Solves #11156 (#11160)

* Solves #11156

* Check file size for all file format. Disable APPEND if file is empty

* Add test for APPEND mode

modules/core/src/persistence_c.cpp

@@ -206,8 +206,12 @@ cvOpenFileStorage( const char* query, CvMemStorage* dststorage, int flags, const
         // and factor=4 for YAML ( as we use 4 bytes for non ASCII characters (e.g. \xAB))
         int buf_size = CV_FS_MAX_LEN*(fs->fmt == CV_STORAGE_FORMAT_XML ? 6 : 4) + 1024;
 
-        if( append )
+        if (append)
+        {
             fseek( fs->file, 0, SEEK_END );
+            if (ftell(fs->file) == 0)
+                append = false;
+        }
 
         fs->write_stack = cvCreateSeq( 0, sizeof(CvSeq), fs->fmt == CV_STORAGE_FORMAT_XML ?
                 sizeof(CvXMLStackRecord) : sizeof(int), fs->memstorage );
@@ -301,7 +305,7 @@ cvOpenFileStorage( const char* query, CvMemStorage* dststorage, int flags, const
         }
         else if( fs->fmt == CV_STORAGE_FORMAT_YAML )
         {
-            if( !append )
+            if( !append)
                 icvPuts( fs, "%YAML:1.0\n---\n" );
             else
                 icvPuts( fs, "...\n---\n" );

modules/core/test/test_io.cpp

@@ -602,6 +602,24 @@ TEST(Core_InputOutput, FileStorageSpaces)
         EXPECT_NO_THROW(f2[cv::format("key%d", i)] >> valuesRead[i]);
         ASSERT_STREQ(values[i].c_str(), valuesRead[i].c_str());
     }
+    std::string fileName = cv::tempfile(".xml");
+    cv::FileStorage g1(fileName, cv::FileStorage::WRITE);
+    for (size_t i = 0; i < 2; i++) {
+        EXPECT_NO_THROW(g1 << cv::format("key%d", i) << values[i]);
+    }
+    g1.release();
+    cv::FileStorage g2(fileName, cv::FileStorage::APPEND);
+    for (size_t i = 2; i < valueCount; i++) {
+        EXPECT_NO_THROW(g2 << cv::format("key%d", i) << values[i]);
+    }
+    g2.release();
+    cv::FileStorage g3(fileName, cv::FileStorage::READ);
+    std::string valuesReadAppend[valueCount];
+    for (size_t i = 0; i < valueCount; i++) {
+        EXPECT_NO_THROW(g3[cv::format("key%d", i)] >> valuesReadAppend[i]);
+        ASSERT_STREQ(values[i].c_str(), valuesReadAppend[i].c_str());
+    }
+    g3.release();
 }
 
 struct data_t