Preserve thread name after attaching it to JVM (#838)

When native thread is attached to JVM, then it's name is taken from
JavaVMAttachArgs. When no JavaVMAttachArgs, or no JavaVMAttachArgs::name
passed, then JVM on its own decides on taming thread. Those names are
not descriptive. To preserve thread name, pass the currently set thread
name in JavaVMAttachArgs::name. pthread_getname_np was introduced in API
26, hence use more generic approach.

Fixes #837
Authored By: Yauheni Khnykin (@Hsilgos)

src/lib/ares_android.c

@@ -26,6 +26,7 @@
 #if defined(ANDROID) || defined(__ANDROID__)
 #  include "ares_private.h"
 #  include <jni.h>
+#  include <sys/prctl.h>
 
 static JavaVM   *android_jvm                  = NULL;
 static jobject   android_connectivity_manager = NULL;
@@ -80,6 +81,23 @@ static jmethodID jni_get_method_id(JNIEnv *env, jclass cls,
   return mid;
 }
 
+static int jvm_attach(JNIEnv **env)
+{
+  char              name[17] = {0};
+
+  JavaVMAttachArgs  args;
+
+  args.version = JNI_VERSION_1_6;
+  if (prctl(PR_GET_NAME, name) == 0) {
+    args.name = name;
+  } else {
+    args.name = NULL;
+  }
+  args.group   = NULL;
+
+  return (*android_jvm)->AttachCurrentThread(android_jvm, env, &args);
+}
+
 void ares_library_init_jvm(JavaVM *jvm)
 {
   android_jvm = jvm;
@@ -100,7 +118,7 @@ int ares_library_init_android(jobject connectivity_manager)
   res = (*android_jvm)->GetEnv(android_jvm, (void **)&env, JNI_VERSION_1_6);
   if (res == JNI_EDETACHED) {
     env          = NULL;
-    res          = (*android_jvm)->AttachCurrentThread(android_jvm, &env, NULL);
+    res          = jvm_attach(&env);
     need_detatch = 1;
   }
   if (res != JNI_OK || env == NULL) {
@@ -233,7 +251,7 @@ void ares_library_cleanup_android(void)
   res = (*android_jvm)->GetEnv(android_jvm, (void **)&env, JNI_VERSION_1_6);
   if (res == JNI_EDETACHED) {
     env          = NULL;
-    res          = (*android_jvm)->AttachCurrentThread(android_jvm, &env, NULL);
+    res          = jvm_attach(&env);
     need_detatch = 1;
   }
   if (res != JNI_OK || env == NULL) {
@@ -285,7 +303,7 @@ char **ares_get_android_server_list(size_t max_servers, size_t *num_servers)
   res = (*android_jvm)->GetEnv(android_jvm, (void **)&env, JNI_VERSION_1_6);
   if (res == JNI_EDETACHED) {
     env          = NULL;
-    res          = (*android_jvm)->AttachCurrentThread(android_jvm, &env, NULL);
+    res          = jvm_attach(&env);
     need_detatch = 1;
   }
   if (res != JNI_OK || env == NULL) {
@@ -403,7 +421,7 @@ char *ares_get_android_search_domains_list(void)
   res = (*android_jvm)->GetEnv(android_jvm, (void **)&env, JNI_VERSION_1_6);
   if (res == JNI_EDETACHED) {
     env          = NULL;
-    res          = (*android_jvm)->AttachCurrentThread(android_jvm, &env, NULL);
+    res          = jvm_attach(&env);
     need_detatch = 1;
   }
   if (res != JNI_OK || env == NULL) {