst

@@ -1741,6 +1741,9 @@ language can only be one of the four following constructs.
 
 This simple four cases notation is called Micheline.
 
+The encoding of a Micheline source file must be UTF-8, and non-ASCII
+characters can only appear in comments and strings.
+
 Constants
 ~~~~~~~~~
 
@@ -1748,12 +1751,12 @@ There are two kinds of constants:
 
 1. Integers or naturals in decimal (no prefix), hexadecimal (0x prefix),
    octal (0o prefix) or binary (0b prefix).
-2. Strings with usual escapes ``\n``, ``\t``, ``\b``, ``\r``, ``\\``,
-   ``\"``. The encoding of a Michelson source file must be UTF-8, and
-   non-ASCII characters can only appear in comments. No line break can
-   appear in a string. Any non-printable characters must be escaped
-   using two hexadecimal characters, as in ``\xHH`` or the
-   predefine escape sequences above..
+2. Strings, with usual escape sequences: ``\n``, ``\t``, ``\b``,
+   ``\r``, ``\\``, ``\"``. Unescaped line breaks (both ``\n`` and ``\r``)
+   cannot appear in the middle of a string.
+
+The current version of Michelson restricts strings to be the printable
+subset of 7-bit ASCII, plus the line break ``\n``.
 
 Primitive applications
 ~~~~~~~~~~~~~~~~~~~~~~

src/lib_micheline/micheline_parser.ml

@@ -255,7 +255,7 @@ let tokenize source =
     | `Uchar c, stop ->
         match uchar_to_char c with
         | Some '"' -> skip (tok () :: acc)
-        | Some '\n' ->
+        | Some ('\n' | '\r') ->
             errors := Unterminated_string { start ; stop } :: !errors ;
             skip (tok () :: acc)
         | Some '\\' ->

src/lib_micheline/micheline_printer.ml

@@ -26,12 +26,13 @@ let print_comment ppf text =
 let print_string ppf text =
   Format.fprintf ppf "\"" ;
   String.iter (function
-      | '"' | 'r' | 'n' | 't' | 'b' | '\\' as c ->
-          Format.fprintf ppf "%c" c
-      | '\x20'..'\x7E' as c ->
-          Format.fprintf ppf "%c" c
-      | c ->
-          Format.fprintf ppf "\\x%02X" (Char.code c))
+      | '"' -> Format.fprintf ppf "\\\""
+      | '\n' -> Format.fprintf ppf "\\n"
+      | '\r' -> Format.fprintf ppf "\\r"
+      | '\b' -> Format.fprintf ppf "\\b"
+      | '\t' -> Format.fprintf ppf "\\t"
+      | '\\' -> Format.fprintf ppf "\\\\"
+      | c -> Format.fprintf ppf "%c" c)
     text ;
   Format.fprintf ppf "\""
 

src/proto_alpha/lib_protocol/src/script_ir_translator.ml

@@ -1109,8 +1109,16 @@ let rec parse_data
         traced (fail (unexpected expr [] Constant_namespace [ D_True ; D_False ]))
     (* Strings *)
     | String_t, String (_, v) ->
-        Lwt.return (Gas.consume ctxt (Typecheck_costs.string (String.length v))) >>|? fun ctxt ->
-        (v, ctxt)
+        Lwt.return (Gas.consume ctxt (Typecheck_costs.string (String.length v))) >>=? fun ctxt ->
+        let rec check_printable_ascii i =
+          if Compare.Int.(i < 0) then true
+          else match String.get v i with
+            | '\n' | '\x20'..'\x7E' -> check_printable_ascii (i - 1)
+            | _ -> false in
+        if check_printable_ascii (String.length v - 1) then
+          return (v, ctxt)
+        else
+          fail (error ())
     | String_t, expr ->
         traced (fail (Invalid_kind (location expr, [ String_kind ], kind expr)))
     (* Integers *)