before this change, scylla's CMake-based system consumes Seastar
library by including it directly. but this failed to address the needs
of linking against Seastar shared libraries in Debug and Dev builds, while
linking against the static libraries in other builds. because Seastar
uses `BUILD_SHARED_LIBS` CMake variable to determine if it builds
shared libraries. and we cannot assign different values to this
CMake variable based on current configure type -- CMake does not
support. see https://gitlab.kitware.com/cmake/cmake/-/issues/19467
in order to address this problem, we have a couple possible
solutions:
- to enable Seastar to build both shared and static libraries in a
pass. without sacrificing the performance, we have to build
all object files twice: once with -fPIC, once without. in order
to accompolish this goal, we need to develop a machinary to
populate the same settings to these two builds. this would
complicate the design of Seastar's building system further.
- to build Seastar libraries twice in scylla, we could use
the ExternalProject module to implement this. but it'd be
complicate to extract the compile options, and link options
previously populated by Seastar's targets with CMake --
we would have to replicate all of them in scylla. this is
out of the question.
- to build Seastar libraries twice before building scylla,
and let scylla to consume them using CMake config files or
.pc files. this is a compromise. it enables scylla to
drive the build of Seastar libraries and to consume
the compile options and link options. the downside is:
* the generated compilation database (compile_commands.json)
does not include the commands building Seastar anymore.
* the building system of scylla does not have finer graind
control on the building process of seastar. for instance,
we cannot specify the build dependency to a certain seastar
library, and just build it instead of building the whole
seastar project.
turns out the last approach is the best one we can have
at this moment. this is also the approach used by the existing
`configure.py`.
in this change, we
- add FindSeastar.cmake to
* detect the preconfigured Seastar builds, and
* extract the build options from .pc files
* expose library targets to be consumed by parent project
- add Seastar as an external project, so we can build it from
the parent project.
this is atypical compared to standard ExternalProject usage:
- Seastar's build system should already be configured at this point.
- We maintain separate project variants for each configuration type.
Benefits of this approach:
- Allows the parent project to consume the compile options exposed by
.pc file. as the compile options vary from one config to another.
- Allows application of config-specific settings
- Enables building Seastar within the parent project's build system
- Facilitates linking of artifacts with the external project target,
establishing proper dependencies between them
we will update `configure.py` to merge the compilation database
of scylla and seastar.
Refs scylladb/scylladb#2717
---
this is a CMake-related change, hence no need to backport.
Closesscylladb/scylladb#21131
* github.com:scylladb/scylladb:
build: cmake: use GENERATOR_IS_MULTI_CONFIG property to detect mult-config
build: cmake: consume Seastar using its .pc files
build: do not use `mode` as the index into `modes`
build: cmake: detect and link against GnuTLS library
build: cmake: detect and link against yaml-cpp
build: cmake: link Seastar with Seastar::<COMPONENT>
build: cmake: define CMake generate helper funcs in scylla
b6da82dba3