Missing support for x86_64 Android Emulator

[andyl]

x86_64 Android emulator support listed in support_matrix.md but no x86_64 OTP tarball is published

Environment

• Host OS: Xubuntu 24.04 (x86_64)
• mob: 0.6.14 (hex)
• mob_dev: 0.5.10 (hex)
• Android SDK: emulator 36.5.11.0, NDK 27.2.12479018
• AVD: system-images;android-34;google_apis;x86_64

Summary

guides/support_matrix.md lists x86_64 (emulator) as a supported Android slice:

▎ | Android | arm64-v8a, x86_64 (emulator) | API 28 / Android 9 | Mob ships pre-built BEAM/erts for these slices only. … |

In practice, x86_64 isn't actually supported — there is no x86_64 OTP tarball published, no x86_64 path in the OTP downloader, and no android_x86_64 target in mix mob.release.otp. On an x86_64 Linux host (where ARM emulation is no longer possible — see "Why this matters" below), this combination is a hard wall: you cannot run a mob app in any emulator at all.

What's actually shipped

$ gh release view otp-d9045670 --repo GenericJam/mob --json assets --jq '.assets[].name'
otp-android-arm32-d9045670.tar.gz
otp-android-d9045670.tar.gz          # arm64-v8a only
otp-ios-device-d9045670.tar.gz
otp-ios-sim-d9045670.tar.gz

No otp-android-x86_64-*.tar.gz.

Code references

• mob_dev/lib/mob_dev/otp_downloader.ex:19-24 — ensure_android/1 only switches on "armeabi-v7a"; every other ABI (including "x86_64") falls through to the arm64 tarball.
• mob_dev/lib/mix/tasks/mob.release.otp.ex — @valid_targets is ["android_arm64", "android_arm32", "ios_sim", "ios_device", "all"]. No x86_64 target exists, so even building locally isn't a supported path.
• mob_new's generated android/app/build.gradle defaults abiFilters to 'arm64-v8a', 'armeabi-v7a' — note armeabi-v7a is in the template but isn't in support_matrix.md's "floor" either, while x86_64 is missing despite being listed.

Reproduce

1. On an x86_64 Linux host, mix mob.new myapp && cd myapp && mix mob.install
2. Create and start an x86_64 AVD:

echo "no" | avdmanager create avd -n mob_test -k "system-images;android-34;google_apis;x86_64" -d "pixel_7"
emulator -avd mob_test -no-snapshot

4. mix mob.deploy --native — APK installs, icon appears, launcher tap succeeds.
5. App exits cleanly within ~ms of nativeStartBeam(). No logcat error, no erl_crash.dump. (Matches Incident Missed some folders in deps #1 in beam_crash.md.)
6. mix mob.connect times out waiting for the node:

ando_android_emulator36x5x11x0@127.0.0.1 ...  ✗
✗ sdk_gphone64_x86_64: timed out waiting for any of [...]

Why this matters

On x86_64 Linux hosts, falling back to an arm64 AVD is no longer an option:

$ emulator -avd mob_test_arm64 -no-snapshot
FATAL | Avd's CPU Architecture 'arm64' is not supported by the QEMU2 emulator
on x86_64 host. System image must match the host architecture.

Modern Android Emulator (36.x) dropped arm64-on-x86_64 emulation. Combined with the missing x86_64 OTP, x86_64 Linux developers have no working emulator path — they're forced onto a physical arm64 device.

This is fine for some, but it makes mob unusable for any contributor without spare arm64 hardware.

Suggested fix

Publish an x86_64 OTP tarball alongside the existing two, and wire it through the toolchain:

1. OTP source: add xcomp/erl-xcomp-x86_64-unknown-linux-android.conf (Erlang/OTP repo — same pattern as the existing arm64/arm32 confs). The macOS-host AR/RANLIB fix from common_fixes.md applies equally.
2. MobDev.Release.OTP: add an :android_x86_64 target. Build OpenSSL for the same triple via mix mob.release.openssl.
3. mix mob.release.otp: add "android_x86_64" to @valid_targets.
4. MobDev.OtpDownloader:
   • Add @android_x86_64_name "otp-android-x86_64-#{@otp_hash}".
   • Extend ensure_android/1 and android_otp_dir/1 to handle "x86_64".
5. mix mob.install: call ensure_android("x86_64") alongside the existing arm64 / arm32 calls.
6. mob_new templates:
   • android/app/build.gradle: default abiFilters to 'arm64-v8a', 'x86_64' (drop armeabi-v7a since the support matrix doesn't promise it either, or document it as an opt-in).
   • android/app/src/main/jni/CMakeLists.txt: add an elseif(ANDROID_ABI STREQUAL "x86_64") branch alongside the existing arm32 one, selecting an OTP_RELEASE_X86_64 path.
   • build.gradle: read mob.otp_release_x86_64 from local.properties / MOB_ANDROID_OTP_RELEASE_X86_64 env.
7. mix mob.install: write mob.otp_release_x86_64 into android/local.properties after download.
8. Publish: upload otp-android-x86_64-d9045670.tar.gz to the otp-d9045670 release.

Workaround in the meantime

Either:

• Develop against a physical arm64 phone over USB (works today).
• Update guides/support_matrix.md to reflect reality — strike x86_64 (emulator) from the Android row, or move it to a "planned, not yet shipped" note — so users on x86_64 Linux hosts know up front that they need physical hardware. The current wording reads like a green-light when it's not, and the silent BEAM exit on x86_64 emulators is a particularly painful failure mode to diagnose (no logcat, no crash file).