[PATCH v2] qcow2: Use a GString in report_unsupported_feature()

[Alberto Garcia]

This is a bit more efficient than having to allocate and free memory
for each item.

The default size (60) is enough for all the existing incompatible
features or the "Unknown incompatible feature" message.

Suggested-by: Philippe Mathieu-Daudé <phi...@redhat.com>
Signed-off-by: Alberto Garcia <be...@igalia.com>
Reviewed-by: Alex Bennée <alex.ben...@linaro.org>
---
 block/qcow2.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

v2: Use g_autoptr and update commit message

diff --git a/block/qcow2.c b/block/qcow2.c
index cef9d72b3a..e29fc07068 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -453,16 +453,15 @@ static void cleanup_unknown_header_ext(BlockDriverState 
*bs)
 static void report_unsupported_feature(Error **errp, Qcow2Feature *table,
                                        uint64_t mask)
 {
-    char *features = g_strdup("");
-    char *old;
+    g_autoptr(GString) features = g_string_sized_new(60);
 
     while (table && table->name[0] != '\0') {
         if (table->type == QCOW2_FEAT_TYPE_INCOMPATIBLE) {
             if (mask & (1ULL << table->bit)) {
-                old = features;
-                features = g_strdup_printf("%s%s%.46s", old, *old ? ", " : "",
-                                           table->name);
-                g_free(old);
+                if (features->len > 0) {
+                    g_string_append(features, ", ");
+                }
+                g_string_append_printf(features, "%.46s", table->name);
                 mask &= ~(1ULL << table->bit);
             }
         }
@@ -470,14 +469,14 @@ static void report_unsupported_feature(Error **errp, 
Qcow2Feature *table,
     }
 
     if (mask) {
-        old = features;
-        features = g_strdup_printf("%s%sUnknown incompatible feature: %" 
PRIx64,
-                                   old, *old ? ", " : "", mask);
-        g_free(old);
+        if (features->len > 0) {
+            g_string_append(features, ", ");
+        }
+        g_string_append_printf(features,
+                               "Unknown incompatible feature: %" PRIx64, mask);
     }
 
-    error_setg(errp, "Unsupported qcow2 feature(s): %s", features);
-    g_free(features);
+    error_setg(errp, "Unsupported qcow2 feature(s): %s", features->str);
 }
 
 /*
-- 
2.20.1