/********************************************************
* ADO.NET 2.0 Data Provider for SQLite Version 3.X
* Written by Robert Simpson (robert@blackcastlesoft.com)
*
* Released to the public domain, use at your own risk!
********************************************************/
namespace System.Data.SQLite
{
using System;
using System.Collections.Generic;
using System.Threading;
internal static class SQLiteConnectionPool
{
/// <summary>
/// Keeps track of connections made on a specified file. The PoolVersion dictates whether old objects get
/// returned to the pool or discarded when no longer in use.
/// </summary>
internal class Pool
{
internal readonly Queue<WeakReference> Queue = new Queue<WeakReference>();
internal int PoolVersion;
internal int MaxPoolSize;
internal Pool(int version, int maxSize)
{
PoolVersion = version;
MaxPoolSize = maxSize;
}
}
/// <summary>
/// The connection pool object
/// </summary>
private static SortedList<string, Pool> _connections = new SortedList<string, Pool>(StringComparer.OrdinalIgnoreCase);
/// <summary>
/// The default version number new pools will get
/// </summary>
private static int _poolVersion = 1;
/// <summary>
/// The number of connections successfully opened from any pool.
/// This value is incremented by the Remove method.
/// </summary>
private static int _poolOpened = 0;
/// <summary>
/// The number of connections successfully closed from any pool.
/// This value is incremented by the Add method.
/// </summary>
private static int _poolClosed = 0;
/// <summary>
/// Counts the number of pool entries matching the specified file name.
/// </summary>
/// <param name="fileName">The file name to match or null to match all files.</param>
/// <param name="counts">The pool entry counts for each matching file.</param>
/// <param name="openCount">The total number of connections successfully opened from any pool.</param>
/// <param name="closeCount">The total number of connections successfully closed from any pool.</param>
/// <param name="totalCount">The total number of pool entries for all matching files.</param>
internal static void GetCounts(
string fileName,
ref Dictionary<string, int> counts,
ref int openCount,
ref int closeCount,
ref int totalCount
)
{
lock (_connections)
{
openCount = _poolOpened;
closeCount = _poolClosed;
if (counts == null)
{
counts = new Dictionary<string, int>(
StringComparer.OrdinalIgnoreCase);
}
if (fileName != null)
{
Pool queue;
if (_connections.TryGetValue(fileName, out queue))
{
Queue<WeakReference> poolQueue = queue.Queue;
int count = (poolQueue != null) ? poolQueue.Count : 0;
counts.Add(fileName, count);
totalCount += count;
}
}
else
{
foreach (KeyValuePair<string, Pool> pair in _connections)
{
if (pair.Value == null)
continue;
Queue<WeakReference> poolQueue = pair.Value.Queue;
int count = (poolQueue != null) ? poolQueue.Count : 0;
counts.Add(pair.Key, count);
totalCount += count;
}
}
}
}
/// <summary>
/// Attempt to pull a pooled connection out of the queue for active duty
/// </summary>
/// <param name="fileName">The filename for a desired connection</param>
/// <param name="maxPoolSize">The maximum size the connection pool for the filename can be</param>
/// <param name="version">The pool version the returned connection will belong to</param>
/// <returns>Returns NULL if no connections were available. Even if none are, the poolversion will still be a valid pool version</returns>
internal static SQLiteConnectionHandle Remove(string fileName, int maxPoolSize, out int version)
{
int localVersion;
Queue<WeakReference> poolQueue;
//
// NOTE: This lock cannot be held while checking the queue for available
// connections because other methods of this class are called from
// the GC finalizer thread and we use the WaitForPendingFinalizers
// method (below). Holding this lock while calling that method
// would therefore result in a deadlock. Instead, this lock is
// held only while a temporary copy of the queue is created, and
// if necessary, when committing changes back to that original
// queue prior to returning from this method.
//
lock (_connections)
{
Pool queue;
// Default to the highest pool version
version = _poolVersion;
// If we didn't find a pool for this file, create one even though it will be empty.
// We have to do this here because otherwise calling ClearPool() on the file will not work for active connections
// that have never seen the pool yet.
if (_connections.TryGetValue(fileName, out queue) == false)
{
queue = new Pool(_poolVersion, maxPoolSize);
_connections.Add(fileName, queue);
return null;
}
// We found a pool for this file, so use its version number
version = localVersion = queue.PoolVersion;
queue.MaxPoolSize = maxPoolSize;
ResizePool(queue, false);
// Try and get a pooled connection from the queue
poolQueue = queue.Queue;
if (poolQueue == null) return null;
//
// NOTE: Temporarily tranfer the queue for this file into a local
// variable. The queue for this file will be modified and
// then committed back to the real pool list (below) prior
// to returning from this method.
//
_connections.Remove(fileName);
poolQueue = new Queue<WeakReference>(poolQueue);
}
try
{
while (poolQueue.Count > 0)
{
WeakReference cnn = poolQueue.Dequeue();
if (cnn == null) continue;
SQLiteConnectionHandle hdl = cnn.Target as SQLiteConnectionHandle;
if (hdl == null) continue;
//
// BUGFIX: For ticket [996d13cd87], step #1. After this point,
// make sure that the finalizer for the connection
// handle just obtained from the queue cannot START
// running (i.e. it may still be pending but it will no
// longer start after this point).
//
GC.SuppressFinalize(hdl);
try
{
//
// BUGFIX: For ticket [996d13cd87], step #2. Now, we must wait
// for all pending finalizers which have STARTED running
// and have not yet COMPLETED. This must be done just
// in case the finalizer for the connection handle just
// obtained from the queue has STARTED running at some
// point before SuppressFinalize was called on it.
