feat(auth): implement legacyFetchSignInWithEmail configuration option

.gitignore

@@ -11,3 +11,4 @@ google-services.json
 crashlytics-build.properties
 auth/src/main/res/values/com_crashlytics_export_strings.xml
 *.log
+.kotlin/

app/scripts/run-demo.sh

@@ -0,0 +1,259 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+APP_ID="com.firebaseui.android.demo"
+MAIN_ACTIVITY="com.firebaseui.android.demo.MainActivity"
+APK_RELATIVE_PATH="app/build/outputs/apk/debug/app-debug.apk"
+EMULATOR_LOG="${TMPDIR:-/tmp}/firebaseui-android-emulator.log"
+EMULATOR_PID=""
+LAUNCHED_EMULATOR=0
+CONNECTED_DEVICES=()
+AVAILABLE_AVDS=()
+KNOWN_DEVICE_SERIALS=()
+
+usage() {
+  cat <<'EOF'
+Usage: run-demo.sh [--help]
+
+Builds, installs, and launches the FirebaseUI Android demo app.
+
+The script lets you:
+1. Use an already connected Android device or emulator.
+2. Start an AVD selected from `emulator -list-avds`.
+EOF
+}
+
+require_command() {
+  local command_name="$1"
+  if ! command -v "$command_name" >/dev/null 2>&1; then
+    echo "Missing required command: $command_name" >&2
+    exit 1
+  fi
+}
+
+collect_connected_devices() {
+  CONNECTED_DEVICES=()
+  while IFS= read -r serial; do
+    if [[ -n "$serial" ]]; then
+      CONNECTED_DEVICES+=("$serial")
+    fi
+  done < <(adb devices | awk 'NR > 1 && $2 == "device" { print $1 }')
+}
+
+collect_available_avds() {
+  AVAILABLE_AVDS=()
+  while IFS= read -r avd_name; do
+    if [[ -n "$avd_name" ]]; then
+      AVAILABLE_AVDS+=("$avd_name")
+    fi
+  done < <(emulator -list-avds 2>/dev/null)
+}
+
+prompt_for_choice() {
+  local prompt="$1"
+  shift
+  local options=("$@")
+  local selection
+  local index=1
+
+  echo "$prompt"
+  for option in "${options[@]}"; do
+    printf "  %d) %s\n" "$index" "$option"
+    index=$((index + 1))
+  done
+
+  while true; do
+    printf "Select an option [1-%d]: " "${#options[@]}"
+    read -r selection
+
+    if [[ "$selection" =~ ^[0-9]+$ ]] && (( selection >= 1 && selection <= ${#options[@]} )); then
+      CHOICE_INDEX=$((selection - 1))
+      return 0
+    fi
+
+    echo "Please enter a number between 1 and ${#options[@]}."
+  done
+}
+
+is_known_device() {
+  local candidate="$1"
+  local known
+  local index
+
+  for (( index=0; index<${#KNOWN_DEVICE_SERIALS[@]}; index++ )); do
+    known="${KNOWN_DEVICE_SERIALS[$index]}"
+    if [[ "$known" == "$candidate" ]]; then
+      return 0
+    fi
+  done
+
+  return 1
+}
+
+cleanup_on_exit() {
+  local exit_code="$?"
+
+  if (( exit_code != 0 )) && (( LAUNCHED_EMULATOR == 1 )) && [[ -n "$EMULATOR_PID" ]]; then
+    echo "Stopping emulator started by this script..." >&2
+    kill "$EMULATOR_PID" 2>/dev/null || true
+  fi
+
+  exit "$exit_code"
+}
+
+wait_for_device_boot() {
+  local serial="$1"
+  local attempt
+  local boot_completed
+
+  adb -s "$serial" wait-for-device >/dev/null
+
+  echo "Waiting for $serial to finish booting..."
+  for (( attempt=1; attempt<=120; attempt++ )); do
+    boot_completed="$(adb -s "$serial" shell getprop sys.boot_completed 2>/dev/null | tr -d '\r')"
+    if [[ "$boot_completed" == "1" ]]; then
+      echo "$serial is ready."
+      return 0
+    fi
+    sleep 2
+  done
+
+  echo "Timed out waiting for $serial to boot." >&2
+  exit 1
+}
+
+start_selected_avd() {
+  local avd_name="$1"
+  local attempt
+  local serial
+  local index
+
+  collect_connected_devices
+  KNOWN_DEVICE_SERIALS=()
+  for (( index=0; index<${#CONNECTED_DEVICES[@]}; index++ )); do
+    KNOWN_DEVICE_SERIALS+=("${CONNECTED_DEVICES[$index]}")
+  done
+
+  echo "Starting emulator '$avd_name'..."
+  echo "Emulator logs: $EMULATOR_LOG"
+  emulator -avd "$avd_name" >"$EMULATOR_LOG" 2>&1 &
+  EMULATOR_PID=$!
+  LAUNCHED_EMULATOR=1
+
+  for (( attempt=1; attempt<=120; attempt++ )); do
+    sleep 2
+    collect_connected_devices
+    for (( index=0; index<${#CONNECTED_DEVICES[@]}; index++ )); do
+      serial="${CONNECTED_DEVICES[$index]}"
+      case "$serial" in
+        emulator-*)
+          if ! is_known_device "$serial"; then
+            TARGET_SERIAL="$serial"
+            wait_for_device_boot "$TARGET_SERIAL"
+            return 0
+          fi
+          ;;
+      esac
+    done
+  done
+
+  echo "Failed to detect the new emulator for AVD '$avd_name'." >&2
+  exit 1
+}
+
+choose_target_device() {
+  local option_labels=()
+  local option_types=()
+  local option_values=()
+  local serial
+  local avd_name
+  local index
+
+  collect_connected_devices
+  collect_available_avds
+
+  for (( index=0; index<${#CONNECTED_DEVICES[@]}; index++ )); do
+    serial="${CONNECTED_DEVICES[$index]}"
+    option_labels+=("Use connected device: $serial")
+    option_types+=("device")
+    option_values+=("$serial")
+  done
+
+  for (( index=0; index<${#AVAILABLE_AVDS[@]}; index++ )); do
+    avd_name="${AVAILABLE_AVDS[$index]}"
+    option_labels+=("Start emulator: $avd_name")
+    option_types+=("avd")
+    option_values+=("$avd_name")
+  done
+
+  if (( ${#option_labels[@]} == 0 )); then
+    cat >&2 <<'EOF'
+No connected Android devices were found, and no AVDs are available.
+Connect a device or create an emulator first, then rerun this script.
+EOF
+    exit 1
+  fi
+
+  prompt_for_choice "Choose a target for the demo app:" "${option_labels[@]}"
+
+  case "${option_types[$CHOICE_INDEX]}" in
+    device)
+      TARGET_SERIAL="${option_values[$CHOICE_INDEX]}"
+      ;;
+    avd)
+      start_selected_avd "${option_values[$CHOICE_INDEX]}"
+      ;;
+  esac
+}
+
+main() {
+  local script_dir
+  local repo_root
+  local apk_path
+  local launch_output
+
+  if [[ "${1:-}" == "--help" ]]; then
+    usage
+    exit 0
+  fi
+
+  trap cleanup_on_exit EXIT
+
+  require_command adb
+  require_command emulator
+
+  script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+  repo_root="$(cd "$script_dir/../.." && pwd)"
+  apk_path="$repo_root/$APK_RELATIVE_PATH"
+
+  if [[ ! -x "$repo_root/gradlew" ]]; then
+    echo "gradlew not found or not executable at $repo_root/gradlew" >&2
+    exit 1
+  fi
+
+  choose_target_device
+
+  echo "Building debug APK..."
+  "$repo_root/gradlew" :app:assembleDebug
+
+  if [[ ! -f "$apk_path" ]]; then
+    echo "APK not found at $apk_path" >&2
+    exit 1
+  fi
+
+  echo "Installing app on $TARGET_SERIAL..."
+  adb -s "$TARGET_SERIAL" install -r "$apk_path"
+
+  echo "Launching demo app on $TARGET_SERIAL..."
+  launch_output="$(adb -s "$TARGET_SERIAL" shell am start -n "$APP_ID/$MAIN_ACTIVITY")"
+  echo "$launch_output"
+  if [[ "$launch_output" == *"Error:"* ]]; then
+    echo "Failed to launch the demo app." >&2
+    exit 1
+  fi
+
+  trap - EXIT
+}
+
+main "$@"

