Fix android compilation

[abique]

See for reference microsoft/vcpkg#44940

[ktmf01]

@robUx4 Could you please take a look at this? This touches your fix from #691 I'd rather not have this break something else again, and I don't have a build system to test it with, neither does CI cover it

[robUx4]

This seems based on bogus assumptions: microsoft/vcpkg#44940 (comment)

The recommended NDK toolchain (aka "legacy") sets CMAKE_SYSTEM_VERSION to 1, so libflac's approach to determine ftello absence is flawed even with vcpkg passing value 28 in.

https://android.googlesource.com/platform/ndk/+/refs/heads/main/build/cmake/android-legacy.toolchain.cmake#38

This is true if ANDROID_USE_LEGACY_TOOLCHAIN_FILE is not set or explicitly set to OFF in the environment. People opting out of the legacy mode will have the proper value. On the other hand, the documentation says people shouldn't use that...

So it's true that the value is not reliable enough. ANDROID_NATIVE_API_LEVEL seems appropriate although I think ANDROID_PLATFORM is the right one to use here. 2k uses vs 1.7k. Also the NDK CMake file puts it in the read-only variables: "We'll keep these around for the existing projects that still use them.".

Note that this is only for people using the recommended cmake toolchain file provided by the NDK. In VLC for example we relied on CMake until recently and still do on VLC 3.0 (stable branch). Neither ANDROID_PLATFORM nor ANDROID_NATIVE_API_LEVEL exist there. In this case CMAKE_SYSTEM_VERSION seems the only way to tell the NDK target version.

TLDR: I think both values need to be checked. The PR in its current state will break the VLC 3.0 build.

[abique]

I'm not expert here, I'm just forwarding what we've done in VCPKG.

[robUx4]

This is one of the downside of VCPKG, they make patch locally and don't send them upstream for review. It's good that you submit it here @abique. That patch might be enough for VCPKG that may force the use of the android.toolchain.cmake file.

For the more general case I think the ANDROID_PLATFORM and CMAKE_SYSTEM_VERSION need to be read first (priority to ANDROID_PLATFORM). For VLC the CMAKE_SYSTEM_VERSION set to 1 is not an issue because we don't build for anything higher than API 21 (so the check for < 24 is always true).

[dg0yt]

I authored the original vcpkg patch, and I came here for upstreaming. I'm surprised about the negative judgements in the comments ("bogus assumptions", "don't send upstream") while confirming at the same time that this judgement is wrong ("documentation", this PR).

I don't claim to know much about flac, but I do have some familarity with vcpkg and most target platforms it supports. The original patch was intentionally minimal, tailored for vcpkg. When authoring the patch, I did research the changes which lead to the build problem, and I did assume that there was testing with a different CMake toolchain. So I did expect that "upstreaming" would mean adjustments.

So how to continue from here in a positive way?

[dg0yt]

There is one ugly thing about ANDROID_PLATFORM: It comes with an android- prefix in front of the number.
Even ANDROID_NATIVE_API_LEVEL could have this prefix.
At least this is what initialization says:
https://android.googlesource.com/platform/ndk/+/refs/heads/main/build/cmake/android-legacy.toolchain.cmake#119

[robUx4]

I verified that when using $ANDROID_NDK/build/cmake/android.toolchain.cmake, as one should, and -DANDROID_PLATFORM=21 the CMAKE_SYSTEM_VERSION stays to 1. With the following code:

message(FATAL_ERROR "CMAKE_SYSTEM_VERSION=${CMAKE_SYSTEM_VERSION} / ANDROID_NATIVE_API_LEVEL=${ANDROID_NATIVE_API_LEVEL}")
...
CMAKE_SYSTEM_VERSION=1 / ANDROID_NATIVE_API_LEVEL=21

Testing without and setting -DCMAKE_SYSTEM_NAME=Android (as one does when cross-compiling) and -DANDROID_PLATFORM=21 I get the following:

CMAKE_SYSTEM_VERSION=21 / ANDROID_NATIVE_API_LEVEL=

That's with CMake 3.22.1 on Ubuntu.

But there's a plot twist. If I change the ANDROID_PLATFORM value with the android.toolchain.cmake the ANDROID_NATIVE_API_LEVEL reflects the value unless the value is bogus and not handled by the NDK. In that case there's an error. Changing ANDROID_PLATFORM value with the -DCMAKE_SYSTEM_NAME=Android does not change CMAKE_SYSTEM_VERSION value, it's stuck to 21, the minimum supported by the NDK. This is actually in line with CMAKE_SYSTEM_VERSION documentation that says:

When the CMAKE_SYSTEM_NAME variable is set explicitly to enable cross compiling then the value of CMAKE_SYSTEM_VERSION must also be set explicitly to specify the target system version.

So a user should either use android.toolchain.cmake and -DANDROID_PLATFORM=xxx or -DCMAKE_SYSTEM_NAME=Android and -DCMAKE_SYSTEM_VERSION=xxx to target the proper Android version.

There is one ugly thing about ANDROID_PLATFORM: It comes with an android- prefix in front of the number.
Even ANDROID_NATIVE_API_LEVEL could have this prefix.

That line (119) is used in case the user sets ANDROID_NATIVE_API_LEVEL manually. But later in the script (681) the value is forced/reset to ANDROID_PLATFORM_LEVEL. This seems to be a number.

With all that being, to properly support builds with and without android.toolchain.cmake both CMAKE_SYSTEM_VERSION and ANDROID_NATIVE_API_LEVEL need to be checked. The ANDROID_NATIVE_API_LEVEL should only be used if CMAKE_SYSTEM_VERSION is 1, as it means `android.toolchain.cmake is used.

You could use something like this instead:

if(CMAKE_SYSTEM_VERSION EQUAL 1)
    set(CMAKE_SYSTEM_VERSION ${ANDROID_NATIVE_API_LEVEL})
endif()

In both usage it will have the proper Android target value.

[robUx4]

Actually it's not entirely clean as it should only be done for Android targets, CMAKE_SYSTEM_VERSION may be 1 on other platforms.

Here are 2 ways to do it depending if you want to check ANDROID_NATIVE_API_LEVEL or CMAKE_SYSTEM_VERSION (I have no preference):

if(ANDROID AND CMAKE_SYSTEM_VERSION EQUAL 1)
    # android.toolchain.cmake resets CMAKE_SYSTEM_VERSION to 1
    set(CMAKE_SYSTEM_VERSION ${ANDROID_NATIVE_API_LEVEL})
endif()

if(ANDROID AND NOT ANDROID_NATIVE_API_LEVEL)
    set(ANDROID_NATIVE_API_LEVEL ${CMAKE_SYSTEM_VERSION})
endif()

Or simply switch the test to

if(ANDROID AND $<IF:CMAKE_SYSTEM_VERSION EQUAL 1,ANDROID_NATIVE_API_LEVEL,CMAKE_SYSTEM_VERSION> VERSION_LESS "24")

[dg0yt]

I'm creating another PR.