MOB-89: cpp_archive NDK resolution honors ANDROID_HOME

lib/mob_dev/native_build.ex

@@ -495,20 +495,10 @@ defmodule MobDev.NativeBuild do
     end
   end
 
-  defp ndk_sysroot do
-    # NDK ships only one prebuilt — darwin-x86_64 even on Apple Silicon
-    # (Apple's Rosetta 2 covers it). On Linux it'd be linux-x86_64.
-    host =
-      case :os.type() do
-        {:unix, :darwin} -> "darwin-x86_64"
-        {:unix, :linux} -> "linux-x86_64"
-        other -> raise "unsupported host for NDK: #{inspect(other)}"
-      end
-
-    sdk_root = System.get_env("ANDROID_HOME") || Path.expand("~/Library/Android/sdk")
-    ndk_version = MobDev.NdkVersion.effective()
-    Path.join([sdk_root, "ndk", ndk_version, "toolchains", "llvm", "prebuilt", host, "sysroot"])
-  end
+  # Single source of truth in MobDev.NdkVersion (honors ANDROID_HOME /
+  # ANDROID_SDK_ROOT + host detection) — shared with cpp_archive / nx_eigen_nif
+  # so the NDK path can't diverge again (MOB-89).
+  defp ndk_sysroot, do: MobDev.NdkVersion.sysroot()
 
   defp resolve_driver_tab_android(mob_dir) do
     resolve_driver_tab(mob_dir, "android", ["android", "jni"])

lib/mob_dev/ndk_version.ex

@@ -189,15 +189,47 @@ defmodule MobDev.NdkVersion do
       default_sdk_root_for_os()
   end
 
-  defp default_sdk_root_for_os do
+  @doc false
+  # The Android NDK toolchain host tag — the NDK ships a single prebuilt
+  # (darwin-x86_64 even on Apple Silicon; Apple's Rosetta 2 covers it).
+  @spec host() :: String.t()
+  def host do
     case :os.type() do
-      {:unix, :darwin} -> Path.expand("~/Library/Android/sdk")
-      {:unix, _} -> Path.expand("~/Android/Sdk")
-      _ -> nil
+      {:unix, :darwin} -> "darwin-x86_64"
+      {:unix, :linux} -> "linux-x86_64"
+      other -> raise "unsupported host for NDK: #{inspect(other)}"
     end
-    |> then(fn path ->
-      if path && File.dir?(path), do: path, else: nil
-    end)
+  end
+
+  @doc false
+  # NDK root for the effective version, honoring ANDROID_HOME / ANDROID_SDK_ROOT
+  # (falls back to the OS-conventional SDK dir even when it doesn't exist, so a
+  # "toolchain not found at <path>" error still names a sensible location). The
+  # single source of truth for `native_build`, `cpp_archive`, and `nx_eigen_nif`
+  # — none of them should re-derive this (see MOB-89).
+  @spec root() :: String.t()
+  def root, do: Path.join([sdk_root() || os_default_sdk_dir(), "ndk", effective()])
+
+  @doc false
+  @spec toolchain_bin() :: String.t()
+  def toolchain_bin, do: Path.join([root(), "toolchains", "llvm", "prebuilt", host(), "bin"])
+
+  @doc false
+  @spec sysroot() :: String.t()
+  def sysroot, do: Path.join([root(), "toolchains", "llvm", "prebuilt", host(), "sysroot"])
+
+  # The OS-conventional SDK dir (path only, no existence check) — used by
+  # `root/0` as the last-resort fallback so error messages name a real path.
+  defp os_default_sdk_dir do
+    case :os.type() do
+      {:unix, :linux} -> Path.expand("~/Android/Sdk")
+      _ -> Path.expand("~/Library/Android/sdk")
+    end
+  end
+
+  defp default_sdk_root_for_os do
+    path = os_default_sdk_dir()
+    if File.dir?(path), do: path, else: nil
   end
 
   @doc """

lib/mob_dev/nx_eigen_nif.ex

@@ -455,12 +455,13 @@ defmodule MobDev.NxEigenNif do
   defp ios_min_version_flag(:ios_sim), do: "-mios-simulator-version-min=#{@ios_min_version}"
   defp ios_min_version_flag(:ios_device), do: "-miphoneos-version-min=#{@ios_min_version}"
 
