Update Android API level to 21 in CI

[de-vri-es]

I ran into a problem with #78572 because the Android API level in CI is pretty old. The accept4() function is exposed by the libc crate, but it is added in Android 5.0 with API level 21 (released in June 2014). Currently, the CI is still on API level 14 in most cases.

So I was wondering: should we just update the API level in CI to 21? I think the alternative would be to remove the accept4() function from the libc crate for Android, but that would break crates that use it.

[rust-highfive]

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[de-vri-es]

I also implemented an alternative fix for libc to keep supporting old Android: rust-lang/libc#1968

If that is preferable, these is no need to merge this.

[est31]

Does this mean that people who use the android related Rust targets can't target older Android devices any more? If so, I think this shouldn't be merged, because Android distribution numbers show that API levels < 21 were still in use by 5.9% of deployed devices:

Related: rust-lang/release-team#2

[de-vri-es]

Well, as it stands, there is at-least one issue in the libc crate for older Android targets with the accept4() function not being present in the native libc. Impact is probably quite limited at the moment though. As long as an application doesn't directly or indirectly call accept4() things will probably compile and link fine.

If the aim is to keep supporting older Android targets then it doesn't make sense to merge this. In that case I think rust-lang/libc#1968 is a better solution. That will fix accept4() for older Android targets.

So it really boils down to: should Rust keep supporting older Android targets?

[est31]

I think there are three questions here:

1. The question I asked above, whether this update will mean that people won't be able to target older androids at all unless they custom-build std artifacts for it. As in, can they use the generated native artifacts and distribute them in apps that have a minimum SDK version of lower than 21.

2. Whether, as of Use SOCK_CLOEXEC and accept4() on more platforms. #78572, breaking code that tries to listen on network connections is a concern or not

3. What this update means for the future. Maybe it's decided that listening on the network is not a common thing to do on Android, thus it's acceptable but generally one wants to keep support for those old androids. In that case, how can one ensure potential breakage is identified early in the process?

The last increase was in #45580 , oct 2017 when the level was changed from 9 to 14. This post says that in nov 2017, API version 10 was used by 0.5% of users. It was the only listed API version before 15, and given honeycomb has barely been deployed at all, I guess it amounted to all users. So back when the update to 14 happened, basically >99% of deployed devices already had API level 14.

If you use this "two nines" rule, requiring that at least 99% of users support the new minimum version, then one could only update to API level 17.

If the aim is to keep supporting older Android targets then it doesn't make sense to merge this. In that case I think rust-lang/libc#1968 is a better solution. That will fix accept4() for older Android targets.

According to man, the accept4 syscall was added in Linux 2.6.28. According to Wikipedia, Android 1.6 (API 4) already used this kernel version by default. I presume that OEMs do ship older kernel versions with newer OS/API versions, so maybe newer phones might still use the kernel, or the OEM's kernel fork might just have updated the version number. But I doubt that this is going to be a problem here. Edit: forgot the conclusion: I think it's fine to call the syscall directly and make the android target use it. If it turns out to be a problem one can always reconsider.

[kennytm]

note: we tried to raised to API 17 in #71123 half a year ago, but the PR was closed due to inactivity.

[de-vri-es]

Considering that rust-lang/libc#1968 is almost certainly getting merged, and this seems to be a hairy topic, I'm closing this :)