Set socket keep alive

app/CMakeLists.txt

@@ -3,8 +3,8 @@
 
 cmake_minimum_required(VERSION 3.4.1)
 
-add_library(compact_enc_det SHARED
-	src/main/jni/charset.cc
+add_library(fairemail SHARED
+	src/main/jni/fairemail.cc
 	src/main/jni/compact_enc_det/compact_enc_det/compact_enc_det.cc
 	src/main/jni/compact_enc_det/compact_enc_det/compact_enc_det_hint_code.cc
 	src/main/jni/compact_enc_det/util/encodings/encodings.cc
@@ -17,6 +17,6 @@ find_library(log-lib log)
 
 add_definitions(-DHAVE_MEMRCHR)
 
-target_compile_options(compact_enc_det PRIVATE -Wno-c++11-narrowing)
+target_compile_options(fairemail PRIVATE -Wno-c++11-narrowing)
 
-target_link_libraries(compact_enc_det ${log-lib})
+target_link_libraries(fairemail ${log-lib})

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

@@ -40,7 +40,7 @@ class CharsetHelper {
     ));
 
     static {
-        System.loadLibrary("compact_enc_det");
+        System.loadLibrary("fairemail");
     }
 
     private static native DetectResult jni_detect(byte[] octets);

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

@@ -21,6 +21,7 @@ package eu.faircode.email;
 
 import android.content.Context;
 import android.content.SharedPreferences;
+import android.os.ParcelFileDescriptor;
 import android.security.KeyChain;
 import android.text.TextUtils;
 
@@ -144,6 +145,12 @@ public class EmailService implements AutoCloseable {
     // TLS_FALLBACK_SCSV https://tools.ietf.org/html/rfc7507
     // TLS_EMPTY_RENEGOTIATION_INFO_SCSV https://tools.ietf.org/html/rfc5746
 
+    static {
+        System.loadLibrary("fairemail");
+    }
+
+    private static native int jni_socket_keep_alive(int fd, int seconds);
+
     private EmailService() {
         // Prevent instantiation
     }
@@ -1032,13 +1039,18 @@ public class EmailService implements AutoCloseable {
 
         if (keepAlive) {
             Log.e("Socket keep-alive=" + keepAlive);
-            socket.setKeepAlive(false);
+            socket.setKeepAlive(false); // sets SOL_SOCKET/SO_KEEPALIVE
         }
 
         if (linger >= 0) {
             Log.e("Socket linger=" + linger);
             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);
     }
 
     class UntrustedException extends MessagingException {

app/src/main/jni/fairemail.cc

@@ -1,6 +1,11 @@
 #include <jni.h>
 #include <android/log.h>
 #include <cstdio>
+
+#include <errno.h>
+#include <sys/socket.h>
+#include <netinet/tcp.h>
+
 #include "compact_enc_det/compact_enc_det.h"
 
 void log_android(int prio, const char *fmt, ...) {
@@ -51,3 +56,20 @@ Java_eu_faircode_email_CharsetHelper_jni_1detect(JNIEnv *env, jclass type, jbyte
             (jint) bytes_consumed,
             (jboolean) is_reliable);
 }
+
+extern "C"
+JNIEXPORT jint JNICALL
+Java_eu_faircode_email_EmailService_jni_1socket_1keep_1alive(
+        JNIEnv *env, jclass clazz,
+        jint fd, jint seconds) {
+    // https://linux.die.net/man/2/setsockopt
+    // https://linux.die.net/man/3/setsockopt
+    // https://tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/#setsockopt
+    int value = seconds;
+
+    int res = setsockopt(fd, SOL_TCP, TCP_KEEPIDLE, (void *) &value, sizeof(value));
+    if (res < 0)
+        res = errno;
+
+    return res;
+}