//
// After this point, checking properties of the
// connection handle (e.g. IsClosed) should work
// reliably without having to worry that they will
// (due to the finalizer) change out from under us.
//
GC.WaitForPendingFinalizers();
//
// BUGFIX: For ticket [996d13cd87], step #3. Next, verify that
// the connection handle is actually valid and [still?]
// not closed prior to actually returning it to our
// caller.
//
if (!hdl.IsClosed && !hdl.IsInvalid)
{
Interlocked.Increment(ref _poolOpened);
return hdl;
}
}
finally
{
//
// BUGFIX: For ticket [996d13cd87], step #4. Next, we must
// re-register the connection handle for finalization
// now that we have a strong reference to it (i.e. the
// finalizer will not run at least until the connection
// is subsequently closed).
//
GC.ReRegisterForFinalize(hdl);
}
GC.KeepAlive(hdl);
}
}
finally
{
//
// BUGFIX: For ticket [996d13cd87], step #5. Finally, commit any
// changes to the pool/queue for this database file.
//
lock (_connections)
{
//
// NOTE: We must check [again] if a pool exists for this file
// because one may have been added while the search for
// an available connection was in progress (above).
//
Pool queue;
Queue<WeakReference> newPoolQueue;
bool addPool;
if (_connections.TryGetValue(fileName, out queue))
{
addPool = false;
}
else
{
addPool = true;
queue = new Pool(localVersion, maxPoolSize);
}
newPoolQueue = queue.Queue;
while (poolQueue.Count > 0)
newPoolQueue.Enqueue(poolQueue.Dequeue());
ResizePool(queue, false);
if (addPool)
_connections.Add(fileName, queue);
}
}
return null;
}
/// <summary>
/// Clears out all pooled connections and rev's up the default pool version to force all old active objects
/// not in the pool to get discarded rather than returned to their pools.
/// </summary>
internal static void ClearAllPools()
{
lock (_connections)
{
foreach (KeyValuePair<string, Pool> pair in _connections)
{
if (pair.Value == null)
continue;
Queue<WeakReference> poolQueue = pair.Value.Queue;
while (poolQueue.Count > 0)
{
WeakReference cnn = poolQueue.Dequeue();
if (cnn == null) continue;
SQLiteConnectionHandle hdl = cnn.Target as SQLiteConnectionHandle;
if (hdl != null)
{
hdl.Dispose();
}
GC.KeepAlive(hdl);
}
// Keep track of the highest revision so we can go one higher when we're finished
if (_poolVersion <= pair.Value.PoolVersion)
_poolVersion = pair.Value.PoolVersion + 1;
}
// All pools are cleared and we have a new highest version number to force all old version active items to get discarded
// instead of going back to the queue when they are closed.
// We can get away with this because we've pumped up the _poolVersion out of range of all active connections, so they
// will all get discarded when they try to put themselves back in their pool.
_connections.Clear();
}
}
/// <summary>
/// Clear a given pool for a given filename. Discards anything in the pool for the given file, and revs the pool
/// version so current active objects on the old version of the pool will get discarded rather than be returned to the pool.
/// </summary>
/// <param name="fileName">The filename of the pool to clear</param>
internal static void ClearPool(string fileName)
{
lock (_connections)
{
Pool queue;
if (_connections.TryGetValue(fileName, out queue) == true)
{
queue.PoolVersion++;
Queue<WeakReference> poolQueue = queue.Queue;
if (poolQueue == null) return;
while (poolQueue.Count > 0)
{
WeakReference cnn = poolQueue.Dequeue();
if (cnn == null) continue;
SQLiteConnectionHandle hdl = cnn.Target as SQLiteConnectionHandle;
if (hdl != null)
{
hdl.Dispose();
}
GC.KeepAlive(hdl);
}
}
}
}
/// <summary>
/// Return a connection to the pool for someone else to use.
/// </summary>
/// <param name="fileName">The filename of the pool to use</param>
/// <param name="hdl">The connection handle to pool</param>
/// <param name="version">The pool version the handle was created under</param>
/// <remarks>
/// If the version numbers don't match between the connection and the pool, then the handle is discarded.
/// </remarks>
internal static void Add(string fileName, SQLiteConnectionHandle hdl, int version)
{
lock (_connections)
{
// If the queue doesn't exist in the pool, then it must've been cleared sometime after the connection was created.
Pool queue;
if (_connections.TryGetValue(fileName, out queue) == true && version == queue.PoolVersion)
{
ResizePool(queue, true);
Queue<WeakReference> poolQueue = queue.Queue;
if (poolQueue == null) return;
poolQueue.Enqueue(new WeakReference(hdl, false));
Interlocked.Increment(ref _poolClosed);
}
else
{
hdl.Close();
}
GC.KeepAlive(hdl);
}
}
/// <summary>
/// We don't have to thread-lock anything in this function, because it's only called by other functions above
/// which already have a thread-safe lock.
/// </summary>
/// <param name="queue">The queue to resize</param>
/// <param name="forAdding">If a function intends to add to the pool, this is true, which forces the resize
/// to take one more than it needs from the pool</param>
private static void ResizePool(Pool queue, bool forAdding)
{
int target = queue.MaxPoolSize;
if (forAdding && target > 0) target--;
Queue<WeakReference> poolQueue = queue.Queue;
if (poolQueue == null) return;
while (poolQueue.Count > target)
{
WeakReference cnn = poolQueue.Dequeue();
if (cnn == null) continue;
SQLiteConnectionHandle hdl = cnn.Target as SQLiteConnectionHandle;
if (hdl != null)
{
hdl.Dispose();
}
GC.KeepAlive(hdl);
}
}
}
}