Implement magic code validation with Modal volume integration

- Add Base64 decoding of magic codes to retrieve epoch folder names
- Mount MODERATION_MODAL_VOLUME and verify folder existence
- Filter sub-directories by UUID pattern (RFC 4122 format)
- Return segment list to populate Segment IDs dropdown
- Handle errors: invalid codes, missing folders, volume access failures
- Display error messages directly in dropdown label on failure

Addresses all acceptance criteria for magic code backend validation.

app.py

@@ -6,8 +6,10 @@
 
 
 import os
+import re
 import cv2
 import time
+import base64
 import modal
 import logging
 import gradio as gr
@@ -91,34 +93,136 @@ def process_video(video_path, notes, email, company_name) -> str:
         return "Unexpected error during video processing."
 
 
-# Placeholder functions for Segmentation Editing tab (future backend integration)
+# UUID regex pattern for filtering segment sub-directories
+UUID_PATTERN = re.compile(r'^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$')
+
+
 def submit_magic_code(magic_code):
     """
-    Placeholder function to validate magic code and retrieve segment list.
-    TODO: Connect to backend API endpoint for magic code validation.
+    Validate magic code and retrieve segment list from Modal volume.
+
+    Decodes the Base64 magic code to obtain the epoch-based folder name,
+    mounts the moderation_volume, checks folder existence, and lists
+    UUID-pattern sub-directories.
+
+    Returns:
+        Tuple of (dropdown_update, status_message, row2_visibility, row3_visibility, row4_visibility, row5_visibility)
     """
     if not magic_code:
         return (
-            gr.update(choices=[]),  # dropdown
-            "Please enter a magic code",  # status message
-            gr.update(visible=False),  # row2
-            gr.update(visible=False),  # row3
-            gr.update(visible=False),  # row4
-            gr.update(visible=False)   # row5
+            gr.update(choices=[], label="Segment IDs"),
+            "Please enter a magic code",
+            gr.update(visible=False),
+            gr.update(visible=False),
+            gr.update(visible=False),
+            gr.update(visible=False)
         )
 
-    # Placeholder: Simulate retrieving segment IDs
-    logger.info(f"Magic code submitted: {magic_code}")
-    sample_segments = ["Segment_001", "Segment_002", "Segment_003"]
-
-    return (
-        gr.update(choices=sample_segments, value=sample_segments[0]),  # dropdown
-        f"Loaded {len(sample_segments)} segments",  # status message
-        gr.update(visible=True),   # row2
-        gr.update(visible=True),   # row3
-        gr.update(visible=True),   # row4
-        gr.update(visible=True)    # row5
-    )
+    try:
+        # Decode Base64 magic code to get folder name
+        try:
+            decoded_bytes = base64.b64decode(magic_code, validate=True)
+            folder_name = decoded_bytes.decode('utf-8').strip()
+            logger.info(f"Magic code decoded to folder: {folder_name}")
+        except Exception as e:
+            logger.error(f"Failed to decode magic code: {e}")
+            return (
+                gr.update(choices=[], label="Invalid magic code"),
+                "Invalid magic code format – please verify your code",
+                gr.update(visible=False),
+                gr.update(visible=False),
+                gr.update(visible=False),
+                gr.update(visible=False)
+            )
+
+        # Get volume name from environment
+        try:
+            modal_volume_name = os.environ['MODERATION_MODAL_VOLUME']
+        except KeyError:
+            logger.error("MODERATION_MODAL_VOLUME environment variable not set")
+            return (
+                gr.update(choices=[], label="Configuration error"),
+                "Server configuration error – contact support",
+                gr.update(visible=False),
+                gr.update(visible=False),
+                gr.update(visible=False),
+                gr.update(visible=False)
+            )
+
+        # Mount volume and check folder existence
+        try:
+            volume = modal.Volume.from_name(modal_volume_name)
+
+            # List contents at the folder path
+            folder_path = f"/{folder_name}"
+            try:
+                # Use listdir to check if folder exists and list contents
+                entries = volume.listdir(folder_path)
+            except Exception as list_error:
+                logger.error(f"Folder not found or inaccessible: {folder_path} - {list_error}")
+                return (
+                    gr.update(choices=[], label="Folder not found"),
+                    "Folder not found – please verify your magic code",
+                    gr.update(visible=False),
+                    gr.update(visible=False),
+                    gr.update(visible=False),
+                    gr.update(visible=False)
+                )
+
+            # Filter for UUID-pattern sub-directories
+            uuid_segments = []
+            for entry in entries:
+                # Extract just the name (last component of path)
+                entry_name = entry.path.rstrip('/').split('/')[-1]
+
+                # Check if it's a directory and matches UUID pattern
+                if entry.type == modal.volume.FileEntryType.DIRECTORY and UUID_PATTERN.match(entry_name):
+                    uuid_segments.append(entry_name)
+
+            uuid_segments.sort()  # Sort alphabetically for consistent display
+
+            if not uuid_segments:
+                logger.warning(f"No UUID segments found in folder: {folder_name}")
+                return (
+                    gr.update(choices=[], label="No segments found"),
+                    "No segment IDs found in this folder",
+                    gr.update(visible=False),
+                    gr.update(visible=False),
+                    gr.update(visible=False),
+                    gr.update(visible=False)
+                )
+
+            logger.info(f"Found {len(uuid_segments)} UUID segments in folder {folder_name}")
+            return (
+                gr.update(choices=uuid_segments, value=uuid_segments[0], label="Segment IDs"),
+                f"Loaded {len(uuid_segments)} segment(s)",
+                gr.update(visible=True),
+                gr.update(visible=True),
+                gr.update(visible=True),
+                gr.update(visible=True)
+            )
+
+        except Exception as e:
+            logger.error(f"Error accessing Modal volume: {e}")
+            return (
+                gr.update(choices=[], label="Volume access error"),
+                f"Error accessing storage: {str(e)}",
+                gr.update(visible=False),
+                gr.update(visible=False),
+                gr.update(visible=False),
+                gr.update(visible=False)
+            )
+
+    except Exception as e:
+        logger.error(f"Unexpected error in submit_magic_code: {e}")
+        return (
+            gr.update(choices=[], label="Error"),
+            "Unexpected error – please try again",
+            gr.update(visible=False),
+            gr.update(visible=False),
+            gr.update(visible=False),
+            gr.update(visible=False)
+        )
 
 
 def load_segment_frame(segment_id, frame_number):