fs::hard_link: use linkat on Android

[RalfJung]

According to 6249cda, linkat needs API level 21. That is ancient (2014, Android 5). Our target page does not say which minimum Android version or API level we support, it just says:

Rust will support the most recent Long Term Support (LTS) Android Native Development Kit (NDK). By default Rust will support all API levels supported by the NDK, but a higher minimum API level may be required if deemed necessary.

That's pretty useless as I have no idea how I'd figure out what the "API levels support by the NDK" are.
Cc @chriswailes @jfgoog @maurer @pirama-arumuga-nainar

Fixes rust-lang/miri#5080

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miri is developed in its own repository. If possible, consider making this change to rust-lang/miri instead.

cc @rust-lang/miri

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[RalfJung]

I found https://github.com/android/ndk/wiki/Compatibility which does not mention API level 21. Does that mean we still support Android 5...?

[Joel-Wwalker]

The Compatibility page is a "last NDK that supported it" table, so API 21 not appearing there means no NDK has dropped it yet. The place the current floor got set is the r26 changelog: "KitKat (APIs 19 and 20) is no longer supported. The minimum OS supported by the NDK is Lollipop (API level 21)." So the minimum for r26 and everything after is exactly 21, meaning Android 5 is still supported and is the oldest thing that is.

For what Rust actually builds with: the dist images use NDK r26d (src/ci/docker/host-x86_64/dist-android/Dockerfile), which can't target anything below 21 anyway. So assuming linkat here should be safe under the current policy.

[RalfJung]

API level 20 also doesn't appear on that page so apparently has not been dropped yet? But that contradicts what you quote from the r26 changelog.

How do I find out what the "most recent Long Term Support (LTS) Android Native Development Kit (NDK)" is?

[Joel-Wwalker]

API 20 is Android 4.4W, the wearables-only KitKat release (API level table). It never existed as an NDK platform level: meta/platforms.json in both the r26 and r27 release branches has "min": 21 and maps "20": 19 in its alias table. The build system docs spell out the rule: "Not every API level includes new NDK APIs. ... applications with a minSdkVersion of 20 should use API 19 for their NDK target." (API 25 is aliased the same way.) So the Compatibility page has no row for 20 because there was never an NDK level 20 to drop; the r26 changelog's "KitKat (APIs 19 and 20)" is just naming the whole KitKat generation.

For the LTS: the NDK wiki front page lists current releases and support windows. Right now r29 is the newest release and r27d is the LTS ("2024 LTS", "supported until r30 is released"; r30 is in beta). The API 21 floor has been in place since r26, which covers both the r26d that Rust CI builds with and the current LTS.

[RalfJung]

Okay, thanks a lot!

IIUC that means we're good depending on linkat, as the latest LTS release only supports API levels that include linkat.

[maurer]

Sorry for the slow response, I'm on leave at the moment. @Joel-Wwalker is correct though, API 21 is currently last supported and does support linkat.

Re: compatibility, if you want a less glitzy description of when native APIs were introduced, you can look here
The introduced comment, if present, indicates the version it was added in. If there's no comment, it means it's been there since 21 or earlier, and you can pretend it's always been there. If you see arches listed, it means that support is only available on those arches.

[RalfJung]

@the8472 maybe I could get a review from you?

[the8472]

Are those miri tests executed anywhere on an android system/emulator?
All the hard_link tests in the std testsuite are guarded with this:

#[cfg_attr(target_os = "android", ignore = "Android SELinux rules prevent creating hardlinks")]

This was introduced in #102757
So we have nominal hardlink support on android but it's not available to regular apps. OS components might be able to use it, but we're not testing that at the moment.

View changes since the review

[RalfJung]

No, those tests are only run on Linux natively, and inside Miri on various targets.

[the8472]

So Miri is simulating the android target and the restrictions don't apply, ok.

[the8472]

It's unfortunate that we don't have test coverage, but on the other hand android is tier2 and from the "android is just a weird linux" angle this PR looks fine. I opened #159739 about reenabling those tests.

@bors r+ rollup

[rust-bors]

📌 Commit 038a55a has been approved by the8472

It is now in the queue for this repository.