string-output-visitor: Fix to use sufficient precision

The string output visitor should serialize numbers so that the string
input visitor deserializes them back to the same number.  It fails to
do so.

print_type_number() uses format %f.  This is prone to nasty rounding
errors.  For instance, numbers between 0 and 0.0000005 get flushed to
zero.

We currently use this visitor only for HMP info migrate, info network,
info qtree, and info memdev.  No double values occur there as far as I
can tell.

Fix anyway by formatting with %.17g.  17 decimal digits always suffice
for IEEE double.

See also recent commit "qobject: Fix qnum_to_string() to use
sufficient precision".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201210161452.2813491-9-armbru@redhat.com>

diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
index b74aa4d44c46abbe326c384c12ac7b71963ca25f..5506c933deef012d4b6349af6a9a7fb8f8c80017 100644 (file)
--- a/qapi/string-output-visitor.c
+++ b/qapi/string-output-visitor.c
@@ -258,7 +258,7 @@ static bool print_type_number(Visitor *v, const char *name, double *obj,
                               Error **errp)
 {
     StringOutputVisitor *sov = to_sov(v);
-    string_output_set(sov, g_strdup_printf("%f", *obj));
+    string_output_set(sov, g_strdup_printf("%.17g", *obj));
     return true;
 }
 

diff --git a/tests/test-string-output-visitor.c b/tests/test-string-output-visitor.c
index cec20848ea2d0f2b04f2b5d8433dd6910fa19d3c..0dae04b960303af84e30ef6d34ffe9de3b42e666 100644 (file)
--- a/tests/test-string-output-visitor.c
+++ b/tests/test-string-output-visitor.c
@@ -136,7 +136,7 @@ static void test_visitor_out_number(TestOutputVisitorData *data,
     visit_type_number(data->ov, NULL, &value, &error_abort);
 
     str = visitor_get(data);
-    g_assert_cmpstr(str, ==, "3.141593");
+    g_assert_cmpstr(str, ==, "3.1415926535897931");
 }
 
 static void test_visitor_out_string(TestOutputVisitorData *data,