libc::sendmmsg is not available on old Android phones

[link2xt]

sendmmsg was added in Linux 3.0 and recvmmsg was added in Linux 2.6.33 according to their man pages.

It is used in

quinn/quinn-udp/src/unix.rs

Line 197 in 389d7ce

let n = unsafe { libc::sendmmsg(io.as_raw_fd(), msgs.as_mut_ptr(), num_transmits as _, 0) };

[djc]

Ugh, how old are these Android phones that you are targeting?

If it's not too much complexity, we're happy to take a PR to enable support on older kernels.

[link2xt]

Ugh, how old are these Android phones that you are targeting?

We support Android API 16, so the oldest phones are roughly 10 years old.

I have created a PR for sendmmsg: #1504

[Ralith]

Fixed by #1504.

[mxinden]

I wonder whether the patch merged via #1504 is worth the trouble it is causing. In other words, I would like to discuss switching back to calling libc::recvmmsg instead of libc::syscall(libc::SYS_recvmmsg.

• On Android x86 calling libc::syscall(libc::SYS_recvmmsg instead of libc::recvmmsg leads to a SECCOMP panic. See fix(udp): use libc::recvmmsg on Android x86 #1964.
• At this point Android API Level 16 is 12 years old. Are there any known use-cases running quinn on such old devices? With the security concerns alone, I don't see a use-case outside a completely isolated environment.
• Android 4 (up to API Level 20) has been end-of-life'd 01 Oct 2017. Should quinn support an OS that has been unsupported for 7 years?

//CC @link2xt, @flub and @dignifiedquire (since you have been active on deltachat/deltachat-core-rust#4007)

[link2xt]

Feel free to drop Android 4 support and bump API level to 21 (Android 5.0) if supporting it is too much trouble. This is what tokio did already and we already bump to API level 19 in deltachat/deltachat-android#3215
Slightly better if you can support Android 4.4 (API level 19), but not a big deal if it is bumped to API level 21.

As for real use cases, Delta Chat has at least a few hundred Android 4 users according to Google Play statistics.