musepack: Fix build for Linux

[iMichka]

Fixes:
./libmpcdec/libmpcdec.so: undefined reference to `pow'

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---

[iMichka]

  [ 63%] Linking C static library libmpcenc_static.a
  ../libmpcdec/libmpcdec.so: undefined reference to `pow'
  ../libmpcdec/libmpcdec.so: undefined reference to `log10'
  collect2: error: ld returned 1 exit status
  mpcdec/CMakeFiles/mpcdec_cmd.dir/build.make:98: recipe for target 'mpcdec/mpcdec' failed

[SMillerDev]

has this been reported upstream?

[iMichka]

No. I guess @alebcay just found this out. We need to upstream this.

[alebcay]

Agreed. Just wanted to make sure this works. Not sure how this has compiled for so long without this (unless they only build/test on platforms where the math routines are contained in libc).

I suspect the fix for upstream will be more than a one-liner though, as it would be better to check if linkage to libm is needed at all (or if libm even exists) rather than blindly adding a link library to the target.

[iMichka]

Do you know how to write that patch or should I help investigate?

[alebcay]

I'm working on writing the needed patch(es). They have both Autotools and CMake build systems available, so I'm trying to add the patch for both.

[alebcay]

My schedule has been busy so I don't think I will be able to help further with the patch for the time being. But I will provide here what I have so far. Adding this snippet to the end of libmpcdec/CMakeLists.txt should be a more well-rounded patch:

include(CheckCSourceCompiles)
set(LIBM_TEST_SOURCE "#include<math.h>\ndouble x,y;int main(){log10(x);pow(x,y);return 0;}")

check_c_source_compiles("${LIBM_TEST_SOURCE}" HAVE_BUILTIN_MATH)
if(NOT HAVE_BUILTIN_MATH)
  set(CMAKE_REQUIRED_LIBRARIES m)
  check_c_source_compiles("${LIBM_TEST_SOURCE}" HAVE_LIBM_MATH)
  unset(CMAKE_REQUIRED_LIBRARIES)
  if(NOT HAVE_LIBM_MATH)
    message(FATAL_ERROR "Unable to find math library functions")
  endif()
  target_link_libraries(mpcdec m)
endif()

I tried to include as many of the math functions in libmpcdec as I could find in LIBM_TEST_SOURCE. But maybe it's overkill - or maybe there's more that should be added.

The Autotools workflow is trickier to patch because it seems there is math library usage in multiple subprojects, and the Autotools files do not work as-is. AC_SEARCH_LIBS can be used in the configure.in file (which should be named configure.ac by convention) to attain this. Like in the CMake approach, I'm unsure if we should check just one of the math functions used, or all of them.

[iMichka]

Thinking more about this, I think we might have an issue to upstream our changes anyway.
The last released were 11 years ago and upstream sort of looks unmaintained (but I might be wrong):
https://www.musepack.net/index.php?pg=src

The download count for Musepack is high, but mostly due to gst-plugins-bad having it as a dependency. I am unsure how important this package is. Do we have any audio experts here?

[github-actions]

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