[Qemu-devel] [PATCH v7 01/31] qobject: Document more shortcomings in our number handling

[Eric Blake <eblake@redhat.com>]

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
risks truncating floating point numbers to the point of losing
significant precision.

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>

---
v7: new patch
---
 qobject/json-parser.c | 4 +++-
 qobject/qjson.c       | 8 +++++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/qobject/json-parser.c b/qobject/json-parser.c
index 3c5d35d..6ab98a7 100644
--- a/qobject/json-parser.c
+++ b/qobject/json-parser.c
@@ -1,5 +1,5 @@
 /*
- * JSON Parser 
+ * JSON Parser
  *
  * Copyright IBM, Corp. 2009
  *
@@ -519,6 +519,8 @@ static QObject *parse_literal(JSONParserContext *ctxt)
     }
     case JSON_FLOAT:
         /* FIXME dependent on locale */
+        /* FIXME our lexer matches RFC7159 in forbidding Inf or NaN,
+         * but those might be useful extensions beyond JSON */
         return QOBJECT(qfloat_from_double(strtod(token->str, NULL)));
     default:
         abort();
diff --git a/qobject/qjson.c b/qobject/qjson.c
index a3e6a7c..41d9d65 100644
--- a/qobject/qjson.c
+++ b/qobject/qjson.c
@@ -237,6 +237,12 @@ 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. */
+        /* FIXME: This risks printing Inf or NaN, which are not valid
+         * JSON values. */
+        /* FIXME: the default precision of %f may be insufficient to
+         * tell this number apart from others. */
         len = snprintf(buffer, sizeof(buffer), "%f", qfloat_get_double(val));
         while (len > 0 && buffer[len - 1] == '0') {
             len--;
@@ -247,7 +253,7 @@ static void to_json(const QObject *obj, QString *str, int pretty, int indent)
         } else {
             buffer[len] = 0;
         }
-        
+
         qstring_append(str, buffer);
         break;
     }
-- 
2.4.3