Fix FTP download reporting success on 0-byte files

download_to_file() returned True when retrbinary transferred 0 bytes
without raising an exception. This caused the /cover endpoint to hit
the empty file check and raise HTTP 500 instead of retrying or
returning 404.

Now treats 0-byte downloads as failures, allowing the cover endpoint's
retry logic to work and falling back to 404 if all attempts fail.

CHANGELOG.md

@@ -11,6 +11,7 @@ All notable changes to Bambuddy will be documented in this file.
   - Changed all FTP exception handlers from `except (OSError, ftplib.error_reply)` to `except (OSError, ftplib.Error)` to catch all FTP error types
 - **HTTP 500 on Reprint and Print Endpoints** — Fixed 500 errors on `/api/v1/archives/{id}/reprint` and `/api/v1/library/files/{id}/print` caused by the FTP failure above
 - **Exception Handling Reverted** — Reverted overly-narrow exception handling introduced in 0.1.8 that could cause uncaught errors in archive parsing, HTTP clients, 3MF/ZIP processing, Home Assistant, and firmware checks
+- **HTTP 500 on Printer Cover Image** — Fixed 500 error on `/api/v1/printers/{id}/cover` when FTP download returned 0 bytes but reported success; now retries and falls back to 404
 - **4-Segment Version Support** — Version parser now supports patch releases like `0.1.8.1` for hotfixes without incrementing the minor version
 
 ## [0.1.8] - 2026-02-06

backend/app/services/bambu_ftp.py

@@ -269,6 +269,11 @@ class BambuFTPClient:
                 f.flush()
                 os.fsync(f.fileno())
             file_size = local_path.stat().st_size if local_path.exists() else 0
+            if file_size == 0:
+                logger.warning("FTP download returned 0 bytes for %s", remote_path)
+                if local_path.exists():
+                    local_path.unlink()
+                return False
             logger.info("Successfully downloaded %s to %s (%s bytes)", remote_path, local_path, file_size)
             return True
         except (OSError, ftplib.Error) as e: