Re: [PATCH 7/6] Makefile: Make Makefile depend on generated qga files, too

[Eric Blake <eblake@redhat.com>]

On 12/4/19 12:56 AM, Markus Armbruster wrote:

>>> +++ b/Makefile
>>> @@ -130,6 +130,15 @@ GENERATED_QAPI_FILES += qapi/qapi-doc.texi
>>>      generated-files-y += $(GENERATED_QAPI_FILES)
>>>    +GENERATED_QGA_FILES := qga-qapi-types.c qga-qapi-types.h
>>> +GENERATED_QGA_FILES += qga-qapi-visit.c qga-qapi-visit.h
>>> +GENERATED_QGA_FILES += qga-qapi-commands.h qga-qapi-commands.c
>>> +GENERATED_QGA_FILES += qga-qapi-init-commands.h qga-qapi-init-commands.c
>>> +GENERATED_QGA_FILES += qga-qapi-doc.texi
>>> +GENERATED_QGA_FILES := $(addprefix qga/qapi-generated/, $(GENERATED_QGA_FILES))
>>
>> Would it be worth using two separate variable names (maybe
>> GENERATED_QGA_BASEFILES for the first list) rather than exploiting the
>> arcane knowledge that consecutive use of := causes GNU make to rewrite
>> an existing variable with new contents?
> 
> Our rules.mak relies on this already.  It's full of magic, which
> admittedly diminishes its suitability to serve as a good example.
> 
> Your worry might be rooted in old "=" burns.  "=" makes the variable
> recursively expanded, and
> 
>      GENERATED_QGA_FILES = $(addprefix qga/qapi-generated/, $(GENERATED_QGA_FILES))

Indeed, but I have to refer to the manual to remind myself of whether = 
or := is what I want in a given situation.

> 
> would be an infinite loop.  ":=" makes it simply expanded; there's not
> even a loop, let alone an infinite one.  The GNU Make manual explains
> this clearly at
> https://www.gnu.org/software/make/manual/html_node/Flavors.html
> 
> Aside: there's a reason one of the two flavors is called "simple".  It
> could additionally be called "not as slow".  One of my pet makefile
> peeves: unthinking use of recursively expanded variables, complicating
> semantics and slowing down builds.
> 
> Back to this patch.  I had started to write the thing in longhand, but
> got tired of repeating qga/qapi-generated/, so I factored that out.
> Would longhand be easier to understand?

It's more verbose.  My suggestion was more:

GENERATED_QGA_BASENAMES := qga-qapi-types.c qga-qapi-types.h
GENERATED_QGA_BASENAMES += qga-qapi-visit.c qga-qapi-visit.h
...
GENERATED_QGA_FILES := $(addprefix qga/qapi-generated/, 
$(GENERATED_QGA_BASENAMES))

to avoid the reassignment-to-self issue altogether, while still 
remaining concise compared to longhand.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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