GH-48750: [CI][Documentation] Disable Unity build for OpenTelemetry

[HyukjinKwon]

Rationale for this change

After commit cbd48a4 migrated OpenTelemetry from ExternalProject to FetchContent, the documentation build started failing with symbol redefinition errors:

error: redefinition of 'const google::protobuf::internal::MigrationSchema schemas []'

FetchContent integrates OpenTelemetry into the parent CMake configuration, causing it to inherit CMAKE_UNITY_BUILD=ON. Unity builds combine multiple .pb.cc files into a single compilation unit, which causes collision of static symbols (like schemas[] and file_default_instances[]) that exist in each protobuf-generated file.

What changes are included in this PR?

Added set(CMAKE_UNITY_BUILD FALSE) in the build_opentelemetry() function in cpp/cmake_modules/ThirdpartyToolchain.cmake to disallow Unity build in protobuf-generated files.

Are these changes tested?

Has to be tested via CI.

Are there any user-facing changes?

No, dev-only.

• GitHub Issue: [CI][Documentation] Disable Unity builds to fix OpenTelemetry symbol collision in docs build #48750

[github-actions]

⚠️ GitHub issue #48750 has been automatically assigned in GitHub to PR creator.

[HyukjinKwon]

@github-actions crossbow submit test-debian-*-docs

[github-actions]

Revision: 5cbafac

Submitted crossbow builds: ursacomputing/crossbow @ actions-71a6b31f76

Task	Status
test-debian-12-docs

[kou]

Can we use this instead?

diff --git a/cpp/cmake_modules/ThirdpartyToolchain.cmake b/cpp/cmake_modules/ThirdpartyToolchain.cmake
index 8a26b46d0b..d25aa01b62 100644
--- a/cpp/cmake_modules/ThirdpartyToolchain.cmake
+++ b/cpp/cmake_modules/ThirdpartyToolchain.cmake
@@ -3724,6 +3724,8 @@ function(build_opentelemetry)
 
   prepare_fetchcontent()
 
+  # TODO: Add the reason why we need to disable unity build...
+  set(CMAKE_UNITY_BUILD FALSE)
   set(OTELCPP_PROTO_PATH "${opentelemetry_proto_SOURCE_DIR}")
   set(WITH_EXAMPLES OFF)
   set(WITH_OTLP_HTTP ON)

[HyukjinKwon]

Ohh let me try!

[HyukjinKwon]

@github-actions crossbow submit test-debian-*-docs

[github-actions]

Revision: 5703dc4

Submitted crossbow builds: ursacomputing/crossbow @ actions-3921253546

Task	Status
test-debian-12-docs

[raulcd]

@github-actions crossbow submit preview-docs

[raulcd]

Thanks for the change! LGTM.
I am triggering the preview-docs job which seems to fail with the same issue to validate, will merge once the job finishes

[github-actions]

Revision: 5703dc4

Submitted crossbow builds: ursacomputing/crossbow @ actions-0d86636019

Task	Status
preview-docs

[HyukjinKwon]

seems passing fine 👍

[HyukjinKwon]

Awesomeness!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 4edefc3.

There weren't enough matching historic benchmark results to make a call on whether there were regressions.

The full Conbench report has more details.