Added option to enable TCP keep alive

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

@@ -88,6 +88,14 @@ public class ApplicationEx extends Application implements SharedPreferences.OnSh
 
         SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
         final boolean crash_reports = prefs.getBoolean("crash_reports", false);
+
+        try {
+            boolean tcp_keep_alive = prefs.getBoolean("tcp_keep_alive", true);
+            System.setProperty("fairemail.tcp_keep_alive", Boolean.toString(tcp_keep_alive));
+        } catch (Throwable ex) {
+            Log.e(ex);
+        }
+
         prefs.registerOnSharedPreferenceChangeListener(this);
 
         prev = Thread.getDefaultUncaughtExceptionHandler();

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

@@ -23,6 +23,7 @@ import android.content.Context;
 import android.content.SharedPreferences;
 import android.os.ParcelFileDescriptor;
 import android.security.KeyChain;
+import android.system.ErrnoException;
 import android.text.TextUtils;
 
 import androidx.annotation.NonNull;
@@ -131,6 +132,8 @@ public class EmailService implements AutoCloseable {
     private final static int FETCH_SIZE = 1024 * 1024; // bytes, default 16K
     private final static int POOL_TIMEOUT = 45 * 1000; // milliseconds, default 45 sec
 
+    private final static int TCP_KEEP_ALIVE_INTERVAL = 9 * 60; // seconds
+
     private static final int APPEND_BUFFER_SIZE = 4 * 1024 * 1024; // bytes
 
     // https://developer.android.com/reference/javax/net/ssl/SSLSocket.html#protocols
@@ -1047,10 +1050,21 @@ public class EmailService implements AutoCloseable {
             socket.setSoLinger(false, -1);
         }
 
-        int fd = ParcelFileDescriptor.fromSocket(socket).getFd();
-        int errno = jni_socket_keep_alive(fd, 9 * 60);
-        if (errno != 0)
-            Log.e("Socket TCP_KEEPIDLE=" + errno);
+        try {
+            boolean tcp_keep_alive = Boolean.parseBoolean(System.getProperty("fairemail.tcp_keep_alive"));
+            if (tcp_keep_alive) {
+                Log.i("Enabling TCP keep alive");
+
+                int fd = ParcelFileDescriptor.fromSocket(socket).getFd();
+                int errno = jni_socket_keep_alive(fd, TCP_KEEP_ALIVE_INTERVAL);
+                if (errno != 0)
+                    throw new ErrnoException("TCP_KEEPIDLE", errno);
+
+                socket.setKeepAlive(true);
+            }
+        } catch (Throwable ex) {
+            Log.e(ex);
+        }
     }
 
     class UntrustedException extends MessagingException {

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

@@ -61,6 +61,7 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
     private SwitchCompat swRlah;
     private EditText etTimeout;
     private SwitchCompat swPreferIp4;
+    private SwitchCompat swTcpKeepAlive;
     private SwitchCompat swSslHarden;
     private Button btnManage;
     private TextView tvNetworkMetered;
@@ -68,7 +69,7 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
     private TextView tvNetworkInfo;
 
     private final static String[] RESET_OPTIONS = new String[]{
-            "metered", "download", "roaming", "rlah", "timeout", "prefer_ip4", "ssl_harden"
+            "metered", "download", "roaming", "rlah", "timeout", "prefer_ip4", "tcp_keep_alive", "ssl_harden"
     };
 
     @Override
@@ -87,6 +88,7 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
         swRlah = view.findViewById(R.id.swRlah);
         etTimeout = view.findViewById(R.id.etTimeout);
         swPreferIp4 = view.findViewById(R.id.swPreferIp4);
+        swTcpKeepAlive = view.findViewById(R.id.swTcpKeepAlive);
         swSslHarden = view.findViewById(R.id.swSslHarden);
         btnManage = view.findViewById(R.id.btnManage);
 
@@ -166,6 +168,18 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
             }
         });
 
+        swTcpKeepAlive.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
+            @Override
+            public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
+                try {
+                    System.setProperty("fairemail.tcp_keep_alive", Boolean.toString(checked));
+                } catch (Throwable ex) {
+                    Log.e(ex);
+                }
+                prefs.edit().putBoolean("tcp_keep_alive", checked).apply();
+            }
+        });
+
         swSslHarden.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
             @Override
             public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
@@ -281,6 +295,7 @@ public class FragmentOptionsConnection extends FragmentBase implements SharedPre
         etTimeout.setHint(Integer.toString(EmailService.DEFAULT_CONNECT_TIMEOUT));
 
         swPreferIp4.setChecked(prefs.getBoolean("prefer_ip4", true));
+        swTcpKeepAlive.setChecked(prefs.getBoolean("tcp_keep_alive", true));
         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",
-            "prefer_ip4", "ssl_harden", // force reconnect
+            "prefer_ip4", "tcp_keep_alive", "ssl_harden", // force reconnect
             "badge", "unseen_ignored", // force update badge/widget
             "protocol", "debug", // force reconnect
             "auth_plain",