On Fri, Mar 7, 2025 at 4:59 AM Paul Smith <psm...@gnu.org> wrote: > > On Thu, 2025-03-06 at 13:28 -0900, Britton Kerin wrote: > > In this example, I would not expect bar to be updated due to > > actual_source when foo is requested. The timestamp dependency chain > > should be broken between foo and bar and Make should be able to > > figure that out when handling an explicit request for foo. Is this a > > bug? > > > > $ cat Makefile > > foo: | bar > > cp $| $@ > > > > bar: actual_source > > cp $< $@ > > > > clean: > > rm -f foo bar > > Possibly I'm misunderstanding your test case but none of the results > you showed look wrong to me. > > Order-only prerequisites are defined in what I hope is a clear way in > this paragraph: > > https://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html#index-order_002donly-prerequisites > > I'm not sure what you mean exactly by "the timestamp dependency chain > should be broken", but nothing about the order-only status of the > prerequisites of "foo" makes any difference when deciding whether to > build "bar". It only impacts "foo". > > If make considers "bar" for any reason, then it will be rebuilt if it > is out of date. > > The only difference between making "bar" and order-only prereq vs. a > normal prereq of "foo", is whether "foo" is updated or not when "foo" > is out of date with respect to "bar". Every other aspect of this > makefile behaves the same regardless. > > "Order-only" means that the prerequisite is only considered to define > an order of build operation, but doesn't impact the out-of-date > calculations of its target.
What confuses me is that since the explicitly requested foo exists and isn't out of date with respect to any non-order-only prereqs (in the example it doesn't have any) and therefore isn't getting rebuilt, I wouldn't expect there to be any need to rebuild it's order-only prereqs either. I would think "order-only" would only impose an order constraint on the execution of the recipes, and since foo's recipe isn't happening why should bar's need to? There's no order violation if it doesn't, and in fact it's a bit of a misuse of the term "order" to describe the current behavior since (in the example) only one recipe is executing. Make is building slightly too much here which is contrary to its purpose. I've run into need for order-only a number of times and really appreciate the feature. I've read the docs on it many times and it's always sort of bothered me as I felt my understanding was somehow imperfect, as now indeed I find. It's obviously far too late to even consider changing the way this works but I do think the documentation could be improved somehow, though I'm not sure exactly how. It sounds like implementation simplicity (keeping order-only prereqs implementation almost identical to regular rules) is part of why it's like it is. Maybe the simple solution of explicitly documenting that order-only prerequisites are themselves rebuilt if mentioned by a required target even if the "depending" target itself isn't being rebuilt would be best. Britton
