swim: do not return raw member tt_uuid pointer

When a member is deleted, pointer at its struct tt_uuid becomes
invalid. This commit makes member:uuid() create a new tt_uuid
object, cached for subsequent calls.

src/lua/swim.lua

@@ -391,7 +391,15 @@ end
 -- metatable, serialization, string conversions etc.
 --
 local function swim_member_uuid(m)
-    return capi.swim_member_uuid(swim_check_member(m, 'member:uuid()'))
+    local ptr = swim_check_member(m, 'member:uuid()')
+    local u = m.u
+    if not u then
+        ptr = capi.swim_member_uuid(ptr)
+        u = ffi.new('struct tt_uuid')
+        ffi.copy(u, ptr, ffi.sizeof('struct tt_uuid'))
+        rawset(m, 'u', u)
+    end
+    return u
 end
 
 local function swim_member_serialize(m)

test/swim/swim.result

@@ -403,6 +403,10 @@ s:uuid()
 ---
 - 00000000-0000-1000-8000-000000000001
 ...
+s:uuid() == s:uuid()
+---
+- true
+...
 s:uri()
 ---
 - 127.0.0.1:<port>

test/swim/swim.test.lua

@@ -137,6 +137,7 @@ s = s1:self()
 s
 s:status()
 s:uuid()
+s:uuid() == s:uuid()
 s:uri()
 s:incarnation()
 s:payload_cdata()