Horton

Re: Tree-walker callbacks vs -Wdeprecated-non-prototype

Tom Lane <tgl@sss.pgh.pa.us>

View thread View on postgr.es
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2022-09-17T01:08:02Z
Lists: pgsql-hackers

Attachments

I wrote:
> Ugh.  I wonder if we can get away with declaring the walker arguments
> as something like "bool (*walker) (Node *, void *)" without having
> to change all the actual walkers to be exactly that signature.
> Having to insert casts in the walkers would be a major pain-in-the-butt.

No joy on that: both gcc and clang want the walkers to be declared
as taking exactly "void *".

Attached is an incomplete POC patch that suppresses these warnings
in nodeFuncs.c itself and in costsize.c, which I selected at random
as a typical caller.  I'll push forward with converting the other
call sites if this way seems good to people.

In nodeFuncs.c, we can hide the newly-required casts inside macros;
indeed, the mutators barely need any changes because they already
had MUTATE() macros that contained casts.  So on that side, it feels
to me that this is actually a bit nicer than before.

For the callers, we can either do it as I did below:

 static bool
-cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
+cost_qual_eval_walker(Node *node, void *ctx)
 {
+	cost_qual_eval_context *context = (cost_qual_eval_context *) ctx;
+
 	if (node == NULL)
 		return false;

or perhaps like this:

 static bool
-cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
+cost_qual_eval_walker(Node *node, void *context)
 {
+	cost_qual_eval_context *cqctx = (cost_qual_eval_context *) context;
+
 	if (node == NULL)
 		return false;

but the latter would require changing references further down in the
function, so I felt it more invasive.

It's sad to note that this exercise in hoop-jumping actually leaves
us with net LESS type safety, because the outside callers of
cost_qual_eval_walker are no longer constrained to call it with
the appropriate kind of context struct.  Thanks, C committee.

			regards, tom lane

Commits

  1. Remove accidentally added meson.build

  2. Disable clang 16's -Wcast-function-type-strict.

  3. Disable -Wdeprecated-non-prototype in the back branches.

  4. Revise tree-walk APIs to improve spec compliance & silence warnings.

  5. Future-proof the recursion inside ExecShutdownNode().