feat: expose the fd from ItemDataDirectAccessInfo across JNI

lib/src/main/cpp/libzim/item.cpp

@@ -48,9 +48,15 @@ GETTER(jlong, getSize)
 
 METHOD0(jobject, getDirectAccessInformation) {
   jobject directObjInfo = newObject("org/kiwix/libzim/DirectAccessInfo", env);
-  setDaiObjValue("", 0, directObjInfo, env);
+  setDaiObjValue("", 0, -1, directObjInfo, env);
 
+#ifndef _WIN32
+  // getDirectAccessFd(): owned dup()'d fd + offset in one call. See #1119.
+  auto directAccessFd = THIS->getDirectAccessFd();
+  setDaiObjValue("", directAccessFd.offset, directAccessFd.fd, directObjInfo, env);
+#else
   auto cDirectObjInfo = THIS->getDirectAccessInformation();
-  setDaiObjValue(cDirectObjInfo.filename, cDirectObjInfo.offset, directObjInfo, env);
+  setDaiObjValue(cDirectObjInfo.filename, cDirectObjInfo.offset, -1, directObjInfo, env);
+#endif
   return directObjInfo;
 } CATCH_EXCEPTION(nullptr)

lib/src/main/cpp/utils.h

@@ -400,13 +400,15 @@ inline void setBoolObjValue(const bool value, const jobject obj, JNIEnv* env)
 }
 
 inline void setDaiObjValue(const std::string& filename, const long offset,
-                            const jobject obj, JNIEnv* env)
+                            const int fd, const jobject obj, JNIEnv* env)
 {
   jclass objClass = env->GetObjectClass(obj);
   jfieldID filenameFid = env->GetFieldID(objClass, "filename", "Ljava/lang/String;");
   env->SetObjectField(obj, filenameFid, c2jni(filename, env));
   jfieldID offsetFid = env->GetFieldID(objClass, "offset", "J");
   env->SetLongField(obj, offsetFid, offset);
+  jfieldID fdFid = env->GetFieldID(objClass, "fd", "I");
+  env->SetIntField(obj, fdFid, fd);
 }
 
 inline void setMigrationResultValue(long nbMigrated, long nbTotalInvalid, const jobject obj, JNIEnv* env)

lib/src/main/java/org/kiwix/libzim/DirectAccessInfo.java

@@ -23,4 +23,10 @@ public class DirectAccessInfo
 {
   public String filename;
   public long offset;
+
+  /**
+   * Owned fd, or -1 if unavailable. `filename` is only set on Windows;
+   * elsewhere prefer ParcelFileDescriptor.adoptFd(fd). Caller must close it.
+   */
+  public int fd = -1;
 }

lib/src/test/test.java

@@ -95,6 +95,9 @@ private void testArchive(TestArchive archive)
         assertNotEquals("", dai.filename);
         byte[] readData = getFileContentPartial(dai.filename, (int) dai.offset, (int) item.getSize());
         assertTrue(Arrays.equals(faviconData, readData));
+        // JNI dup()s an already-open native descriptor for this archive regardless of
+        // whether it was opened by path or by fd - see openzim/libzim#852.
+        assertTrue(dai.fd >= 0);
 
         // Checking all metadata
         assertTrue(archive.hasNewNamespaceScheme());