Simutrans on Android builds as single threaded

[prissi]

Anyone having a look at that would be nice.

[victor_18993]

The Android build is single-threaded because

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find_package(Threads) is currently placed inside a branch that Android never enters.

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cmake/SimutransFindDependencies.cmake begins with:

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if (NOT ANDROID)

and

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find_package(Threads) is inside that branch. As a result, it is never called when configuring the Android build.
Nothing fails and no warning is emitted. The variable populated by

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FindThreads,

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CMAKE_USE_PTHREADS_INIT, is simply never set.

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SimutransCompileOptions.cmake then checks that exact variable:

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if (CMAKE_USE_PTHREADS_INIT)
    option(SIMUTRANS_MULTI_THREAD "Use multiple threads for drawing" ON)
else ()
    set(SIMUTRANS_MULTI_THREAD OFF)
endif ()

Android therefore always takes the second branch.
Since

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SIMUTRANS_MULTI_THREAD is disabled,

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MULTI_THREAD is never defined, and

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environment.cc falls back to:

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#ifdef MULTI_THREAD
    num_threads = min(MAX_THREADS, dr_get_max_threads());
#else
    num_threads = 1;
#endif

Android is not being built as single-threaded because the platform lacks thread support. It is being built that way because thread support is never detected.
Reproduced with the Android toolchain
I reproduced this using NDK r27c, targeting

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arm64-v8a, configured directly from the current master branch.
These are the definitions passed to the compiler:

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-DCOLOUR_DEPTH=0
-DMSG_LEVEL=3
-DNDEBUG=1
-Dsimutrans_EXPORTS


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MULTI_THREAD is not present.
Fix
Move these two lines outside the

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if (NOT ANDROID) branch so that they run on every platform:

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set(CMAKE_THREAD_PREFER_PTHREAD ON)
find_package(Threads)

That is the entire functional patch.
How thread detection works on Android
I initially expected Android to use the libc detection path in

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FindThreads, since bionic provides the pthread implementation.
That is not what happens:

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-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread - not found
-- Check if compiler accepts -pthread - yes
-- Found Threads: TRUE

The libc probe fails because CMake's test program calls

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pthread_cancel, which bionic does not provide:

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error: call to undeclared function 'pthread_cancel'

Android also does not provide a separate

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libpthread, so the library checks fail as expected.
Thread detection succeeds through the compiler's acceptance of the

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-pthread flag. This results in the thread variables being initialized, including

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CMAKE_USE_PTHREADS_INIT, which in turn enables

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SIMUTRANS_MULTI_THREAD and defines

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MULTI_THREAD.
Simutrans itself does not call

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pthread_cancel, so the failed libc probe does not indicate a problem for the application.
Compatibility with older Android versions
I also checked the barrier implementation because

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pthread_barrier_init is only available from Android API 24 onward.
The existing code already handles this correctly.
Bionic defines:

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#define _POSIX_BARRIERS __BIONIC_POSIX_FEATURE_SINCE(24)

which evaluates according to the configured Android API level.

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simthread.h checks:

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#if defined(_POSIX_BARRIERS) && (_POSIX_BARRIERS > 0)

Below API 24, Simutrans uses its existing mutex-and-condition-variable barrier implementation, the same fallback used on macOS.
From API 24 onward, it uses the native pthread barrier functions.
No additional Android-specific thread code is required.
Verification
With the patch applied,

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MULTI_THREAD reaches the compiler and all 28 translation units that use the threading code, including the multi-threaded drawing implementation, compile successfully for:

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API 21, arm64-v8a     fallback barrier implementation
API 24, arm64-v8a     native pthread barriers
API 21, armeabi-v7a   32-bit build

I did not perform the final link step because that requires the prebuilt libraries included in the Gradle project, such as zlib, libpng, bzip2, zstd, FreeType and FluidSynth. None of those dependencies affects the thread detection or compilation result.
The Android CI build should now report

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SIMUTRANS_MULTI_THREAD as enabled and include

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MULTI_THREAD in the compiler definitions.
The desktop build is unaffected.

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SIMUTRANS_MULTI_THREAD remains enabled and compiles as before.
Separate observation
The preceding line:

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set(CMAKE_THREAD_PREFER_PTHREAD ON)

does not appear to have any effect.

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FindThreads reads

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THREADS_PREFER_PTHREAD_FLAG, not

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CMAKE_THREAD_PREFER_PTHREAD.
I have deliberately left this unchanged rather than include an unrelated cleanup in the bug fix. It could be removed or corrected separately.

[prissi]

Thank you very much. This was very helpful.

[victor_18993]

You're welcome! I'm glad it helped.
I can also test the resulting Android build if needed.