GH-40577: [C++] Ensure pkg-config flags include -ldl for static builds

[bkietz]

Rationale for this change

When linking statically, pkg-config doesn't pick up the new dependency on libdl introduced by #39067

This produces unresolved symbol errors

What changes are included in this PR?

Addition of -ldl to ARROW_PC_LIBS_PRIVATE to ensure linkage to the necessary library

Are these changes tested?

yes

Are there any user-facing changes?

no

• GitHub Issue: [C++] pkg-config flags don't include -ldl for static builds #40577

[github-actions]

⚠️ GitHub issue #40577 has no components, please add labels for components.

[kou]

If we put this to this block, this flag isn't used when -DARROW_BUILD_SHARED=ON -DARROW_BUILD_STATIC=ON is used. Does this work?

diff --git a/cpp/src/arrow/CMakeLists.txt b/cpp/src/arrow/CMakeLists.txt
index c5449d9956..4329d66821 100644
--- a/cpp/src/arrow/CMakeLists.txt
+++ b/cpp/src/arrow/CMakeLists.txt
@@ -140,9 +140,10 @@ if(ARROW_ENABLE_THREADING)
   list(APPEND ARROW_STATIC_INSTALL_INTERFACE_LIBS Threads::Threads)
 endif()
 
-if(NOT MSVC_TOOLCHAIN)
+if(CMAKE_DL_LIBS)
   list(APPEND ARROW_SHARED_INSTALL_INTERFACE_LIBS ${CMAKE_DL_LIBS})
   list(APPEND ARROW_STATIC_INSTALL_INTERFACE_LIBS ${CMAKE_DL_LIBS})
+  string(APPEND ARROW_PC_LIBS_PRIVATE " -l${CMAKE_DL_LIBS}")
 endif()
 
 set(ARROW_TEST_LINK_TOOLCHAIN ${ARROW_GTEST_GMOCK} ${ARROW_GTEST_GTEST_MAIN})

[bkietz]

That seems reasonable. Out of curiosity, what's the utility of building both shared and static and libraries?

[kou]

If we build Apache Arrow C++ for single propose (e.g. creating a PyArrow wheel), we don't need to build shared and static libraries. But if we want to use multiple proposes (e.g. using it from Python and R), we may need shared and static libraries.

Most packaging systems (deb, rpm and so on) provide both of shared and static libraries to support multiple proposes. If Apache Arrow C++ supports building both shared and static library, they don't need to build Apache Arrow C++ twice to build shared and static libraries.

[bkietz]

If Apache Arrow C++ supports building both shared and static library, they don't need to build Apache Arrow C++ twice

I think you'd only be able to reuse the object files, which should be doable by having adjacent build directories both using ccache. We can't even reuse object files on WIN32 because we add -DARROW_STATIC there and so the object files linked into arrow_static are different from those linked into arrow_shared.

[kou]

As a packager view, reusability of object files isn't so important. Composing shared/static libraries to one package family (libarrow15 and libarrow-dev for .deb) is important. Building twice for shared and static libraries isn't easy to compose...

For example:

• Both of cmake --install for shared library and cmake --install for static library install not only library files but also header files, data files and so on. Most of header files will be same but some files may be different. In our case, arrow/util/config.h may be different if we have some #cmakedefines that depend on library type. If there are different files, package for shared library and package for static library will be conflicted.
• CMake packages for shared library and static library are different. We need to use different CMake package names (e.g. ArrowShared for shared library and ArrowStatic for static library) to avoid conflicting them.
• ...

GoogleTest is a product that doesn't support building both of shared and static libraries at once.
MSYS2 package of it building GoogleTest twice:

• https://github.com/msys2/MINGW-packages/blob/ad99b751d11d027345f6da4c91b4c33d5ad1ac7d/mingw-w64-gtest/PKGBUILD#L36-L60
• https://github.com/msys2/MINGW-packages/blob/ad99b751d11d027345f6da4c91b4c33d5ad1ac7d/mingw-w64-gtest/PKGBUILD#L64-L68

The package provides only CMake package for shared library: https://github.com/msys2/MINGW-packages/blob/ad99b751d11d027345f6da4c91b4c33d5ad1ac7d/mingw-w64-gtest/PKGBUILD#L70-L71
It means that users can't use static library version via its CMake package.

[assignUser]

We should really at some point move a way from the multiple list with dozens of targets or other lists in them ^_^ but that's of course a topic for some other time.

[assignUser]

@github-actions crossbow submit example-cpp-minimal-build-static*

[github-actions]

Revision: 00c4b06

Submitted crossbow builds: ursacomputing/crossbow @ actions-fa21affe42

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 7 benchmarking runs that have been run so far on merge-commit 2224a29.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 67 possible false positives for unstable benchmarks that are known to sometimes produce them.