Added option to prefer IPv4

app/src/main/java/eu/faircode/email/EmailService.java

@@ -431,6 +431,18 @@ public class EmailService implements AutoCloseable {
             //            new SocketConnectException("Debug", new IOException("Test"), host, port, 0));
 
             main = InetAddress.getByName(host);
+
+            SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
+            boolean prefer_ip4 = prefs.getBoolean("prefer_ip4", false);
+            if (prefer_ip4 && main instanceof Inet6Address) {
+                for (InetAddress iaddr : InetAddress.getAllByName(host))
+                    if (iaddr instanceof Inet4Address) {
+                        main = iaddr;
+                        EntityLog.log(context, "Preferring=" + main);
+                        break;
+                    }
+            }
+
             EntityLog.log(context, "Connecting to " + main);
             _connect(main, port, require_id, user, authenticator, factory);
         } catch (UnknownHostException ex) {

app/src/main/java/eu/faircode/email/FragmentOptionsConnection.java

@@ -58,13 +58,14 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
     private SwitchCompat swRoaming;
     private SwitchCompat swRlah;
     private EditText etTimeout;
+    private SwitchCompat swPreferIp4;
     private SwitchCompat swSslHarden;
     private Button btnManage;
     private TextView tvConnectionType;
     private TextView tvConnectionRoaming;
 
     private final static String[] RESET_OPTIONS = new String[]{
-            "metered", "download", "roaming", "rlah", "timeout", "ssl_harden"
+            "metered", "download", "roaming", "rlah", "timeout", "prefer_ip4", "ssl_harden"
     };
 
     @Override
@@ -82,6 +83,7 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
         swRoaming = view.findViewById(R.id.swRoaming);
         swRlah = view.findViewById(R.id.swRlah);
         etTimeout = view.findViewById(R.id.etTimeout);
+        swPreferIp4 = view.findViewById(R.id.swPreferIp4);
         swSslHarden = view.findViewById(R.id.swSslHarden);
         btnManage = view.findViewById(R.id.btnManage);
 
@@ -153,6 +155,13 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
             }
         });
 
+        swPreferIp4.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
+            @Override
+            public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
+                prefs.edit().putBoolean("prefer_ip4", checked).apply();
+            }
+        });
+
         swSslHarden.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
             @Override
             public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
@@ -266,6 +275,7 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
         etTimeout.setText(timeout == 0 ? null : Integer.toString(timeout));
         etTimeout.setHint(Integer.toString(EmailService.DEFAULT_CONNECT_TIMEOUT));
 
+        swPreferIp4.setChecked(prefs.getBoolean("prefer_ip4", false));
         swSslHarden.setChecked(prefs.getBoolean("ssl_harden", false));
     }
 

app/src/main/java/eu/faircode/email/ServiceSynchronize.java

@@ -134,7 +134,7 @@ public class ServiceSynchronize extends ServiceBase implements SharedPreferences
             "sync_kept",
             "sync_folders",
             "sync_shared_folders",
-            "ssl_harden", // force reconnect
+            "prefer_ip4", "ssl_harden", // force reconnect
             "badge", "unseen_ignored", // force update badge/widget
             "protocol", "debug", // force reconnect
             "auth_plain",

app/src/main/res/layout/fragment_options_connection.xml

@@ -184,6 +184,17 @@
             app:layout_constraintStart_toStartOf="parent"
             app:layout_constraintTop_toBottomOf="@id/tvTimeoutHint" />
 
+        <androidx.appcompat.widget.SwitchCompat
+            android:id="@+id/swPreferIp4"
+            android:layout_width="0dp"
+            android:layout_height="wrap_content"
+            android:layout_marginTop="12dp"
+            android:text="@string/title_advanced_prefer_ip4"
+            app:layout_constraintEnd_toEndOf="parent"
+            app:layout_constraintStart_toStartOf="parent"
+            app:layout_constraintTop_toBottomOf="@id/etTimeout"
+            app:switchPadding="12dp" />
+
         <androidx.appcompat.widget.SwitchCompat
             android:id="@+id/swSslHarden"
             android:layout_width="0dp"
@@ -193,7 +204,7 @@
             android:text="@string/title_advanced_ssl_harden"
             app:layout_constraintEnd_toEndOf="parent"
             app:layout_constraintStart_toStartOf="parent"
-            app:layout_constraintTop_toBottomOf="@id/etTimeout"
+            app:layout_constraintTop_toBottomOf="@id/swPreferIp4"
             app:switchPadding="12dp" />
 
         <eu.faircode.email.FixedTextView

app/src/main/res/values/strings.xml

@@ -328,6 +328,7 @@
     <string name="title_advanced_roaming">Download messages and attachments while roaming</string>
     <string name="title_advanced_rlah">Roam like at home</string>
     <string name="title_advanced_timeout">Connection timeout (seconds)</string>
+    <string name="title_advanced_prefer_ip4">Prefer IPv4 over IPv6</string>
     <string name="title_advanced_ssl_harden">Harden SSL connections</string>
     <string name="title_advanced_manage_connectivity">Manage connectivity</string>