Re: Enumize logical replication message actions

On Fri, Oct 23, 2020 at 6:26 PM Ashutosh Bapat
<ashutosh(dot)bapat(at)2ndquadrant(dot)com> wrote:
>
>
>
> On Fri, 23 Oct 2020 at 18:23, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>
>> On Fri, Oct 23, 2020 at 11:50 AM Kyotaro Horiguchi
>> <horikyota(dot)ntt(at)gmail(dot)com> wrote:
>> >
>> > At Thu, 22 Oct 2020 22:31:41 -0300, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote in
>> > > On 2020-Oct-22, Ashutosh Bapat wrote:
>> > >
>> > > > On Thu, 22 Oct 2020 at 14:46, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
>> > > > wrote:
>> > >
>> > > > > pg_send_logicalrep_msg_type() looks somewhat too-much. If we need
>> > > > > something like that we shouldn't do this refactoring, I think.
>> > > >
>> > > > Enum is an integer, and we want to send byte. The function asserts that the
>> > > > enum fits a byte. If there's a way to declare byte long enums I would use
>> > > > that. But I didn't find a way to do that.
>> > >
>> > > I didn't look at the code, but maybe it's sufficient to add a
>> > > StaticAssert?
>> >
>> > That check needs to visit all symbols in a enum and confirm that each
>> > of them is in a certain range.
>> >
>>
>> Can we define something like LOGICAL_REP_MSG_LAST (also add a comment
>> indicating this is a fake message and must be the last one) as the
>> last and just check that?
>>
>
> I don't think that's required once I applied suggestions from Kyotaro and Peter. Please check the latest patch.
> Usually LAST is added to an enum when we need to cap the number of symbols or want to find the number of symbols. None of that is necessary here. Do you see any other use?
>

You mentioned in the beginning that you prefer to use Enum instead of
define so that switch cases can detect any remaining items but I have
tried adding extra enum value at the end and didn't handle that in
switch case but I didn't get any compilation warning or error. Do we
need something else to detect that at compile time?

Some comments assuming we want to use enum either because I am missing
something or due to some other reason we have not discussed yet.

1.
+ LOGICAL_REP_MSG_STREAM_ABORT = 'A',
+} LogicalRepMsgType;

There is no need for a comma after the last message.

2.
+/*
+ * Logical message types
+ *
+ * Used by logical replication wire protocol.
+ *
+ * Note: though this is an enum it should fit a single byte and should be a
+ * printable character.
+ */
+typedef enum
+{

I think we can expand the comments to probably say why we need these
to fit in a single byte or what problem it can cause if that rule is
disobeyed. This is to make the next person clear why we are imposing
such a rule.

3.
+typedef enum
+{
..
+} LogicalRepMsgType;

There are places in code where we use the enum name
(LogicalRepMsgType) both in the start and end. See TypeCat,
CoercionMethod, CoercionCodes, etc. I see places where we use the way
you have in the code. I would prefer the way we have used at places
like TypeCat as that makes it easier to read.

4.
switch (action)
{
- /* BEGIN */
- case 'B':
+ case LOGICAL_REP_MSG_BEGIN:
apply_handle_begin(s);
- break;
- /* COMMIT */
- case 'C':
+ return;

I think we can simply use 'return apply_handle_begin;' instead of
adding return in another line. Again, I think we changed this handling
in apply_dispatch() to improve the case where we can detect at the
compile time any missing enum but at this stage it is not clear to me
if that is true.

--
With Regards,
Amit Kapila.