Add diagram caching module with POST support

- Implement content-based hashing for cache keys
- POST diagram content to Kroki server instead of URL encoding
- Store generated SVGs in local filesystem cache
- Add cache clearing functionality

diagramm_proxy/diagram_cache.py

@@ -0,0 +1,91 @@
+import hashlib
+import os
+import requests
+from pathlib import Path
+from django.conf import settings
+from django.core.files.storage import default_storage
+from django.core.files.base import ContentFile
+import logging
+
+logger = logging.getLogger(__name__)
+
+# Configure cache directory
+CACHE_DIR = getattr(settings, 'DIAGRAM_CACHE_DIR', 'diagram_cache')
+KROKI_UPSTREAM = "http://svckroki:8000"
+
+def get_cache_path(diagram_type, content_hash):
+    """Generate cache file path for a diagram."""
+    return os.path.join(CACHE_DIR, diagram_type, f"{content_hash}.svg")
+
+def compute_hash(content):
+    """Compute SHA256 hash of diagram content."""
+    return hashlib.sha256(content.encode('utf-8')).hexdigest()
+
+def get_cached_diagram(diagram_type, diagram_content):
+    """
+    Retrieve diagram from cache or generate it via POST.
+    
+    Args:
+        diagram_type: Type of diagram (e.g., 'plantuml', 'mermaid')
+        diagram_content: Raw diagram content
+        
+    Returns:
+        Path to cached diagram file (relative to MEDIA_ROOT)
+    """
+    content_hash = compute_hash(diagram_content)
+    cache_path = get_cache_path(diagram_type, content_hash)
+    
+    # Check if diagram exists in cache
+    if default_storage.exists(cache_path):
+        logger.debug(f"Cache hit for {diagram_type} diagram: {content_hash[:8]}")
+        return cache_path
+    
+    # Generate diagram via POST request
+    logger.info(f"Cache miss for {diagram_type} diagram: {content_hash[:8]}, generating...")
+    try:
+        url = f"{KROKI_UPSTREAM}/{diagram_type}/svg"
+        response = requests.post(
+            url,
+            data=diagram_content.encode('utf-8'),
+            headers={'Content-Type': 'text/plain'},
+            timeout=30
+        )
+        response.raise_for_status()
+        
+        # Ensure cache directory exists
+        cache_dir = os.path.dirname(default_storage.path(cache_path))
+        os.makedirs(cache_dir, exist_ok=True)
+        
+        # Save to cache
+        default_storage.save(cache_path, ContentFile(response.content))
+        logger.info(f"Diagram cached successfully: {cache_path}")
+        
+        return cache_path
+        
+    except requests.RequestException as e:
+        logger.error(f"Error generating diagram: {e}")
+        raise
+
+def clear_cache(diagram_type=None):
+    """
+    Clear cached diagrams.
+    
+    Args:
+        diagram_type: If specified, only clear diagrams of this type
+    """
+    if diagram_type:
+        cache_path = os.path.join(CACHE_DIR, diagram_type)
+    else:
+        cache_path = CACHE_DIR
+        
+    if default_storage.exists(cache_path):
+        full_path = default_storage.path(cache_path)
+        # Walk through and delete files
+        for root, dirs, files in os.walk(full_path):
+            for file in files:
+                file_path = os.path.join(root, file)
+                try:
+                    os.remove(file_path)
+                    logger.info(f"Deleted cached diagram: {file_path}")
+                except OSError as e:
+                    logger.error(f"Error deleting {file_path}: {e}")