fix: reduce unclosed sessions in lazily loaded xarrays

Architecture.md

@@ -0,0 +1,213 @@
+# py-hamt Library Architecture Analysis
+
+## Overview
+
+The py-hamt library is a Python implementation of a Hash Array Mapped Trie (HAMT) designed for content-addressed storage systems like IPFS. It provides efficient storage and retrieval of large key-value mappings using the IPLD data model.
+
+## Core Architecture
+
+### 1. HAMT Structure (`hamt.py`)
+
+The core HAMT implementation follows a hierarchical trie structure where:
+- **Nodes** contain 256 buckets (indexed 0-255)
+- Each bucket can either be:
+  - A dictionary containing key-value mappings (when bucket size ≤ max_bucket_size)
+  - A link to a child Node (when bucket overflows)
+- Hash values are consumed 8 bits at a time to determine bucket indices
+
+#### Key Components:
+
+**Node Class** (`hamt.py:62`)
+- Fixed array of 256 elements representing buckets
+- Uses `list[IPLDKind]` where empty dicts `{}` represent empty buckets
+- Links are stored as single-element lists `[link_id]`
+- Serializes to DAG-CBOR format for content addressing
+
+**HAMT Class** (`hamt.py:287`)
+- Main interface for trie operations
+- Supports both read-only and read-write modes
+- Uses asyncio locks for thread safety in write mode
+- Implements two node storage strategies via NodeStore abstraction
+
+### 2. Storage Abstraction (`store.py`)
+
+**ContentAddressedStore** (`store.py:11`)
+- Abstract base class for content-addressed storage backends
+- Returns immutable IDs (IPLDKind) for stored content
+- Two codec types: "raw" for data, "dag-cbor" for structured content
+
+**KuboCAS** (`store.py:74`)
+- IPFS implementation using Kubo daemon
+- Uses RPC API for writes (`/api/v0/add`)
+- Uses HTTP Gateway for reads (`/ipfs/{cid}`)
+- Supports authentication and custom headers
+- Implements connection pooling and concurrency limiting
+
+**InMemoryCAS** (`store.py:37`)
+- Testing implementation using in-memory dictionary
+- Content-addressed via Blake3 hashing
+
+### 3. Node Storage Strategies
+
+**ReadCacheStore** (`hamt.py:150`)
+- Used in read-only mode
+- Implements LRU-style caching of loaded nodes
+- Cannot perform writes (throws exception)
+- Optimized for concurrent read operations
+
+**InMemoryTreeStore** (`hamt.py:180`)
+- Used in read-write mode
+- Maintains modified nodes in memory buffer
+- Uses UUID4 integers as temporary node IDs
+- Implements sophisticated flush algorithm during `vacate()`
+
+### 4. Zarr Integration (`zarr_hamt_store.py`)
+
+**ZarrHAMTStore** (`zarr_hamt_store.py:11`)
+- Implements Zarr v3 Store interface
+- Provides metadata caching for `zarr.json` files
+- Supports directory listing operations with efficient prefix matching
+- Key insight: Zarr keys map directly to HAMT keys without transformation
+
+**SimpleEncryptedZarrHAMTStore** (`encryption_hamt_store.py:12`)
+- Extends ZarrHAMTStore with ChaCha20-Poly1305 encryption
+- Encrypts all data including metadata
+- Uses 24-byte nonces and 16-byte authentication tags
+
+## Value Setting Mechanism & Data Flow
+
+### Setting Values (`_set_pointer` in `hamt.py:505`)
+
+1. **Hash and Navigate**: Key is hashed, bits extracted to determine path
+2. **Queue-based Insertion**: Uses FIFO queue to handle bucket overflows
+3. **Bucket Overflow Handling**: When bucket exceeds max_bucket_size:
+   - All existing items moved to queue for reinsertion
+   - New child node created and linked
+   - Continues insertion process at deeper level
+4. **Tree Rebalancing**: After insertion, `_reserialize_and_link` updates all affected nodes
+
+### Getting Values (`_get_pointer` in `hamt.py:618`)
+
+1. **Hash Traversal**: Follow hash-determined path through trie
+2. **Bucket Search**: Check final bucket for key
+3. **Pointer Resolution**: Return content-addressed pointer
+4. **Value Retrieval**: Load actual value from CAS using pointer
+
+### Memory Management
+
+**Read-Write Mode**:
+- Uses `InMemoryTreeStore` with UUID-based temporary IDs
+- Modified nodes stay in memory until `make_read_only()` or `cache_vacate()`
+- Flush algorithm uses DFS to preserve parent-child relationships
+
+**Read-Only Mode**:
+- Switches to `ReadCacheStore` for better read performance
+- Allows concurrent operations without locks
+- Cache size can be monitored and manually vacated
+
+## Architecture Gotchas & Edge Cases
+
+### 1. Mode Switching Complexity
+- **Problem**: HAMT can switch between read-only and read-write modes
+- **Gotcha**: In read-write mode, `root_node_id` is invalid until `make_read_only()`
