[CMake] Dependencies to protobuf sources

[Stephan Menzel]

Hello all,

I am a long term user of CMake, protobuf and grpc on Windows (MSVC15) and
Linux.
Roughly in the beginning of this year I started to notice that dependencies
of targets using generated sources to their appropriate protobuf files
didn't work anymore. Basically, when I change a grpc source, the build
system notices the change, re-runs the protoc and grpc compiler and builds.
As it should. When I change a pure protobuf source however, nothing gets
rebuilt. Which resulted in some nasty bugs lately.

I thought it might be a temporary bug and waited for several updates to
protobuf, grpc, visual studio, cmake and everything else but so far it
hasn't changed and now I finally decided I wanted to solve this as it
slowly turns from a mild nuisance to a perilous situation in our team.

While doing so, I took a look at the 'new' way protobuf wants to be used.
as opposed to what they call 'legacy' aka PROTOBUF_GENERATE_CPP and friends.

Nowadays they have this:

find_package(protobuf CONFIG REQUIRED)

add_library(my_protobuf_lib STATIC
   SomeOfMySources.cpp
   Protosource.proto      // they expect the protos to be included right
here
)

and then, judging from the source it should be enough to do this:

protobuf_generate(TARGET my_protobuf_lib)

Which looks nice and clean and I switched to this, hoping it may also solve
the dependency bug. And it works in general, but the dependency still
doesn't.
So I tried to add the dependency by force like this:

add_custom_target(my_protobuf_lib_protos DEPENDS Protosource.proto)
add_dependencies(my_protobuf_lib my_protobuf_lib_protos)

Still nothing. Right now I'm quite out of ideas of what else I could try so
I thought I might ask here. The way I understand it those dependencies are
supposed to work. And they have for many years until at some point in
January 2018.
Unfortunately I cannot trace down the exact thing or date that broke it
because it took me a while to notice. Since then, pretty much everything
involved in the system has changed and upgraded multiple times. Protobuf is
built externally and installed using its INSTALL target, then pulled into
this project. Not built in tree. GRPC the same. The problem occurs
everywhere. Windows, Linux, all distributions I build for. Ninja and make.
I have tried all ways of building the protos available to me. 'Legacy',
this new way, roll-your-own, using old cmake files from a time before the
bug... nothing brought me anywhere.
Right now all I know is there is what feels like a thousand ways to
generate and build with protobuf but none of them have the dependencies in
order. It might be worth noticing that Qt's auto-uic feature broke as well.
Perhaps there is a connection. Manually generating the UICs still works and
detects changes in the ui files but they are not recognized if I use
auto-uic. This broke only recently though. Summer-ish.

Anybody has any ideas where I could look?

Thanks...
Stephan
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