Fixed InMemoryFile#copyTo

* Does always copy the full contents
* Does always overwrite all contents in the target file
* Added file invariant test to test write after read returns written
bytes

main/filesystem-inmemory/src/main/java/org/cryptomator/filesystem/inmem/InMemoryReadableFile.java

@@ -37,7 +37,8 @@ class InMemoryReadableFile implements ReadableFile {
 	@Override
 	public void copyTo(WritableFile other) throws UncheckedIOException {
 		ByteBuffer source = contentGetter.get().asReadOnlyBuffer();
-		source.position(position);
+		source.position(0);
+		other.truncate();
 		this.position += other.write(source);
 	}
 

main/filesystem-invariants-tests/src/test/java/org/cryptomator/filesystem/invariants/FileTests.java

@@ -0,0 +1,54 @@
+package org.cryptomator.filesystem.invariants;
+
+import static org.cryptomator.filesystem.invariants.matchers.NodeMatchers.hasContent;
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+import static org.junit.Assume.assumeThat;
+
+import java.nio.ByteBuffer;
+
+import org.cryptomator.filesystem.File;
+import org.cryptomator.filesystem.FileSystem;
+import org.cryptomator.filesystem.WritableFile;
+import org.cryptomator.filesystem.invariants.FileSystemFactories.FileSystemFactory;
+import org.cryptomator.filesystem.invariants.WaysToObtainAFile.WayToObtainAFile;
+import org.cryptomator.filesystem.invariants.WaysToObtainAFolder.WayToObtainAFolder;
+import org.junit.Rule;
+import org.junit.experimental.theories.DataPoints;
+import org.junit.experimental.theories.Theories;
+import org.junit.experimental.theories.Theory;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+
+@RunWith(Theories.class)
+public class FileTests {
+
+	@DataPoints
+	public static final Iterable<FileSystemFactory> FILE_SYSTEM_FACTORIES = new FileSystemFactories();
+
+	@DataPoints
+	public static final Iterable<WayToObtainAFolder> WAYS_TO_OBTAIN_A_FOLDER = new WaysToObtainAFolder();
+
+	@DataPoints
+	public static final Iterable<WayToObtainAFile> WAYS_TO_OBTAIN_A_FILE = new WaysToObtainAFile();
+
+	private static final String FILE_NAME = "fileName";
+
+	@Rule
+	public final ExpectedException thrown = ExpectedException.none();
+
+	@Theory
+	public void testWriteToNonExistingFileCreatesFileWithContent(FileSystemFactory fileSystemFactory, WayToObtainAFile wayToObtainANonExistingFile) {
+		assumeThat(wayToObtainANonExistingFile.returnedFilesExist(), is(false));
+		FileSystem fileSystem = fileSystemFactory.create();
+		File file = wayToObtainANonExistingFile.fileWithName(fileSystem, FILE_NAME);
+		byte[] dataToWrite = new byte[] {42, -43, 111, 104, -3, 83, -99, 30};
+
+		try (WritableFile writable = file.openWritable()) {
+			writable.write(ByteBuffer.wrap(dataToWrite));
+		}
+
+		assertThat(file, hasContent(dataToWrite));
+	}
+
+}