+- **Why**: InMemoryTreeStore uses temporary UUIDs that aren't content-addressed
+- **Solution**: Always call `make_read_only()` before reading `root_node_id`
+
+### 2. Thread Safety Limitations
+- **Problem**: Only async-safe, not thread-safe in write mode
+- **Gotcha**: Multiple threads writing simultaneously can corrupt state
+- **Why**: Uses asyncio.Lock, not threading.Lock
+- **Solution**: Use single event loop for all write operations
+
+### 3. Hash Function Constraints
+- **Problem**: Hash must be multiple of 8 bits (bytes)
+- **Gotcha**: Custom hash functions with odd bit lengths will fail
+- **Why**: `extract_bits` assumes byte-aligned hash values
+- **Solution**: Ensure hash functions return byte-aligned results
+
+### 4. Bucket Size Tuning
+- **Problem**: max_bucket_size affects performance vs memory trade-offs
+- **Gotcha**: Very small bucket sizes (1) force deep trees, large sizes create big nodes
+- **Why**: Small buckets = more CAS operations, large buckets = bigger serialized nodes
+- **Solution**: Test with your specific workload (default 4 is reasonable)
+
+### 5. Empty Node Pruning
+- **Problem**: Deletions can leave empty nodes in tree
+- **Gotcha**: Empty nodes are automatically pruned except root
+- **Why**: Content addressing means identical empty nodes have same hash
+- **Solution**: Pruning is automatic, but be aware root can become empty
+
+### 6. Serialization Edge Cases
+- **Problem**: InMemoryTreeStore nodes contain non-serializable UUID links
+- **Gotcha**: Attempting to serialize buffer nodes directly will fail
+- **Why**: UUID4.int values exceed DAG-CBOR integer limits
+- **Solution**: Always use `vacate()` or `make_read_only()` before accessing serialized form
+
+### 7. CAS ID Immutability Requirement
+- **Problem**: CAS implementations must return immutable IDs
+- **Gotcha**: Using mutable objects (lists, dicts) as IDs breaks assumptions
+- **Why**: HAMT uses ID equality checks and as dictionary keys
+- **Solution**: Ensure CAS returns immutable types (bytes, CID, int, str, etc.)
+
+### 8. Zarr Metadata Caching
+- **Problem**: ZarrHAMTStore caches `zarr.json` files
+- **Gotcha**: Cache isn't invalidated on writes, can become stale
+- **Why**: Zarr frequently re-reads metadata, caching improves performance
+- **Solution**: Cache is updated on writes, but be aware of this optimization
+
+### 9. Concurrent Operations in Read Mode
+- **Problem**: Read-only mode allows concurrent access
+- **Gotcha**: Switching to write mode while reads are happening is unsafe
+- **Why**: Mode switch changes internal data structures
+- **Solution**: Ensure all operations complete before mode switches
+
+### 10. Key Encoding Assumptions
+- **Problem**: Keys are encoded as UTF-8 bytes for hashing
+- **Gotcha**: Non-UTF-8 string keys or binary keys need special handling
+- **Why**: `key.encode()` assumes UTF-8 encoding
+- **Solution**: Ensure keys are valid UTF-8 strings or modify key handling
+
+## Performance Characteristics
+
+### Time Complexity
+- **Get/Set/Delete**: O(log₂₅₆ n) average case, O(depth) worst case
+- **Iteration**: O(n) for all keys/values
+- **Tree depth**: Typically 1-4 levels for most datasets
+
+### Space Complexity
+- **Node overhead**: ~2KB per node (256 × 8-byte pointers)
+- **Memory efficiency**: Improves with higher bucket sizes
+- **CAS efficiency**: Content addressing deduplicates identical subtrees
+
+### Concurrency
+- **Read-only mode**: Full concurrency support
+- **Write mode**: Single-writer, async-safe
+- **Mode switching**: Blocking operation requiring exclusive access
+
+## Integration Patterns
+
+### 1. IPFS Integration
+```python
+kubo_cas = KuboCAS()
+hamt = await HAMT.build(cas=kubo_cas)
+await hamt.set("key", "value")
+await hamt.make_read_only()
+cid = hamt.root_node_id  # IPFS CID
+```
+
+### 2. Zarr Storage
+```python
+hamt = await HAMT.build(cas=kubo_cas, values_are_bytes=True)
+zarr_store = ZarrHAMTStore(hamt, read_only=False)
+dataset.to_zarr(store=zarr_store, zarr_format=3)
+```
+
+### 3. Encrypted Storage
+```python
+encryption_key = get_random_bytes(32)
+encrypted_store = SimpleEncryptedZarrHAMTStore(
+    hamt, read_only=False, encryption_key=encryption_key, header=b"app-name"
+)
+```
+
+The py-hamt library provides a robust, efficient implementation of HAMTs for content-addressed storage with careful attention to memory management, concurrency, and integration patterns. Understanding these architectural details and gotchas is crucial for successful implementation in production systems.

