Improve error reporting when serializing non-Unicode strings to JSON

* src/coding.c (utf8_string_p): New helper function.
(syms_of_coding) <utf-8-unix>: Move from json.c.

* src/json.c (json_check_utf8): New helper function.
(lisp_to_json_toplevel_1, lisp_to_json): Use it.  To save a bit of
time, check for invalid UTF-8 strings only after encountering an
error, since Jansson already rejects them.

* test/src/json-tests.el (json-serialize/invalid-unicode): Adapt
expected error symbol.

src/coding.c

@@ -6360,6 +6360,27 @@ check_utf_8 (struct coding_system *coding)
 }
 
 
+/* Return whether STRING is a valid UTF-8 string.  STRING must be a
+   unibyte string.  */
+
+bool
+utf8_string_p (Lisp_Object string)
+{
+  eassert (!STRING_MULTIBYTE (string));
+  struct coding_system coding;
+  setup_coding_system (Qutf_8_unix, &coding);
+  /* We initialize only the fields that check_utf_8 accesses.  */
+  coding.head_ascii = -1;
+  coding.src_pos = 0;
+  coding.src_pos_byte = 0;
+  coding.src_chars = SCHARS (string);
+  coding.src_bytes = SBYTES (string);
+  coding.src_object = string;
+  coding.eol_seen = EOL_SEEN_NONE;
+  return check_utf_8 (&coding) != -1;
+}
+
+
 /* Detect how end-of-line of a text of length SRC_BYTES pointed by
    SOURCE is encoded.  If CATEGORY is one of
    coding_category_utf_16_XXXX, assume that CR and LF are encoded by
@@ -10846,6 +10867,7 @@ syms_of_coding (void)
   DEFSYM (Qiso_2022, "iso-2022");
 
   DEFSYM (Qutf_8, "utf-8");
+  DEFSYM (Qutf_8_unix, "utf-8-unix");
   DEFSYM (Qutf_8_emacs, "utf-8-emacs");
 
 #if defined (WINDOWSNT) || defined (CYGWIN)

src/coding.h

@@ -665,6 +665,7 @@ struct coding_system
 /* Extern declarations.  */
 extern Lisp_Object code_conversion_save (bool, bool);
 extern bool encode_coding_utf_8 (struct coding_system *);
+extern bool utf8_string_p (Lisp_Object);
 extern void setup_coding_system (Lisp_Object, struct coding_system *);
 extern Lisp_Object coding_charset_list (struct coding_system *);
 extern Lisp_Object coding_system_charset_list (Lisp_Object);

src/json.c

@@ -316,6 +316,15 @@ json_check (json_t *object)
   return object;
 }
 
+/* If STRING is not a valid UTF-8 string, signal an error of type
+   `wrong-type-argument'.  STRING must be a unibyte string.  */
+
+static void
+json_check_utf8 (Lisp_Object string)
+{
+  CHECK_TYPE (utf8_string_p (string), Qutf_8_string_p, string);
+}
+
 static json_t *lisp_to_json (Lisp_Object);
 
 /* Convert a Lisp object to a toplevel JSON object (array or object).
@@ -363,9 +372,12 @@ lisp_to_json_toplevel_1 (Lisp_Object lisp, json_t **json)
             int status = json_object_set_new (*json, key_str,
                                               lisp_to_json (HASH_VALUE (h, i)));
             if (status == -1)
-              /* FIXME: A failure here might also indicate that the
-                 key is not a valid Unicode string.  */
-              json_out_of_memory ();
+              {
+                /* A failure can be caused either by an invalid key or
+                   by low memory.  */
+                json_check_utf8 (key);
+                json_out_of_memory ();
+              }
           }
       clear_unwind_protect (count);
       return unbind_to (count, Qnil);
@@ -447,9 +459,15 @@ lisp_to_json (Lisp_Object lisp)
   else if (STRINGP (lisp))
     {
       Lisp_Object encoded = json_encode (lisp);
-      /* FIXME: We might throw an out-of-memory error here if the
-         string is not valid Unicode.  */
-      return json_check (json_stringn (SSDATA (encoded), SBYTES (encoded)));
+      json_t *json = json_stringn (SSDATA (encoded), SBYTES (encoded));
+      if (json == NULL)
+        {
+          /* A failure can be caused either by an invalid string or by
+             low memory.  */
+          json_check_utf8 (encoded);
+          json_out_of_memory ();
+        }
+      return json;
     }
 
   /* LISP now must be a vector, hashtable, or alist.  */
@@ -863,8 +881,7 @@ syms_of_json (void)
 
   DEFSYM (Qstring_without_embedded_nulls_p, "string-without-embedded-nulls-p");
   DEFSYM (Qjson_value_p, "json-value-p");
-
-  DEFSYM (Qutf_8_unix, "utf-8-unix");
+  DEFSYM (Qutf_8_string_p, "utf-8-string-p");
 
   DEFSYM (Qjson_error, "json-error");
   DEFSYM (Qjson_out_of_memory, "json-out-of-memory");

test/src/json-tests.el

@@ -108,13 +108,11 @@
 
 (ert-deftest json-serialize/invalid-unicode ()
   (skip-unless (fboundp 'json-serialize))
-  ;; FIXME: "out of memory" is the wrong error signal, but we don't
-  ;; currently distinguish between error types when serializing.
-  (should-error (json-serialize ["a\uDBBBb"]) :type 'json-out-of-memory)
-  (should-error (json-serialize ["u\x110000v"]) :type 'json-out-of-memory)
-  (should-error (json-serialize ["u\x3FFFFFv"]) :type 'json-out-of-memory)
-  (should-error (json-serialize ["u\xCCv"]) :type 'json-out-of-memory)
-  (should-error (json-serialize ["u\u00C4\xCCv"]) :type 'json-out-of-memory))
+  (should-error (json-serialize ["a\uDBBBb"]) :type 'wrong-type-argument)
+  (should-error (json-serialize ["u\x110000v"]) :type 'wrong-type-argument)
+  (should-error (json-serialize ["u\x3FFFFFv"]) :type 'wrong-type-argument)
+  (should-error (json-serialize ["u\xCCv"]) :type 'wrong-type-argument)
+  (should-error (json-serialize ["u\u00C4\xCCv"]) :type 'wrong-type-argument))
 
 (ert-deftest json-parse-string/null ()
   (skip-unless (fboundp 'json-parse-string))