cmake: support narrowly constrained lite-only runtime builds

[dudantas]

Summary

Implements the narrowly constrained lite-only runtime build discussed in #27231.

This adds one new CMake option:

option(protobuf_BUILD_LIBPROTOBUF "Build libprotobuf" ON)

The default behavior remains unchanged. With defaults, Protobuf still builds and installs both libprotobuf-lite and libprotobuf.

When protobuf_BUILD_LIBPROTOBUF=OFF, the build is only accepted in a strict target-side lite-only configuration:

• protobuf_BUILD_PROTOC_BINARIES=OFF
• protobuf_BUILD_LIBPROTOC=OFF
• protobuf_BUILD_LIBUPB=OFF
• protobuf_BUILD_TESTS=OFF
• protobuf_BUILD_CONFORMANCE=OFF
• protobuf_BUILD_EXAMPLES=OFF

In that mode, CMake still builds, installs, and exports protobuf::libprotobuf-lite, but does not build/install/export protobuf::libprotobuf, libprotoc, protoc, or libupb.

Motivation

Some downstream/package-manager workflows use host protoc for code generation while the target package only needs the lite C++ runtime.

For example, generated C++ sources may use:

option optimize_for = LITE_RUNTIME;

and the target consumer only links:

target_link_libraries(app PRIVATE protobuf::libprotobuf-lite)

This PR does not try to support every possible CMake combination. It only adds a strict lite-only target-runtime mode where the full runtime and compiler-side artifacts are explicitly disabled.

Scope

This PR intentionally does not:

• change Bazel;
• change C++ runtime sources;
• prune installed headers;
• add protobuf_BUILD_LIBPROTOBUF_LITE;
• add telemetry/devtools/docs from the local PoC;
• support full-runtime consumers in lite-only mode;
• support protoc, libprotoc, tests, conformance, examples, or libupb when protobuf_BUILD_LIBPROTOBUF=OFF.

CONFIG mode is the primary validated contract:

find_package(protobuf CONFIG REQUIRED)
target_link_libraries(app PRIVATE protobuf::libprotobuf-lite)

Implementation notes

CMakeLists.txt

• Adds protobuf_BUILD_LIBPROTOBUF, defaulting to ON.
• Keeps libprotobuf-lite included whenever protobuf_BUILD_PROTOBUF_BINARIES=ON.
• Includes libprotobuf.cmake only when protobuf_BUILD_LIBPROTOBUF=ON.
• Rejects unsupported lite-only combinations with explicit FATAL_ERROR messages.

cmake/install.cmake

• Installs/exports only targets that actually exist.
• Generates/installs protobuf-lite.pc only when libprotobuf-lite exists.
• Generates/installs protobuf.pc only when libprotobuf exists.
• Generates/installs upb.pc only when libupb exists.

Validation

Validated locally on Windows/MSVC.

Default build

cmake -S . -B build-default -Dprotobuf_BUILD_TESTS=OFF
cmake --build build-default --config Release --parallel 1
cmake --install build-default --config Release --prefix install-default

Default install contained:

• libprotobuf
• libprotobuf-lite
• libprotoc
• protoc
• libupb
• protobuf.pc
• protobuf-lite.pc
• upb.pc

Lite-only build

cmake -S . -B build-lite-only `
  -Dprotobuf_BUILD_TESTS=OFF `
  -Dprotobuf_BUILD_CONFORMANCE=OFF `
  -Dprotobuf_BUILD_EXAMPLES=OFF `
  -Dprotobuf_BUILD_PROTOC_BINARIES=OFF `
  -Dprotobuf_BUILD_LIBPROTOC=OFF `
  -Dprotobuf_BUILD_LIBUPB=OFF `
  -Dprotobuf_BUILD_LIBPROTOBUF=OFF

cmake --build build-lite-only --config Release --parallel 1
cmake --install build-lite-only --config Release --prefix install-lite-only

Lite-only install contained:

• libprotobuf-lite
• protobuf-lite.pc

Lite-only install did not contain:

• libprotobuf
• libprotoc
• protoc
• libupb
• protobuf.pc

CONFIG consumer

A minimal external consumer passed with:

find_package(protobuf CONFIG REQUIRED)

if(NOT TARGET protobuf::libprotobuf-lite)
  message(FATAL_ERROR "protobuf::libprotobuf-lite was not exported")
endif()

if(TARGET protobuf::libprotobuf)
  message(FATAL_ERROR "protobuf::libprotobuf should not exist in lite-only install")
endif()

add_executable(protobuf_lite_consumer main.cc)
target_link_libraries(protobuf_lite_consumer PRIVATE protobuf::libprotobuf-lite)

The consumer included:

#include <google/protobuf/message_lite.h>
#include <google/protobuf/stubs/common.h>

int main() {
  google::protobuf::ShutdownProtobufLibrary();
  return 0;
}

Invalid configuration checks

All expected invalid combinations failed at configure time with clear FATAL_ERROR messages:

• protobuf_BUILD_LIBPROTOBUF=OFF + protobuf_BUILD_PROTOC_BINARIES=ON
• protobuf_BUILD_LIBPROTOBUF=OFF + protobuf_BUILD_TESTS=ON
• protobuf_BUILD_LIBPROTOBUF=OFF + protobuf_BUILD_CONFORMANCE=ON
• protobuf_BUILD_LIBPROTOBUF=OFF + protobuf_BUILD_EXAMPLES=ON
• protobuf_BUILD_LIBPROTOBUF=OFF + protobuf_BUILD_LIBUPB=ON
• protobuf_BUILD_LIBPROTOBUF=OFF + protobuf_BUILD_LIBPROTOC=ON

Local PoC measurements

The earlier local PoC measured the following on Windows/MSVC:

Variant	Configure	Build	Install	Install size
Default	21.299s	508.063s	7.699s	113754 KB
Lite-only	49.902s	55.648s	4.363s	21579 KB

The lite-only configure step was slower in that local run, but the build step dominated the total time and dropped from 508.063s to 55.648s.

Related

Resolves #27231

[google-cla]

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[dudantas]

@esrauchg sorry, I'm not familiar with this repository checks. Do you need anything else from me?

[zhangskz]

Tests seem to be passing (there may have been a spurious failure earlier), @esrauchg will review with comments if additional action is needed.

[dudantas]

Tests seem to be passing (there may have been a spurious failure earlier), @esrauchg will review with comments if additional action is needed.

Ok, thanks.

[esrauchg]

Thanks for iterating on this!