[Tarantool-patches] [PATCH vshard 1/1] router: bucket_id_strcrc32 and bucket_id_mpcrc32

[Vladislav Shpilevoy]

Thanks for the comments!

> On 26/02/2020 02:52, Vladislav Shpilevoy wrote:
>> Closes #207
>>
>> @TarantoolBot document
>> Title: vshard.router.bucket_id_strcrc32() and .bucket_id_mpcrc32()
>> vshard.router.bucket_id() is deprecated, each its usage logs a
>> warning. It still works, but will be deleted in future.
> 
> I think it's quite agressive to print message in logs for each time. Moreover it could lead to unexpected behaviour for some people.
> IMO the most proper way print such message only once. Consider using something like that - https://github.com/tarantool/errors/blob/master/errors/deprecate.lua#L21

The aforementioned module won't print a warning only once. It will
print all the errors again once a code reload happens, because
'local errors_issued = {}' will be called again on reload, and
will clear 'errors_issued' table. But perhaps it was done
intentionally.

I made it the same way. Intentionally. In that case it will help
user to replace all bucket_id() calls during reloads, and after each
reload he will be able to validate whether the message is printed
again anywhere.

====================
+local router_bucket_id_deprecated_warn = true
 local function router_bucket_id(router, key)
     if key == nil then
         error("Usage: vshard.router.bucket_id(key)")
     end
-    log.warn('vshard.router.bucket_id() is deprecated, use '..
-             'vshard.router.bucket_id_strcrc32() or '..
-             'vshard.router.bucket_id_mpcrc32()')
+    if router_bucket_id_deprecated_warn then
+        router_bucket_id_deprecated_warn = false
+        log.warn('vshard.router.bucket_id() is deprecated, use '..
+                 'vshard.router.bucket_id_strcrc32() or '..
+                 'vshard.router.bucket_id_mpcrc32()')
+    end
     return lhash.strcrc32(key) % router.total_bucket_count + 1
 end
====================

>> Behaviour of the old bucket_id() function is now available as
>> vshard.router.bucket_id_strcrc32(). It works exactly like the old
>> function, but does not log a warning.
>>
>> The reason why there is a new function bucket_id_mpcrc32() is that
>> the old bucket_id() and the new bucket_id_strcrc32() are not
>> consistent for cdata numbers. In particular, they return 3
>> different values for normal Lua numbers like 123, for unsigned
>> long long cdata (like 123ULL, or ffi.cast('unsigned long long',
>> 123)), and for signed long long cdata (like 123LL, or
>> ffi.cast('long long', 123)). Note, this is important!
>>
>>      vshard.router.bucket_id(123)
>>      vshard.router.bucket_id(123LL)
>>      vshard.router.bucket_id(123ULL)
>>
>>      Return 3 different values!!!
>>
>> For float and double cdata (ffi.cast('float', number),
>> ffi.cast('double', number)) these functions return different
>> values even for the same numbers of the same floating point type.
>> This is because tostring() on a floating point cdata number
>> returns not the number, but a pointer at it. Different on each
>> call.
>>
>> vshard.router.bucket_id_strcrc32() behaves exactly the same.
>>
>> vshard.router.bucket_id_mpcrc32() is safer. 
> 
> Looks like it's not enough safe for new "double" type:
> 
> | tarantool> box.space.test:create_index('double', {type='tree', parts = {1, 'double'}})
> | tarantool> box.space.test:insert({ffi.cast('double', 1)}) -- cdata
> | ---
> | - [1]
> | ...
> | tarantool> type(box.space.test:select()[1][1])
> | ---
> | - number
> | ...
> 
> And results of msgpack.encode are different.
> 
> I think you can return to your idea (https://github.com/tarantool/vshard/issues/207#issuecomment-589976204) with "tonumber":
> |tarantool> tonumber(ffi.cast('double', 100))
> | ---
> | - 100
> | ...
> 
> Such approach will lead to precision lost for huge unsigned/signed integers. But it will be predictable and determined for all cases.

I added that example to docbot request, slightly modified.