py_hamt/store.py

@@ -212,13 +212,22 @@ def __init__(
     # helper: get or create the session bound to the current running loop   #
     # --------------------------------------------------------------------- #
     def _loop_session(self) -> aiohttp.ClientSession:
+        """Get or create a session for the current event loop with improved cleanup."""
         loop: asyncio.AbstractEventLoop = asyncio.get_running_loop()
         try:
             return self._session_per_loop[loop]
         except KeyError:
+            # Create a session with a connector that closes more quickly
+            connector = aiohttp.TCPConnector(
+                limit=64,
+                limit_per_host=32,
+                force_close=True,  # Force close connections
+                enable_cleanup_closed=True,  # Enable cleanup of closed connections
+            )
+
             sess = aiohttp.ClientSession(
                 timeout=aiohttp.ClientTimeout(total=60),
-                connector=aiohttp.TCPConnector(limit=64, limit_per_host=32),
+                connector=connector,
                 headers=self._default_headers,
                 auth=self._default_auth,
             )
@@ -229,11 +238,23 @@ def _loop_session(self) -> aiohttp.ClientSession:
     # graceful shutdown: close **all** sessions we own                      #
     # --------------------------------------------------------------------- #
     async def aclose(self) -> None:
-        if self._owns_session:
-            for sess in list(self._session_per_loop.values()):
-                if not sess.closed:
+        """Close all internally-created sessions."""
+        if not self._owns_session:
+            # User supplied the session; they are responsible for closing it.
+            return
+
+        for sess in list(self._session_per_loop.values()):
+            if not sess.closed:
+                try:
                     await sess.close()
+                except Exception:
+                    # Best-effort cleanup; ignore errors during shutdown
+                    pass
+
+        self._session_per_loop.clear()
 
+    # At this point, _session_per_loop should be empty or only contain
+    # sessions from loops we haven't seen (which shouldn't happen in practice)
     async def __aenter__(self) -> "KuboCAS":
         return self