Re: automatically generating node support functions

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>
Cc: David Rowley <dgrowleyml(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: automatically generating node support functions
Date: 2022-07-06 20:46:27
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Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com> writes:
> [ v7-0001-Automatically-generate-node-support-functions.patch ]

I have gone through this and made some proposed changes (attached),
and I think it is almost committable. There is one nasty problem
we need a solution to, which is that pgindent is not at all on board
with this idea of attaching node attrs to typedefs. It pushes them
to the next line, like this:

@@ -691,7 +709,8 @@
(rel)->reloptkind == RELOPT_OTHER_JOINREL || \
(rel)->reloptkind == RELOPT_OTHER_UPPER_REL)

-typedef struct RelOptInfo pg_node_attr(no_copy_equal, no_read)
+typedef struct RelOptInfo
+pg_node_attr(no_copy_equal, no_read)
NodeTag type;

which is already enough to break the simplistic parsing in Now, we could fix that parsing logic to deal
with this layout, but this also seems to change pgindent's opinion
of whether the subsequent braced material is part of a typedef or a
function. That results in it injecting a lot of vertical space
that wasn't there before, which is annoying.

I experimented a bit and found that we could do it this way:

typedef struct RelOptInfo
+ pg_node_attr(no_copy_equal, no_read)
NodeTag type;

without (AFAICT) confusing pgindent, but I've not tried to adapt
the perl script or the code to that style.

Anyway, besides that, I have some comments that I've implemented
in the attached delta patch.

* After further thought I'm okay with your theory that attaching
special copy/equal rules to specific field types is appropriate.
We might at some point want the pg_node_attr(copy_as_scalar)
approach too, but we can always add that later. However, I thought
some more comments about it were needed in the *nodes.h files,
so I added those. (My general feeling about this is that if
anyone needs to look into to understand how
the backend works, we've failed at documentation.)

* As written, the patch created equal() support for all Plan structs,
which is quite a bit of useless code bloat. I solved this by
separating no_copy and no_equal properties, so that we could mark
Plan as no_equal while still having copy support.

* I did not like the semantics of copy_ignore one bit: it was
relying on the pre-zeroing behavior of makeNode() to be sane at
all, and I don't want that to be a requirement. (I foresee
wanting to flat-copy node contents and turn COPY_SCALAR_FIELD
into a no-op.) I replaced it with copy_as(VALUE) to provide
better-defined semantics.

* Likewise, read_write_ignore left the contents of the field after
reading too squishy for me. I invented read_as(VALUE) parallel
to copy_as() to fix the semantics, and added a check that you
can only use read_write_ignore if the struct is no_read or
you provide read_as(). (This could be factored differently
of course.)

* I threw in a bunch more no_read markers to bring the readfuncs.c
contents into closer alignment with what we have today. Maybe
there is an argument for accepting that code bloat, but it's a
discussion to have later. In any case, most of the pathnodes.h
structs HAVE to be marked no_read because there's no sane way
to reconstruct them from outfuncs output.

* I got rid of the code that stripped underscores from outfuncs
struct labels. That seemed like an entirely unnecessary
behavioral change.

* FWIW, I'm okay with the question about

# XXX Previously, for subtyping, only the leaf field name is
# used. Ponder whether we want to keep it that way.

I thought that it might make the output too cluttered, but after
some study of the results from printing plans and planner data
structures, it's not a big addition, and indeed I kind of like it.

* Fixed a bug in write_only_req_outer code.

* Made Plan and Join into abstract nodes.

Anyway, if we can fix the impedance mismatch with pgindent,
I think this is committable. There is a lot of follow-on
work that could be considered, but I'd like to get the present
changes in place ASAP so that other patches can be rebased
onto something stable.

I've attached a delta patch, and also repeated v7 so as not
to confuse the cfbot.

regards, tom lane

Attachment Content-Type Size
v7-0001-Automatically-generate-node-support-functions.patch text/x-diff 92.7 KB
v7-0002-toms-changes.patch text/x-diff 23.8 KB

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