auth/src/main/java/com/firebase/ui/auth/AuthException.kt

@@ -200,6 +200,26 @@ abstract class AuthException(
         cause: Throwable? = null
     ) : AuthException(message, cause)
 
+    /**
+     * A different sign-in method should be used for this email address.
+     *
+     * This exception is used for the opt-in legacy recovery path backed by
+     * `fetchSignInMethodsForEmail`, allowing the UI to guide users toward a previously
+     * used provider when email enumeration protection has been disabled.
+     *
+     * @property email The email address being recovered
+     * @property signInMethods The sign-in methods returned by Firebase Auth
+     * @property suggestedSignInMethod The preferred method the UI should direct the user toward
+     * @property cause The underlying authentication failure that triggered the lookup
+     */
+    class DifferentSignInMethodRequiredException(
+        message: String,
+        val email: String,
+        val signInMethods: List<String>,
+        val suggestedSignInMethod: String,
+        cause: Throwable? = null
+    ) : AuthException(message, cause)
+
     /**
      * Authentication was cancelled by the user.
      *

auth/src/main/java/com/firebase/ui/auth/configuration/AuthUIConfiguration.kt

@@ -49,6 +49,7 @@ class AuthUIConfigurationBuilder {
     var isNewEmailAccountsAllowed: Boolean = true
     var isDisplayNameRequired: Boolean = true
     var isProviderChoiceAlwaysShown: Boolean = false
+    var legacyFetchSignInWithEmail: Boolean = false
     var transitions: AuthUITransitions? = null
 
     fun providers(block: AuthProvidersBuilder.() -> Unit) =
@@ -114,6 +115,7 @@ class AuthUIConfigurationBuilder {
             isNewEmailAccountsAllowed = isNewEmailAccountsAllowed,
             isDisplayNameRequired = isDisplayNameRequired,
             isProviderChoiceAlwaysShown = isProviderChoiceAlwaysShown,
+            legacyFetchSignInWithEmail = legacyFetchSignInWithEmail,
             transitions = transitions
         )
     }
@@ -199,6 +201,15 @@ class AuthUIConfiguration(
      */
     val isProviderChoiceAlwaysShown: Boolean = false,
 
+    /**
+     * Enables legacy provider recovery via `fetchSignInMethodsForEmail`.
+     *
+     * This should only be enabled when email enumeration protection is disabled for the
+     * Firebase project and the application explicitly wants to use the legacy API to
+     * recover from email/password attempts made with the wrong provider.
+     */
+    val legacyFetchSignInWithEmail: Boolean = false,
+
     /**
      * Custom screen transition animations.
      * If null, uses default fade in/out transitions.