feat(deploy): preserve app data on --native when signing key matches

adb_install_all ran an unconditional 'adb uninstall' before 'adb install',
wiping MOB_DATA_DIR (on-device identity, screen stores) on every native deploy
— even an in-place update signed with the same committed debug keystore.

Try 'adb install -r' first (preserves data when signatures match) and only
fall back to uninstall+install when the in-place update is genuinely rejected
(INSTALL_FAILED_UPDATE_INCOMPATIBLE, INSTALL_FAILED_VERSION_DOWNGRADE, ...).
Decision extracted to NativeBuild.needs_clean_reinstall?/2 and unit-tested.

Verified empirically: on the same device, 'mix mob.deploy --native' wiped the
seeded identity while a manual 'adb install -r' of the same keystore-signed APK
preserved it (firstInstallTime unchanged) — confirming the uninstall, not a
signature mismatch, was the cause.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: GenericJam

CHANGELOG.md

@@ -8,6 +8,23 @@ Full module documentation: [hexdocs.pm/mob_dev](https://hexdocs.pm/mob_dev).
 
 ---
 
+## [Unreleased]
+
+### Changed
+- **`mix mob.deploy --native` now preserves on-device app data when the signing
+  key matches.** The Android install path previously ran an unconditional
+  `adb uninstall` before `adb install`, which wiped `MOB_DATA_DIR` (on-device
+  identity, screen stores) on **every** native deploy — even an in-place update
+  signed with the same (e.g. committed) debug keystore. It now attempts
+  `adb install -r` first and only falls back to uninstall + install when the
+  in-place update is genuinely rejected (`INSTALL_FAILED_UPDATE_INCOMPATIBLE`
+  from a signature mismatch, `INSTALL_FAILED_VERSION_DOWNGRADE`, etc.). Apps that
+  pin a committed debug keystore now keep their identity across `--native`
+  redeploys. Decision logic extracted to `NativeBuild.needs_clean_reinstall?/2`
+  and unit-tested.
+
+---
+
 ## [0.6.12] - 2026-06-19
 
 ### Fixed

lib/mob_dev/native_build.ex

@@ -1232,6 +1232,20 @@ defmodule MobDev.NativeBuild do
     end)
   end
 
+  @doc """
+  Decide whether an `adb install -r` result forces a clean (uninstall + install)
+  reinstall.
+
+  True when the in-place update was rejected — a non-zero exit or an
+  `INSTALL_FAILED_*` line (signature mismatch, version downgrade, etc.). A clean
+  reinstall wipes app data (on-device identity, screen stores), so the caller
+  only falls back to it when the in-place update genuinely cannot apply.
+  """
+  @spec needs_clean_reinstall?(String.t(), integer()) :: boolean()
+  def needs_clean_reinstall?(install_output, exit_code) do
+    exit_code != 0 or String.contains?(install_output, "INSTALL_FAILED")
+  end
+
   defp adb_install_all(apk, bundle_id, device_id) do
     case System.cmd("adb", ["devices"], stderr_to_stdout: true) do
       {output, 0} ->
@@ -1250,10 +1264,26 @@ defmodule MobDev.NativeBuild do
             stderr_to_stdout: true
           )
 
-          System.cmd("adb", ["-s", serial, "uninstall", bundle_id], stderr_to_stdout: true)
+          # Try an in-place reinstall first (`install -r`): it preserves app data
+          # (on-device identity, screen stores) when the signing key matches —
+          # the common case once an app pins a committed debug keystore. Only
+          # when the package can't be updated in place (signature mismatch,
+          # version downgrade) do we uninstall + install, which clears app data.
+          {first_out, first_rc} =
+            System.cmd("adb", ["-s", serial, "install", "-r", apk], stderr_to_stdout: true)
 
           {install_out, install_rc} =
-            System.cmd("adb", ["-s", serial, "install", apk], stderr_to_stdout: true)
+            if needs_clean_reinstall?(first_out, first_rc) do
+              IO.puts(
+                "  #{IO.ANSI.yellow()}In-place update rejected — reinstalling clean " <>
+                  "(app data will be cleared)#{IO.ANSI.reset()}"
+              )
+
+              System.cmd("adb", ["-s", serial, "uninstall", bundle_id], stderr_to_stdout: true)
+              System.cmd("adb", ["-s", serial, "install", apk], stderr_to_stdout: true)
+            else
+              {first_out, first_rc}
+            end
 
           if install_rc != 0 or String.contains?(install_out, "INSTALL_FAILED") do
             reason =

test/mob_dev/native_build_test.exs

@@ -1742,4 +1742,24 @@ defmodule MobDev.NativeBuildTest do
       refute NativeBuild.zig_required_message() =~ "—"
     end
   end
+
+  describe "needs_clean_reinstall?/2 (in-place install -r vs uninstall fallback)" do
+    test "false on a successful in-place reinstall — app data is preserved" do
+      refute NativeBuild.needs_clean_reinstall?("Success\n", 0)
+    end
+
+    test "true on signature mismatch (INSTALL_FAILED_UPDATE_INCOMPATIBLE)" do
+      out = "adb: failed to install app.apk: Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE]"
+      assert NativeBuild.needs_clean_reinstall?(out, 1)
+    end
+
+    test "true on version downgrade (INSTALL_FAILED_VERSION_DOWNGRADE)" do
+      out = "Failure [INSTALL_FAILED_VERSION_DOWNGRADE]"
+      assert NativeBuild.needs_clean_reinstall?(out, 1)
+    end
+
+    test "true on a non-zero exit even without an INSTALL_FAILED line" do
+      assert NativeBuild.needs_clean_reinstall?("error: device offline", 1)
+    end
+  end
 end