From 46437bbbbc3f8ed8e3af663e8a10c980404103bc Mon Sep 17 00:00:00 2001
From: Adam Hathcock <adam@hathcock.uk>
Date: Tue, 13 Oct 2026 10:21:45 +0100
Subject: [PATCH] Return pooled memory stream blocks when construction fails

---
 src/SharpCompress/IO/PooledMemoryStream.cs    | 22 ++++++++++++++--
 .../Streams/PooledMemoryStreamTests.cs        | 25 +++++++++++++++++++
 2 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/src/SharpCompress/IO/PooledMemoryStream.cs b/src/SharpCompress/IO/PooledMemoryStream.cs
index 3116386f..58633a7c 100644
--- a/src/SharpCompress/IO/PooledMemoryStream.cs
+++ b/src/SharpCompress/IO/PooledMemoryStream.cs
@@ -54,7 +54,15 @@ public sealed class PooledMemoryStream : MemoryStream
         _length = 0;
         _capacity = capacity;
 
-        EnsureSegmentedAllocated(capacity);
+        try
+        {
+            EnsureSegmentedAllocated(capacity);
+        }
+        catch
+        {
+            ReturnPooledBuffers();
+            throw;
+        }
     }
 
     public override bool CanRead => _isOpen;
@@ -565,7 +573,17 @@ public sealed class PooledMemoryStream : MemoryStream
 
         while (_blocks.Count < requiredBlocks)
         {
-            _blocks.Add(_arrayPool.Rent(_blockSize));
+            var block = _arrayPool.Rent(_blockSize);
+            try
+            {
+                _blocks.Add(block);
+            }
+            catch
+            {
+                // A failed list expansion must not lose the rental either.
+                _arrayPool.Return(block);
+                throw;
+            }
         }
 
         while (_blocks.Count > requiredBlocks)
diff --git a/tests/SharpCompress.Test/Streams/PooledMemoryStreamTests.cs b/tests/SharpCompress.Test/Streams/PooledMemoryStreamTests.cs
index d2b0996c..0f318947 100644
--- a/tests/SharpCompress.Test/Streams/PooledMemoryStreamTests.cs
+++ b/tests/SharpCompress.Test/Streams/PooledMemoryStreamTests.cs
@@ -8,6 +8,31 @@ namespace SharpCompress.Test.Streams;
 
 public class PooledMemoryStreamTests
 {
+    [Fact]
+    public void FailedConstructorReturnsPreviouslyRentedBlocks()
+    {
+        var pool = new FailingArrayPool();
+        Assert.Throws<IOException>(() => new PooledMemoryStream(16, 8, pool));
+        Assert.Equal(1, pool.ReturnCount);
+    }
+
+    private sealed class FailingArrayPool : ArrayPool<byte>
+    {
+        private int rentCount;
+        public int ReturnCount { get; private set; }
+
+        public override byte[] Rent(int minimumLength)
+        {
+            if (++rentCount == 2)
+            {
+                throw new IOException("Injected rental failure");
+            }
+            return new byte[minimumLength];
+        }
+
+        public override void Return(byte[] array, bool clearArray = false) => ReturnCount++;
+    }
+
     [Fact]
     public void GrowsUsingFixedSizeBlocks()
     {