[local] Write files atomically to avoid 500s on concurrent reads (#30)

Author: JornC

source/file-server/src/main/java/nl/aerius/fileserver/local/LocalFileStorageSevice.java

@@ -66,7 +66,17 @@ public void putFile(final String uuid, final String filename, final long size, f
       Files.createDirectory(uuidPath);
     }
     final Path file = filePath(uuidPath, filename);
-    Files.copy(in, file, StandardCopyOption.REPLACE_EXISTING);
+    // Write to a temporary file in the same directory and then atomically move it into place. A plain
+    // Files.copy(REPLACE_EXISTING) deletes the target before recreating it, which exposes a window where a
+    // concurrent read sees the file missing or partially written and fails with a 500. ATOMIC_MOVE (a rename)
+    // ensures readers always observe either the complete old file or the complete new one.
+    final Path tempFile = Files.createTempFile(uuidPath, filename + ".", ".tmp");
+    try {
+      Files.copy(in, tempFile, StandardCopyOption.REPLACE_EXISTING);
+      Files.move(tempFile, file, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING);
+    } finally {
+      Files.deleteIfExists(tempFile);
+    }
   }
 
   @Override

source/file-server/src/test/java/nl/aerius/fileserver/local/LocalFileStorageSeviceTest.java

@@ -16,6 +16,7 @@
  */
 package nl.aerius.fileserver.local;
 
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
@@ -82,6 +83,16 @@ void testPutFileOverwrite() throws IOException {
     assertEquals(overwriteContent, Files.readString(expectedFile.toPath()), "Content of file should be as expected.");
   }
 
+  @Test
+  void testPutFileLeavesNoTempFile() throws IOException {
+    // The atomic-write path stages the content in a temp file before moving it into place; ensure that
+    // temp file is always cleaned up so it can't leak into directory listings or repeated overwrites.
+    service.putFile(UUID_CODE, FILENAME, 10, null, new ByteArrayInputStream(CONTENT.getBytes()));
+    service.putFile(UUID_CODE, FILENAME, 0, null, new ByteArrayInputStream(CONTENT.getBytes()));
+    final String[] storedFiles = expectedFile.getParentFile().list();
+    assertArrayEquals(new String[] {FILENAME}, storedFiles, "Only the target file should remain, no temp residue.");
+  }
+
   @Test
   void testGetFile() throws IOException {
     writeTempFile();