-  defp default_ndk_root do
-    Path.join([System.user_home!(), "Library/Android/sdk/ndk", NdkVersion.effective()])
-  end
+  # Honors ANDROID_HOME / ANDROID_SDK_ROOT via the shared NdkVersion helper (MOB-89).
+  defp default_ndk_root, do: NdkVersion.root()
 
   defp android_toolchain_bin(opts) do
-    ndk_root = opts[:ndk_root] || default_ndk_root()
-    Path.join([ndk_root, "toolchains/llvm/prebuilt/darwin-x86_64/bin"])
+    case opts[:ndk_root] do
+      nil -> NdkVersion.toolchain_bin()
+      root -> Path.join([root, "toolchains", "llvm", "prebuilt", NdkVersion.host(), "bin"])
+    end
   end
 end

lib/mob_dev/plugin/cpp_archive.ex

@@ -289,12 +289,15 @@ defmodule MobDev.Plugin.CppArchive do
   defp ios_min_flag(:ios_sim), do: "-mios-simulator-version-min=#{@ios_min_version}"
   defp ios_min_flag(:ios_device), do: "-miphoneos-version-min=#{@ios_min_version}"
 
-  defp default_ndk_root do
-    Path.join([System.user_home!(), "Library/Android/sdk/ndk", NdkVersion.effective()])
-  end
+  # Honors ANDROID_HOME / ANDROID_SDK_ROOT via the shared NdkVersion helper —
+  # do NOT re-derive the SDK path here (MOB-89: this used to hardcode
+  # ~/Library/Android/sdk, breaking builds where the NDK lives elsewhere).
+  defp default_ndk_root, do: NdkVersion.root()
 
   defp android_toolchain_bin(opts) do
-    ndk_root = opts[:ndk_root] || default_ndk_root()
-    Path.join([ndk_root, "toolchains/llvm/prebuilt/darwin-x86_64/bin"])
+    case opts[:ndk_root] do
+      nil -> NdkVersion.toolchain_bin()
+      root -> Path.join([root, "toolchains", "llvm", "prebuilt", NdkVersion.host(), "bin"])
+    end
   end
 end

test/mob_dev/ndk_version_test.exs

@@ -149,4 +149,51 @@ defmodule MobDev.NdkVersionTest do
       assert cmd =~ NdkVersion.recommended()
     end
   end
+
+  # The single source of truth cpp_archive / nx_eigen_nif / native_build all use.
+  # MOB-89: cpp_archive + nx_eigen_nif used to hardcode ~/Library/Android/sdk,
+  # ignoring ANDROID_HOME — so a cpp_archive build failed wherever the NDK lived
+  # elsewhere. These pin that root/sysroot/toolchain honor the SDK env.
+  describe "root/0 + host/0 + sysroot/0 (shared NDK path — MOB-89)" do
+    setup do
+      prev_home = System.get_env("ANDROID_HOME")
+      prev_root = System.get_env("ANDROID_SDK_ROOT")
+
+      on_exit(fn ->
+        restore = fn k, v -> if v, do: System.put_env(k, v), else: System.delete_env(k) end
+        restore.("ANDROID_HOME", prev_home)
+        restore.("ANDROID_SDK_ROOT", prev_root)
+      end)
+
+      :ok
+    end
+
+    test "root/0 honors ANDROID_HOME, not a hardcoded ~/Library path" do
+      System.delete_env("ANDROID_SDK_ROOT")
+      System.put_env("ANDROID_HOME", "/opt/custom-sdk")
+
+      assert NdkVersion.root() == Path.join(["/opt/custom-sdk", "ndk", NdkVersion.effective()])
+      refute NdkVersion.root() =~ "Library/Android/sdk"
+    end
+
+    test "root/0 falls back to ANDROID_SDK_ROOT when ANDROID_HOME is unset" do
+      System.delete_env("ANDROID_HOME")
+      System.put_env("ANDROID_SDK_ROOT", "/opt/sdkroot")
+
+      assert NdkVersion.root() =~ "/opt/sdkroot/ndk/"
+    end
+
+    test "host/0 is the single NDK prebuilt tag for this OS" do
+      assert NdkVersion.host() in ["darwin-x86_64", "linux-x86_64"]
+    end
+
+    test "sysroot/0 and toolchain_bin/0 compose root + host" do
+      System.delete_env("ANDROID_SDK_ROOT")
+      System.put_env("ANDROID_HOME", "/opt/custom-sdk")
+      base = Path.join([NdkVersion.root(), "toolchains", "llvm", "prebuilt", NdkVersion.host()])
+
+      assert NdkVersion.sysroot() == Path.join(base, "sysroot")
+      assert NdkVersion.toolchain_bin() == Path.join(base, "bin")
+    end
+  end
 end