[CMake] protobuf and imports relative to root (and --proto_path)

[Alexander Neundorf]

On 2018 M08 23, Thu 12:50:14 CEST David Jobet wrote:
> Hello,
> 
> I'm trying to port an existing project from premake to cmake.
> I'm trying to avoid modifying the source files while doing so.
> 
> Right now, we have several libraries (read in different directories) using
> proto files with imports between them.
> All imports are made relative to the root of the project.
> 
> e.g :
> work/lib1/sublib1/a.proto
> work/lib1/sublib1/b.proto
> 
> with a.proto having a link to b.proto like this "import
> lib1/sublib1/b.proto"
> 
> If I compile this with an unchanged FindProtobuf.cmake, I do this :
> 
> protobuf_generate_cpp(PROTO_SRCS PROTO_HRDS a.proto b.proto)
> add_library(...)
> 
> the problem being that it looks like the "namespaces" generated by protoc
> in b.proto do not match the one used in a.proto : it does not compile.

do you mean protoc fails, or that the C++ compiler fails when compiling the 
generated file ?
I'm also using proto files with imports between them and don't have problems 
with finding them.
Are you setting the PROTOBUF_IMPORT_DIRS variable ?

What I do have a problem is rerunning protoc automatically on proto-files which 
import proto files that have been modified.
How do you handle this ?

> Is there a way to make this work ?
> 
> Otherwise, I made the patch below which solves my problem. Do you think it
> could be included in cmake ?
> it defines a new option PROTO_PATH, so the decl above becomes
> protobuf_generate_cpp(PROTO_SRCS PROTO_HRDS PROTO_PATH ${CMAKE_SOURCE_DIR}
> a.proto b.proto)
> 
> With regards
> 
> David
> 
> $ diff share/cmake-3.10/Modules/FindProtobuf.cmake.orig
> share/cmake-3.10/Modules/FindProtobuf.cmake

can you please use diff -bup ?
This makes the patch easier to read.

Thanks
Alex