[Scons-dev] Accommodating dependency cycles.

[William Blevins]

Here is another one. I assume this round of issues is because they updated
SCons on the latest Ubuntu. This one actually makes sense. Someone else
posted this one. "test2.h" explicitly depends on "test.h" via Command
Builder, and "test.h" implicitly depends on "test2.h" via scanner.

# SConstruct
Command('test2.h', 'test.h', 'echo "" > test2.h')
Library('test.c')

// test.c
#include "test.h"
#include "test2.h"

// test.h
#include "test2.h"


With 2.3.4, this ran successfully.

With 2.5.0 we have this:
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
scons: done building targets.

scons: *** Found dependency cycle(s):
  test2.h -> test2.h

On Wed, Mar 22, 2017 at 10:43 PM, William Blevins <wblevins001 at gmail.com>
wrote:

> Oh, that's true. It depends on itself. It is not a chain per say: A <-->
> A. Yeah, that is bizarre. I assume he messed up the example, but I copied
> it verbatim.
>
> On Wed, Mar 22, 2017 at 7:39 PM, Gary Oberbrunner <garyo at oberbrunner.com>
> wrote:
>
>>
>> On Wed, Mar 22, 2017 at 6:15 PM, William Blevins <wblevins001 at gmail.com>
>> wrote:
>>
>>> //source.cpp
>>> #include "source.gen.h"
>>> int main() { return 0; }
>>>
>>> //SConstruct
>>> e = Environment()
>>> e.Command("source.gen.h", "source.cpp", Copy('$TARGET', '$SOURCE')) #
>>> "generator"
>>> e.Program("source.cpp")
>>>
>>> This code works fine in Scons 2.1, but Scons 2.5.1 produce error:
>>> scons: *** Found dependency cycle(s):
>>>   source.gen.h -> source.gen.h
>>>
>>> If I'm reading this correctly, I don't see a cycle.
>> "source" depends on source.cpp and source.gen.h.
>> source.gen.h depends on source.cpp.
>> source.cpp is a source, doesn't depend on anything.
>> Now source.gen.h includes itself, which is weird, but shouldn't all by
>> itself cause a loop as long as the scanner knows it's already seen that
>> file.
>> source.gen.h should definitely not depend on itself.
>>
>> (Not saying the current code doesn't detect a cycle, just that in the
>> abstract there shouldn't actually be one.)
>>
>>
>> --
>> Gary
>>
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>>
>>
>
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