build(android): drop the x86_64 ABI, which no Android can run

The fat APK's second half was 31 MB that cannot execute on any Android
device. modernc.org/libc's Xlstat64 issues a raw lstat syscall on
linux/amd64, and Android's seccomp policy forbids it because bionic
never issues it, so the process takes SIGSYS the first time anything
touches the database -- which for this app is startup. That is every
x86_64 Android, x86 Chromebooks included, not merely the emulator.
arm64 is structurally unaffected: the architecture has no lstat syscall
at all, so modernc routes through fstatat.

27,059,130 bytes to 15,898,465, and one lib/ entry.

Three places had to agree, and the third is what would have made this a
silent no-op: abiFilters (what Gradle packages), android:package rather
than package:fat (what Go *compiles* -- otherwise the library is still
built and then discarded), and the native-code assertion in CI. That
assertion is anchored, `native-code: 'arm64-v8a'$`, because without the
anchor it also matches the fat APK's line and would pass on exactly the
thing it exists to catch. Checked against a real artifact.

Adding the ABI back, if modernc ever fixes Xlstat64, is those same
three edits.

.gitea/workflows/android-apk.yml

@@ -1,7 +1,7 @@
 name: Build & publish the Android APK
 
 # The fifth workflow, and the second that publishes.  It builds a signed
-# fat APK (arm64-v8a + x86_64) on every version tag and puts it in
+# arm64-v8a APK on every version tag and puts it in
 # Gitea's *generic* package registry, which — unlike the repository — is
 # readable without credentials.  That is what lets an Obtainium client
 # poll a plain URL with no token and no public mirror of the source.
@@ -225,7 +225,7 @@ jobs:
       # `$GITHUB_ENV` — where the `env:` dump is only masked for values
       # that are *verbatim* a secret, so a trimmed one could print in
       # clear — or repeating the trimming logic in both.
-      - name: Build the signed fat APK
+      - name: Build the signed APK
         working-directory: /src
         env:
           KEYSTORE_B64: ${{ secrets.ANDROID_KEYSTORE_B64 }}
@@ -333,9 +333,14 @@ jobs:
           ls -la "$apk"
           "$bt/aapt2" dump badging "$apk" | sed -n '1p;/application-label:/p;/native-code/p'
 
-          # Both ABIs, or the artifact is not the fat APK it claims to be.
-          "$bt/aapt2" dump badging "$apk" | grep -q "native-code: 'arm64-v8a' 'x86_64'" || {
-            echo "the APK does not carry both ABIs" >&2; exit 1; }
+          # arm64 and *only* arm64.  x86_64 Android cannot run this app
+          # (modernc's raw lstat against Android's seccomp filter, which
+          # is every x86_64 device and not merely the emulator), so an
+          # x86_64 slice would be ~31 MB that runs nowhere -- and its
+          # reappearance would mean someone had put the ABI back in
+          # app/build.gradle without knowing that.
+          "$bt/aapt2" dump badging "$apk" | grep -q "native-code: 'arm64-v8a'$" || {
+            echo "the APK's ABI set is not exactly arm64-v8a" >&2; exit 1; }
 
           # The identity the pipeline exists to keep stable.
           "$bt/aapt2" dump badging "$apk" | grep -q "versionCode='${{ steps.version.outputs.code }}'" || {

Makefile

@@ -52,8 +52,13 @@ ANDROID_SDK ?= $(HOME)/Android/Sdk
 ANDROID_NDK ?= /opt/android-ndk
 ANDROID_ENV := ANDROID_HOME=$(ANDROID_SDK) ANDROID_SDK_ROOT=$(ANDROID_SDK) ANDROID_NDK_HOME=$(ANDROID_NDK)
 
-android: build-frontend ## Build the fat APK (arm64 + x86_64) into bin/
-	@$(ANDROID_ENV) PATH="$(TOOLBIN):$$PATH" go tool wails3 task android:package:fat
+# `package`, not `package:fat`: x86_64 Android cannot run this app at
+# all (modernc's raw lstat vs Android's seccomp -- see
+# android-tier.md), so the second ABI was ~31 MB that could not run
+# anywhere.  app/build.gradle's abiFilters says the same thing to
+# Gradle; both have to agree or the .so is built and then dropped.
+android: build-frontend ## Build the arm64 APK into bin/
+	@$(ANDROID_ENV) PATH="$(TOOLBIN):$$PATH" go tool wails3 task android:package
 
 android-setup: ## Install the SDK pieces and create the AVD (once, ~3.5GB)
 	@$(ANDROID_ENV) ./scripts/android-emulator.sh setup

build/android/app/build.gradle

@@ -40,9 +40,21 @@ android {
         versionCode Integer.parseInt(System.getenv("YJ_VERSION_CODE") ?: "1")
         versionName System.getenv("YJ_VERSION") ?: "0.0.0"
 
-        // Configure supported ABIs
+        // **arm64 only, and x86_64 is not a gap.**  `modernc.org/libc`'s
+        // Xlstat64 issues a raw lstat syscall on linux/amd64, which
+        // Android's seccomp policy forbids (bionic never issues it), so
+        // the process takes SIGSYS the first time anything touches the
+        // database -- which for this app is startup.  That is every
+        // x86_64 Android, emulators and x86 Chromebooks alike, not just
+        // some.  arm64 is structurally unaffected: the architecture has
+        // no lstat syscall at all, so modernc routes through fstatat.
+        //
+        // So the second ABI was ~31 MB of an artifact that could not run
+        // anywhere.  If modernc fixes it, adding 'x86_64' back here and
+        // to the native-code assertion in android-apk.yml is the whole
+        // change.
         ndk {
-            abiFilters 'arm64-v8a', 'x86_64'
+            abiFilters 'arm64-v8a'
         }
     }
 
@@ -93,7 +105,6 @@ android {
     packagingOptions {
         // Don't strip Go symbols in debug builds
         doNotStrip '*/arm64-v8a/libwails.so'
-        doNotStrip '*/x86_64/libwails.so'
     }
 }