qobject: Document more shortcomings in our number handling

We've already documented that our JSON parsing is locale dependent;
but we should also document that our JSON output has the same
problem.  Additionally, JSON requires finite values (you have to
upgrade to JSON5 to get support for Inf or NaN), and our output
truncates floating point numbers to the point of losing significant
precision that could cause the receiver to read a different value.

Sadly, this series is not going to be the one that addresses these
problems.

Fix some trailing whitespace I noticed in the vicinity.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1454075341-13658-2-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qobject/json-parser.c

 /*
- * JSON Parser 
+ * JSON Parser
  *
  * Copyright IBM, Corp. 2009
  *
@@ -518,7 +518,9 @@ static QObject *parse_literal(JSONParserContext *ctxt)
         /* fall through to JSON_FLOAT */
     }
     case JSON_FLOAT:
-        /* FIXME dependent on locale */
+        /* FIXME dependent on locale; a pervasive issue in QEMU */
+        /* FIXME our lexer matches RFC 7159 in forbidding Inf or NaN,
+         * but those might be useful extensions beyond JSON */
         return QOBJECT(qfloat_from_double(strtod(token->str, NULL)));
     default:
         abort();

qobject/qjson.c

@@ -238,6 +238,15 @@ static void to_json(const QObject *obj, QString *str, int pretty, int indent)
         char buffer[1024];
         int len;
 
+        /* FIXME: snprintf() is locale dependent; but JSON requires
+         * numbers to be formatted as if in the C locale. Dependence
+         * on C locale is a pervasive issue in QEMU. */
+        /* FIXME: This risks printing Inf or NaN, which are not valid
+         * JSON values. */
+        /* FIXME: the default precision of 6 for %f often causes
+         * rounding errors; we should be using DBL_DECIMAL_DIG (17),
+         * and only rounding to a shorter number if the result would
+         * still produce the same floating point value.  */
         len = snprintf(buffer, sizeof(buffer), "%f", qfloat_get_double(val));
         while (len > 0 && buffer[len - 1] == '0') {
             len--;
@@ -248,7 +257,7 @@ static void to_json(const QObject *obj, QString *str, int pretty, int indent)
         } else {
             buffer[len] = 0;
         }
-        
+
         qstring_append(str, buffer);
         break;
     }