fix: survive permanently broken path_provider channel on Android (#192)

Two changes prevent the crash reported on Samsung S1RXS32.50-13-25:

1. Add WidgetsFlutterBinding.ensureInitialized() to callbackDispatcher so
   that Flutter platform channels (including path_provider) are available
   when WorkManager triggers background sync.  Without it the channel is
   permanently unavailable in that isolate regardless of how long we wait.

2. Add an Android-specific fallback in _resolveDatabasePath: after all
   back-off retries fail, derive the app files-dir path from
   /proc/self/cmdline (the Android process name equals the package name)
   without a platform channel.  This lets the database open on devices
   where path_provider is broken even in the main isolate.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core/sync/background_sync.dart

@@ -6,6 +6,7 @@ import 'package:drift/drift.dart';
 import 'package:drift/native.dart';
 import 'package:enough_mail/enough_mail.dart' as imap;
 import 'package:flutter/services.dart';
+import 'package:flutter/widgets.dart';
 import 'package:path/path.dart' as p;
 import 'package:path_provider/path_provider.dart';
 
@@ -24,6 +25,9 @@ const _kResourceType = 'background_check';
 
 @pragma('vm:entry-point')
 void callbackDispatcher() {
+  // Required so that path_provider and other plugins are available in this
+  // background isolate (issue #192).
+  WidgetsFlutterBinding.ensureInitialized();
   Workmanager().executeTask((_, __) async {
     try {
       await _doBackgroundSync();

lib/data/db/database.dart

@@ -609,6 +609,17 @@ Future<String> _resolveDatabasePath() async {
       await Future<void>.delayed(Duration(milliseconds: ms));
     }
   }
+  // On Android, path_provider can be permanently broken on some devices
+  // regardless of how long we wait (issue #192).  Derive the path from
+  // /proc/self/cmdline (the Android process name == package name) without
+  // a platform channel as a last resort so the app can still open its DB.
+  if (Platform.isAndroid) {
+    final fallback = await _androidFallbackPath();
+    if (fallback != null) {
+      _dbPath = fallback;
+      return _dbPath!;
+    }
+  }
   throw PlatformException(
     code: 'channel-error',
     message: 'path_provider unavailable after ${delays.length + 1} attempts — '
@@ -616,10 +627,44 @@ Future<String> _resolveDatabasePath() async {
   );
 }
 
-// These two functions are only called from unit tests (database_path_test.dart).
+// Reads /proc/self/cmdline to extract the Android package name, then
+// constructs the standard app files-dir path without a platform channel.
+// Returns null when the path cannot be determined or created.
+Future<String?> _androidFallbackPath() async {
+  try {
+    final bytes = await File('/proc/self/cmdline').readAsBytes();
+    final end = bytes.indexOf(0);
+    final packageName = String.fromCharCodes(
+      end >= 0 ? bytes.sublist(0, end) : bytes,
+    ).trim();
+    // A valid Android package name contains dots but not slashes.
+    if (packageName.isEmpty ||
+        !packageName.contains('.') ||
+        packageName.contains('/')) {
+      return null;
+    }
+    for (final base in [
+      '/data/user/0/$packageName/files',
+      '/data/data/$packageName/files',
+    ]) {
+      try {
+        await Directory(base).create(recursive: true);
+        return p.join(base, 'sharedinbox.db');
+      } catch (_) {
+        continue;
+      }
+    }
+    return null;
+  } catch (_) {
+    return null;
+  }
+}
+
+// These functions are only called from unit tests (database_path_test.dart).
 // They expose internals that cannot be reached via the public API.
 Future<String> resolveDatabasePathForTesting() => _resolveDatabasePath();
 void resetDatabasePathForTesting() => _dbPath = null;
+Future<String?> androidFallbackPathForTesting() => _androidFallbackPath();
 
 LazyDatabase _openConnection() {
   return LazyDatabase(() async {

test/unit/database_path_test.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:io';
 
 import 'package:fake_async/fake_async.dart';
 import 'package:flutter/services.dart';
@@ -129,5 +130,27 @@ void main() {
         );
       });
     },
+    // The Android fallback runs only on Android, so on the host machine the
+    // exception is still thrown after all retries.  Skip on Android to avoid
+    // depending on /data/user/0/... being absent in the test environment.
+    skip: Platform.isAndroid,
+  );
+
+  // Regression test for issue #192: _androidFallbackPath must return null when
+  // the process cmdline does not look like an Android package name (e.g. on
+  // the host test machine where the process is the Dart executable).
+  test(
+    '_androidFallbackPath returns null when process name is not a package name',
+    () async {
+      // On non-Android platforms the host process cmdline is a file-system path
+      // (starts with '/'), which the fallback correctly rejects.  On Android
+      // the process IS named after the package — the fallback is free to
+      // succeed or return null depending on the device state; we do not assert
+      // here so as not to constrain Android behaviour.
+      if (!Platform.isAndroid) {
+        final result = await androidFallbackPathForTesting();
+        expect(result, isNull);
+      }
+    },
   );
 }