round up the expand-path pointer in msgpack_zone_malloc

include/msgpack/zone.h

@@ -125,7 +125,13 @@ static inline void* msgpack_zone_malloc(msgpack_zone* zone, size_t size)
     {
         void* ptr = msgpack_zone_malloc_expand(zone, size + (MSGPACK_ZONE_ALIGN - 1));
         if (ptr) {
-            return (char*)((uintptr_t)(ptr) & ~(uintptr_t)(MSGPACK_ZONE_ALIGN - 1));
+            /* round up into the padding reserved above; rounding down
+             * would hand out the new chunk's header */
+            return (char*)(
+                (uintptr_t)(
+                    (char*)ptr + (MSGPACK_ZONE_ALIGN - 1)
+                ) & ~(uintptr_t)(MSGPACK_ZONE_ALIGN - 1)
+            );
         }
     }
     return NULL;

test/CMakeLists.txt

@@ -13,6 +13,7 @@ SET (check_PROGRAMS
     msgpack_c.cpp
     pack_unpack_c.cpp
     streaming_c.cpp
+    zone_align_c.cpp
     zone_c.cpp
 )
 

test/zone_align_c.cpp

@@ -0,0 +1,29 @@
+/* msgpack_zone_malloc() is inline, so its alignment is fixed per translation
+ * unit. Use one stricter than the zone chunk header, as CMakeLists.txt does
+ * on sparc (MSGPACK_ZONE_ALIGN=8 with a 4-byte header), so that an object
+ * served from a newly expanded chunk needs padding. */
+#undef MSGPACK_ZONE_ALIGN
+#define MSGPACK_ZONE_ALIGN (2 * sizeof(void*))
+
+#include <msgpack/zone.h>
+
+#include <gtest/gtest.h>
+
+#include <string.h>
+
+TEST(zone, zone_c_malloc_expand_align)
+{
+    msgpack_zone z;
+    const size_t size = 4096;
+    EXPECT_TRUE(msgpack_zone_init(&z, 2048));
+    /* larger than the chunk size: served by msgpack_zone_malloc_expand() */
+    char* p = (char*)msgpack_zone_malloc(&z, size);
+    ASSERT_TRUE(p != NULL);
+    EXPECT_TRUE((uintptr_t)p % MSGPACK_ZONE_ALIGN == 0);
+    /* the object has to stay inside the size + MSGPACK_ZONE_ALIGN - 1 bytes
+     * reserved by the expansion, not overlap the new chunk's header */
+    ASSERT_TRUE(p >= z.chunk_list.ptr - (size + MSGPACK_ZONE_ALIGN - 1));
+    ASSERT_TRUE(p + size <= z.chunk_list.ptr);
+    memset(p, 0xff, size);
+    msgpack_zone_destroy(&z);
+}